ELECTRONICS 101-PROGRAMMING WITH PYTHON-3 (ADVANCED)

Python Object Oriented

Python has been an object-oriented language from day one. Because of this, creating and using classes and objects are downright easy. This chapter helps you become an expert in using Python’s object-oriented programming support.

If you don’t have any previous experience with object-oriented (OO) programming, you may want to consult an introductory course on it or at least a tutorial of some sort so that you have a grasp of the basic concepts.

However, here is small introduction of Object-Oriented Programming (OOP) to bring you at speed:

Overview of OOP Terminology

  • Class: A user-defined prototype for an object that defines a set of attributes that characterize any object of the class. The attributes are data members (class variables and instance variables) and methods, accessed via dot notation.
  • Class variable: A variable that is shared by all instances of a class. Class variables are defined within a class but outside any of the class’s methods. Class variables aren’t used as frequently as instance variables are.
  • Data member: A class variable or instance variable that holds data associated with a class and its objects.
  • Function overloading: The assignment of more than one behavior to a particular function. The operation performed varies by the types of objects (arguments) involved.
  • Instance variable: A variable that is defined inside a method and belongs only to the current instance of a class.
  • Inheritance : The transfer of the characteristics of a class to other classes that are derived from it.
  • Instance: An individual object of a certain class. An object obj that belongs to a class Circle, for example, is an instance of the class Circle.
  • Instantiation : The creation of an instance of a class.
  • Method : A special kind of function that is defined in a class definition.
  • Object : A unique instance of a data structure that’s defined by its class. An object comprises both data members (class variables and instance variables) and methods.
  • Operator overloading: The assignment of more than one function to a particular operator.

Creating Classes:

The class statement creates a new class definition. The name of the class immediately follows the keyword class followed by a colon as follows:

class ClassName:
   'Optional class documentation string'
   class_suite
  • The class has a documentation string, which can be accessed via ClassName.__doc__.
  • The class_suite consists of all the component statements defining class members, data attributes and functions.

Example:

Following is the example of a simple Python class:

class Employee:
   'Common base class for all employees'
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print "Total Employee %d" % Employee.empCount

   def displayEmployee(self):
      print "Name : ", self.name,  ", Salary: ", self.salary
  • The variable empCount is a class variable whose value would be shared among all instances of a this class. This can be accessed as Employee.empCount from inside the class or outside the class.
  • The first method __init__() is a special method, which is called class constructor or initialization method that Python calls when you create a new instance of this class.
  • You declare other class methods like normal functions with the exception that the first argument to each method is self. Python adds the self argument to the list for you; you don’t need to include it when you call the methods.

Creating instance objects:

To create instances of a class, you call the class using class name and pass in whatever arguments its __init__ method accepts.

"This would create first object of Employee class"
emp1 = Employee("Zara", 2000)
"This would create second object of Employee class"
emp2 = Employee("Manni", 5000)

Accessing attributes:

You access the object’s attributes using the dot operator with object. Class variable would be accessed using class name as follows:

emp1.displayEmployee()
emp2.displayEmployee()
print "Total Employee %d" % Employee.empCount

Now, putting all the concepts together:

#!/usr/bin/python

class Employee:
   'Common base class for all employees'
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print "Total Employee %d" % Employee.empCount

   def displayEmployee(self):
      print "Name : ", self.name,  ", Salary: ", self.salary

"This would create first object of Employee class"
emp1 = Employee("Zara", 2000)
"This would create second object of Employee class"
emp2 = Employee("Manni", 5000)
emp1.displayEmployee()
emp2.displayEmployee()
print "Total Employee %d" % Employee.empCount

When the above code is executed, it produces the following result:

Name :  Zara ,Salary:  2000
Name :  Manni ,Salary:  5000
Total Employee 2

You can add, remove or modify attributes of classes and objects at any time:

emp1.age = 7  # Add an 'age' attribute.
emp1.age = 8  # Modify 'age' attribute.
del emp1.age  # Delete 'age' attribute.

Instead of using the normal statements to access attributes, you can use following functions:

  • The getattr(obj, name[, default]) : to access the attribute of object.
  • The hasattr(obj,name) : to check if an attribute exists or not.
  • The setattr(obj,name,value) : to set an attribute. If attribute does not exist, then it would be created.
  • The delattr(obj, name) : to delete an attribute.
hasattr(emp1, 'age')    # Returns true if 'age' attribute exists
getattr(emp1, 'age')    # Returns value of 'age' attribute
setattr(emp1, 'age', 8) # Set attribute 'age' at 8
delattr(empl, 'age')    # Delete attribute 'age'

Built-In Class Attributes:

Every Python class keeps following built-in attributes and they can be accessed using dot operator like any other attribute:

  • __dict__ : Dictionary containing the class’s namespace.
  • __doc__ : Class documentation string or None if undefined.
  • __name__: Class name.
  • __module__: Module name in which the class is defined. This attribute is “__main__” in interactive mode.
  • __bases__ : A possibly empty tuple containing the base classes, in the order of their occurrence in the base class list.

For the above class let’s try to access all these attributes:

#!/usr/bin/python

class Employee:
   'Common base class for all employees'
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print "Total Employee %d" % Employee.empCount

   def displayEmployee(self):
      print "Name : ", self.name,  ", Salary: ", self.salary

print "Employee.__doc__:", Employee.__doc__
print "Employee.__name__:", Employee.__name__
print "Employee.__module__:", Employee.__module__
print "Employee.__bases__:", Employee.__bases__
print "Employee.__dict__:", Employee.__dict__

When the above code is executed, it produces the following result:

Employee.__doc__: Common base class for all employees
Employee.__name__: Employee
Employee.__module__: __main__
Employee.__bases__: ()
Employee.__dict__: {'__module__': '__main__', 'displayCount':
<function displayCount at 0xb7c84994>, 'empCount': 2, 
'displayEmployee': <function displayEmployee at 0xb7c8441c>, 
'__doc__': 'Common base class for all employees', 
'__init__': <function __init__ at 0xb7c846bc>}

Destroying Objects (Garbage Collection):

Python deletes unneeded objects (built-in types or class instances) automatically to free memory space. The process by which Python periodically reclaims blocks of memory that no longer are in use is termed garbage collection.

Python’s garbage collector runs during program execution and is triggered when an object’s reference count reaches zero. An object’s reference count changes as the number of aliases that point to it changes.

An object’s reference count increases when it’s assigned a new name or placed in a container (list, tuple or dictionary). The object’s reference count decreases when it’s deleted with del, its reference is reassigned, or its reference goes out of scope. When an object’s reference count reaches zero, Python collects it automatically.

a = 40      # Create object <40>
b = a       # Increase ref. count  of <40> 
c = [b]     # Increase ref. count  of <40> 

del a       # Decrease ref. count  of <40>
b = 100     # Decrease ref. count  of <40> 
c[0] = -1   # Decrease ref. count  of <40> 

You normally won’t notice when the garbage collector destroys an orphaned instance and reclaims its space. But a class can implement the special method __del__(), called a destructor, that is invoked when the instance is about to be destroyed. This method might be used to clean up any nonmemory resources used by an instance.

Example:

This __del__() destructor prints the class name of an instance that is about to be destroyed:

#!/usr/bin/python

class Point:
   def __init( self, x=0, y=0):
      self.x = x
      self.y = y
   def __del__(self):
      class_name = self.__class__.__name__
      print class_name, "destroyed"

pt1 = Point()
pt2 = pt1
pt3 = pt1
print id(pt1), id(pt2), id(pt3) # prints the ids of the obejcts
del pt1
del pt2
del pt3

When the above code is executed, it produces following result:

3083401324 3083401324 3083401324
Point destroyed

Note: Ideally, you should define your classes in separate file, then you should import them in your main program file using import statement. Kindly check Python – Modules chapter for more details on importing modules and classes.

Class Inheritance:

Instead of starting from scratch, you can create a class by deriving it from a preexisting class by listing the parent class in parentheses after the new class name.

The child class inherits the attributes of its parent class, and you can use those attributes as if they were defined in the child class. A child class can also override data members and methods from the parent.

Syntax:

Derived classes are declared much like their parent class; however, a list of base classes to inherit from are given after the class name:

class SubClassName (ParentClass1[, ParentClass2, ...]):
   'Optional class documentation string'
   class_suite

Example:

#!/usr/bin/python

class Parent:        # define parent class
   parentAttr = 100
   def __init__(self):
      print "Calling parent constructor"

   def parentMethod(self):
      print 'Calling parent method'

   def setAttr(self, attr):
      Parent.parentAttr = attr

   def getAttr(self):
      print "Parent attribute :", Parent.parentAttr

class Child(Parent): # define child class
   def __init__(self):
      print "Calling child constructor"

   def childMethod(self):
      print 'Calling child method'

c = Child()          # instance of child
c.childMethod()      # child calls its method
c.parentMethod()     # calls parent's method
c.setAttr(200)       # again call parent's method
c.getAttr()          # again call parent's method

When the above code is executed, it produces the following result:

Calling child constructor
Calling child method
Calling parent method
Parent attribute : 200

Similar way, you can drive a class from multiple parent classes as follows:

class A:        # define your class A
.....

class B:         # define your calss B
.....

class C(A, B):   # subclass of A and B
.....

You can use issubclass() or isinstance() functions to check a relationships of two classes and instances.

  • The issubclass(sub, sup) boolean function returns true if the given subclass sub is indeed a subclass of the superclass sup.
  • The isinstance(obj, Class) boolean function returns true if obj is an instance of class Class or is an instance of a subclass of Class

Overriding Methods:

You can always override your parent class methods. One reason for overriding parent’s methods is because you may want special or different functionality in your subclass.

Example:

#!/usr/bin/python

class Parent:        # define parent class
   def myMethod(self):
      print 'Calling parent method'

class Child(Parent): # define child class
   def myMethod(self):
      print 'Calling child method'

c = Child()          # instance of child
c.myMethod()         # child calls overridden method

When the above code is executed, it produces the following result:

Calling child method

Base Overloading Methods:

Following table lists some generic functionality that you can override in your own classes:

SN Method, Description & Sample Call
1 __init__ ( self [,args…] )
Constructor (with any optional arguments)
Sample Call : obj = className(args)
2 __del__( self )
Destructor, deletes an object
Sample Call : del obj
3 __repr__( self )
Evaluatable string representation
Sample Call : repr(obj)
4 __str__( self )
Printable string representation
Sample Call : str(obj)
5 __cmp__ ( self, x )
Object comparison
Sample Call : cmp(obj, x)

Overloading Operators:

Suppose you’ve created a Vector class to represent two-dimensional vectors, what happens when you use the plus operator to add them? Most likely Python will yell at you.

You could, however, define the __add__ method in your class to perform vector addition and then the plus operator would behave as per expectation:

Example:

#!/usr/bin/python

class Vector:
   def __init__(self, a, b):
      self.a = a
      self.b = b

   def __str__(self):
      return 'Vector (%d, %d)' % (self.a, self.b)
   
   def __add__(self,other):
      return Vector(self.a + other.a, self.b + other.b)

v1 = Vector(2,10)
v2 = Vector(5,-2)
print v1 + v2

When the above code is executed, it produces the following result:

Vector(7,8)

Data Hiding:

An object’s attributes may or may not be visible outside the class definition. For these cases, you can name attributes with a double underscore prefix, and those attributes will not be directly visible to outsiders.

Example:

#!/usr/bin/python

class JustCounter:
   __secretCount = 0
  
   def count(self):
      self.__secretCount += 1
      print self.__secretCount

counter = JustCounter()
counter.count()
counter.count()
print counter.__secretCount

When the above code is executed, it produces the following result:

1
2
Traceback (most recent call last):
  File "test.py", line 12, in <module>
    print counter.__secretCount
AttributeError: JustCounter instance has no attribute '__secretCount'

Python protects those members by internally changing the name to include the class name. You can access such attributes as object._className__attrName. If you would replace your last line as following, then it would work for you:

.........................
print counter._JustCounter__secretCount

When the above code is executed, it produces the following result:

1
2
2

Python Regular Expressions

A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern. Regular expressions are widely used in UNIX world.

The module re provides full support for Perl-like regular expressions in Python. The re module raises the exception re.error if an error occurs while compiling or using a regular expression.

We would cover two important functions, which would be used to handle regular expressions. But a small thing first: There are various characters, which would have special meaning when they are used in regular expression. To avoid any confusion while dealing with regular expressions, we would use Raw Strings as r’expression’.

The match Function

This function attempts to match RE pattern to string with optional flags.

Here is the syntax for this function:

re.match(pattern, string, flags=0)

Here is the description of the parameters:

Parameter Description
pattern This is the regular expression to be matched.
string This is the string, which would be searched to match the pattern at the beginning of string.
flags You can specify different flags using bitwise OR (|). These are modifiers, which are listed in the table below.

The re.match function returns a match object on success, None on failure. We would use group(num) or groups() function of match object to get matched expression.

Match Object Methods Description
group(num=0) This method returns entire match (or specific subgroup num)
groups() This method returns all matching subgroups in a tuple (empty if there weren’t any)

Example:

#!/usr/bin/python
import re

line = "Cats are smarter than dogs"

matchObj = re.match( r'(.*) are (.*?) .*', line, re.M|re.I)

if matchObj:
   print "matchObj.group() : ", matchObj.group()
   print "matchObj.group(1) : ", matchObj.group(1)
   print "matchObj.group(2) : ", matchObj.group(2)
else:
   print "No match!!"

When the above code is executed, it produces following result:

matchObj.group() :  Cats are smarter than dogs
matchObj.group(1) :  Cats
matchObj.group(2) :  smarter

The search Function

This function searches for first occurrence of RE pattern within string with optional flags.

Here is the syntax for this function:

re.search(pattern, string, flags=0)

Here is the description of the parameters:

Parameter Description
pattern This is the regular expression to be matched.
string This is the string, which would be searched to match the pattern anywhere in the string.
flags You can specify different flags using bitwise OR (|). These are modifiers, which are listed in the table below.

The re.search function returns a match object on success, None on failure. We would use group(num) or groups() function of match object to get matched expression.

Match Object Methods Description
group(num=0) This method returns entire match (or specific subgroup num)
groups() This method returns all matching subgroups in a tuple (empty if there weren’t any)

Example:

#!/usr/bin/python
import re

line = "Cats are smarter than dogs";

searchObj = re.search( r'(.*) are (.*?) .*', line, re.M|re.I)

if searchObj:
   print "searchObj.group() : ", searchObj.group()
   print "searchObj.group(1) : ", searchObj.group(1)
   print "searchObj.group(2) : ", searchObj.group(2)
else:
   print "Nothing found!!"

When the above code is executed, it produces following result:

matchObj.group() :  Cats are smarter than dogs
matchObj.group(1) :  Cats
matchObj.group(2) :  smarter

Matching vs Searching:

Python offers two different primitive operations based on regular expressions: match checks for a match only at the beginning of the string, while search checks for a match anywhere in the string (this is what Perl does by default).

Example:

#!/usr/bin/python
import re

line = "Cats are smarter than dogs";

matchObj = re.match( r'dogs', line, re.M|re.I)
if matchObj:
   print "match --> matchObj.group() : ", matchObj.group()
else:
   print "No match!!"

searchObj = re.search( r'dogs', line, re.M|re.I)
if searchObj:
   print "search --> searchObj.group() : ", searchObj.group()
else:
   print "Nothing found!!"

When the above code is executed, it produces the following result:

No match!!
search --> matchObj.group() :  dogs

Search and Replace:

Some of the most important re methods that use regular expressions is sub.

Syntax:

re.sub(pattern, repl, string, max=0)

This method replaces all occurrences of the RE pattern in string with repl, substituting all occurrences unless max provided. This method would return modified string.

Example:

Following is the example:

#!/usr/bin/python
import re

phone = "2004-959-559 # This is Phone Number"

# Delete Python-style comments
num = re.sub(r'#.*$', "", phone)
print "Phone Num : ", num

# Remove anything other than digits
num = re.sub(r'\D', "", phone)    
print "Phone Num : ", num

When the above code is executed, it produces the following result:

Phone Num :  2004-959-559
Phone Num :  2004959559

Regular-expression Modifiers – Option Flags

Regular expression literals may include an optional modifier to control various aspects of matching. The modifiers are specified as an optional flag. You can provide multiple modifiers using exclusive OR (|), as shown previously and may be represented by one of these:

Modifier Description
re.I Performs case-insensitive matching.
re.L Interprets words according to the current locale. This interpretation affects the alphabetic group (\w and \W), as well as word boundary behavior (\b and \B).
re.M Makes $ match the end of a line (not just the end of the string) and makes ^ match the start of any line (not just the start of the string).
re.S Makes a period (dot) match any character, including a newline.
re.U Interprets letters according to the Unicode character set. This flag affects the behavior of \w, \W, \b, \B.
re.X Permits “cuter” regular expression syntax. It ignores whitespace (except inside a set [] or when escaped by a backslash) and treats unescaped # as a comment marker.

Regular-expression patterns:

Except for control characters, (+ ? . * ^ $ ( ) [ ] { } | \), all characters match themselves. You can escape a control character by preceding it with a backslash.

Following table lists the regular expression syntax that is available in Python:

Pattern Description
^ Matches beginning of line.
$ Matches end of line.
. Matches any single character except newline. Using m option allows it to match newline as well.
[…] Matches any single character in brackets.
[^…] Matches any single character not in brackets
re* Matches 0 or more occurrences of preceding expression.
re+ Matches 1 or more occurrence of preceding expression.
re? Matches 0 or 1 occurrence of preceding expression.
re{ n} Matches exactly n number of occurrences of preceding expression.
re{ n,} Matches n or more occurrences of preceding expression.
re{ n, m} Matches at least n and at most m occurrences of preceding expression.
a| b Matches either a or b.
(re) Groups regular expressions and remembers matched text.
(?imx) Temporarily toggles on i, m, or x options within a regular expression. If in parentheses, only that area is affected.
(?-imx) Temporarily toggles off i, m, or x options within a regular expression. If in parentheses, only that area is affected.
(?: re) Groups regular expressions without remembering matched text.
(?imx: re) Temporarily toggles on i, m, or x options within parentheses.
(?-imx: re) Temporarily toggles off i, m, or x options within parentheses.
(?#…) Comment.
(?= re) Specifies position using a pattern. Doesn’t have a range.
(?! re) Specifies position using pattern negation. Doesn’t have a range.
(?> re) Matches independent pattern without backtracking.
\w Matches word characters.
\W Matches nonword characters.
\s Matches whitespace. Equivalent to [\t\n\r\f].
\S Matches nonwhitespace.
\d Matches digits. Equivalent to [0-9].
\D Matches nondigits.
\A Matches beginning of string.
\Z Matches end of string. If a newline exists, it matches just before newline.
\z Matches end of string.
\G Matches point where last match finished.
\b Matches word boundaries when outside brackets. Matches backspace (0x08) when inside brackets.
\B Matches nonword boundaries.
\n, \t, etc. Matches newlines, carriage returns, tabs, etc.
\1…\9 Matches nth grouped subexpression.
\10 Matches nth grouped subexpression if it matched already. Otherwise refers to the octal representation of a character code.

Regular-expression Examples

Literal characters:

Example Description
python Match “python”.

Character classes:

Example Description
[Pp]ython Match “Python” or “python”
rub[ye] Match “ruby” or “rube”
[aeiou] Match any one lowercase vowel
[0-9] Match any digit; same as [0123456789]
[a-z] Match any lowercase ASCII letter
[A-Z] Match any uppercase ASCII letter
[a-zA-Z0-9] Match any of the above
[^aeiou] Match anything other than a lowercase vowel
[^0-9] Match anything other than a digit

Special Character Classes:

Example Description
. Match any character except newline
\d Match a digit: [0-9]
\D Match a nondigit: [^0-9]
\s Match a whitespace character: [ \t\r\n\f]
\S Match nonwhitespace: [^ \t\r\n\f]
\w Match a single word character: [A-Za-z0-9_]
\W Match a nonword character: [^A-Za-z0-9_]

Repetition Cases:

Example Description
ruby? Match “rub” or “ruby”: the y is optional
ruby* Match “rub” plus 0 or more ys
ruby+ Match “rub” plus 1 or more ys
\d{3} Match exactly 3 digits
\d{3,} Match 3 or more digits
\d{3,5} Match 3, 4, or 5 digits

Nongreedy repetition:

This matches the smallest number of repetitions:

Example Description
<.*> Greedy repetition: matches “<python>perl>”
<.*?> Nongreedy: matches “<python>” in “<python>perl>”

Grouping with parentheses:

Example Description
\D\d+ No group: + repeats \d
(\D\d)+ Grouped: + repeats \D\d pair
([Pp]ython(, )?)+ Match “Python”, “Python, python, python”, etc.

Backreferences:

This matches a previously matched group again:

Example Description
([Pp])ython&\1ails Match python&pails or Python&Pails
([‘”])[^\1]*\1 Single or double-quoted string. \1 matches whatever the 1st group matched . \2 matches whatever the 2nd group matched, etc.

Alternatives:

Example Description
python|perl Match “python” or “perl”
rub(y|le)) Match “ruby” or “ruble”
Python(!+|\?) “Python” followed by one or more ! or one ?

Anchors:

This needs to specify match position.

Example Description
^Python Match “Python” at the start of a string or internal line
Python$ Match “Python” at the end of a string or line
\APython Match “Python” at the start of a string
Python\Z Match “Python” at the end of a string
\bPython\b Match “Python” at a word boundary
\brub\B \B is nonword boundary: match “rub” in “rube” and “ruby” but not alone
Python(?=!) Match “Python”, if followed by an exclamation point
Python(?!!) Match “Python”, if not followed by an exclamation point

Special syntax with parentheses:

Example Description
R(?#comment) Matches “R”. All the rest is a comment
R(?i)uby Case-insensitive while matching “uby”
R(?i:uby) Same as above
rub(?:y|le)) Group only without creating \1 backreference

Python CGI Programming

What is CGI?

  • The Common Gateway Interface, or CGI, is a set of standards that define how information is exchanged between the web server and a custom script.
  • The CGI specs are currently maintained by the NCSA and NCSA defines CGI is as follows:
  • The Common Gateway Interface, or CGI, is a standard for external gateway programs to interface with information servers such as HTTP servers.
  • The current version is CGI/1.1 and CGI/1.2 is under progress.

Web Browsing

To understand the concept of CGI, lets see what happens when we click a hyper link to browse a particular web page or URL.

  • Your browser contacts the HTTP web server and demands for the URL i.e., filename.
  • Web Server will parse the URL and will look for the filename in if it finds that file then sends it back to the browser, otherwise sends an error message indicating that you have requested a wrong file.
  • Web browser takes response from web server and displays either the received file or error message.

However, it is possible to set up the HTTP server so that whenever a file in a certain directory is requested that file is not sent back; instead it is executed as a program, and whatever that program outputs is sent back for your browser to display. This function is called the Common Gateway Interface or CGI and the programs are called CGI scripts. These CGI programs can be a Python Script, PERL Script, Shell Script, C or C++ program, etc.

CGI Architecture Diagram

CGI Architecture

Web Server Support & Configuration

Before you proceed with CGI Programming, make sure that your Web Server supports CGI and it is configured to handle CGI Programs. All the CGI Programs to be executed by the HTTP server are kept in a pre-configured directory. This directory is called CGI Directory and by convention it is named as /var/www/cgi-bin. By convention, CGI files will have extension as .cgi, but you can keep your files with python extension .py as well.

By default, the Linux server is configured to run only the scripts in the cgi-bin directory in /var/www. If you want to specify any other directory to run your CGI scripts, comment the following lines in the httpd.conf file:

<Directory "/var/www/cgi-bin">
   AllowOverride None
   Options ExecCGI
   Order allow,deny
   Allow from all
</Directory>

<Directory "/var/www/cgi-bin">
Options All
</Directory>

Here, I assumed that you have Web Server up and running successfully and you are able to run any other CGI program like Perl or Shell, etc.

First CGI Program

Here is a simple link, which is linked to a CGI script called hello.py. This file is being kept in /var/www/cgi-bin directory and it has following content. Before running your CGI program, make sure you have change mode of file using chmod 755 hello.py UNIX command to make file executable.

#!/usr/bin/python

print "Content-type:text/html\r\n\r\n"
print '<html>'
print '<head>'
print '<title>Hello Word - First CGI Program</title>'
print '</head>'
print '<body>'
print '<h2>Hello Word! This is my first CGI program</h2>'
print '</body>'
print '</html>'

If you click hello.py, then this produces the following output:

Hello Word! This is my first CGI program

This hello.py script is a simple Python script, which is writing its output on STDOUT file i.e., screen. There is one important and extra feature available which is first line to be printed Content-type:text/html\r\n\r\n. This line is sent back to the browser and specifiy the content type to be displayed on the browser screen.

Now, you must have understood basic concept of CGI and you can write many complicated CGI programs using Python. This script can interact with any other external system also to exchange information such as RDBMS.

HTTP Header

The line Content-type:text/html\r\n\r\n is part of HTTP header which is sent to the browser to understand the content. All the HTTP header will be in the following form:

HTTP Field Name: Field Content

For Example
Content-type: text/html\r\n\r\n

There are few other important HTTP headers, which you will use frequently in your CGI Programming.

Header Description
Content-type: A MIME string defining the format of the file being returned. Example is Content-type:text/html
Expires: Date The date the information becomes invalid. This should be used by the browser to decide when a page needs to be refreshed. A valid date string should be in the format 01 Jan 1998 12:00:00 GMT.
Location: URL The URL that should be returned instead of the URL requested. You can use this field to redirect a request to any file.
Last-modified: Date The date of last modification of the resource.
Content-length: N The length, in bytes, of the data being returned. The browser uses this value to report the estimated download time for a file.
Set-Cookie: String Set the cookie passed through the string

CGI Environment Variables

All the CGI program will have access to the following environment variables. These variables play an important role while writing any CGI program.

Variable Name Description
CONTENT_TYPE The data type of the content. Used when the client is sending attached content to the server. For example, file upload, etc.
CONTENT_LENGTH The length of the query information. It’s available only for POST requests.
HTTP_COOKIE Returns the set cookies in the form of key & value pair.
HTTP_USER_AGENT The User-Agent request-header field contains information about the user agent originating the request. Its name of the web browser.
PATH_INFO The path for the CGI script.
QUERY_STRING The URL-encoded information that is sent with GET method request.
REMOTE_ADDR The IP address of the remote host making the request. This can be useful for logging or for authentication purpose.
REMOTE_HOST The fully qualified name of the host making the request. If this information is not available then REMOTE_ADDR can be used to get IR address.
REQUEST_METHOD The method used to make the request. The most common methods are GET and POST.
SCRIPT_FILENAME The full path to the CGI script.
SCRIPT_NAME The name of the CGI script.
SERVER_NAME The server’s hostname or IP Address
SERVER_SOFTWARE The name and version of the software the server is running.

Here is small CGI program to list out all the CGI variables. Click this link to see the result Get Environment

#!/usr/bin/python

import os

print "Content-type: text/html\r\n\r\n";
print "<font size=+1>Environment</font><\br>";
for param in os.environ.keys():
  print "<b>%20s</b>: %s<\br>" % (param, os.environ[param])

GET and POST Methods

You must have come across many situations when you need to pass some information from your browser to web server and ultimately to your CGI Program. Most frequently, browser uses two methods two pass this information to web server. These methods are GET Method and POST Method.

Passing Information using GET method:

The GET method sends the encoded user information appended to the page request. The page and the encoded information are separated by the ? character as follows:

http://www.test.com/cgi-bin/hello.py?key1=value1&key2=value2

The GET method is the default method to pass information from browser to web server and it produces a long string that appears in your browser’s Location:box. Never use GET method if you have password or other sensitive information to pass to the server. The GET method has size limtation: only 1024 characters can be sent in a request string. The GET method sends information using QUERY_STRING header and will be accessible in your CGI Program through QUERY_STRING environment variable.

You can pass information by simply concatenating key and value pairs along with any URL or you can use HTML <FORM> tags to pass information using GET method.

Simple URL Example : Get Method

Here is a simple URL, which will pass two values to hello_get.py program using GET method.

/cgi-bin/hello_get.py?first_name=ZARA&last_name=ALI

Below is hello_get.py script to handle input given by web browser. We are going to use cgi module, which makes it very easy to access passed information:

#!/usr/bin/python

# Import modules for CGI handling 
import cgi, cgitb 

# Create instance of FieldStorage 
form = cgi.FieldStorage() 

# Get data from fields
first_name = form.getvalue('first_name')
last_name  = form.getvalue('last_name')

print "Content-type:text/html\r\n\r\n"
print "<html>"
print "<head>"
print "<title>Hello - Second CGI Program</title>"
print "</head>"
print "<body>"
print "<h2>Hello %s %s</h2>" % (first_name, last_name)
print "</body>"
print "</html>"

This would generate the following result:

Hello ZARA ALI

Simple FORM Example: GET Method

Here is a simple example which passes two values using HTML FORM and submit button. We are going to use same CGI script hello_get.py to handle this imput.

<form action="/cgi-bin/hello_get.py" method="get">
First Name: <input type="text" name="first_name">  <br />

Last Name: <input type="text" name="last_name" />
<input type="submit" value="Submit" />
</form>

Here is the actual output of the above form, You enter First and Last Name and then click submit button to see the result.

First Name:
Last Name:

Passing Information using POST method:

A generally more reliable method of passing information to a CGI program is the POST method. This packages the information in exactly the same way as GET methods, but instead of sending it as a text string after a ? in the URL it sends it as a separate message. This message comes into the CGI script in the form of the standard input.

Below is same hello_get.py script which handles GET as well as POST method.

#!/usr/bin/python

# Import modules for CGI handling 
import cgi, cgitb 

# Create instance of FieldStorage 
form = cgi.FieldStorage() 

# Get data from fields
first_name = form.getvalue('first_name')
last_name  = form.getvalue('last_name')

print "Content-type:text/html\r\n\r\n"
print "<html>"
print "<head>"
print "<title>Hello - Second CGI Program</title>"
print "</head>"
print "<body>"
print "<h2>Hello %s %s</h2>" % (first_name, last_name)
print "</body>"
print "</html>"

Let us take again same example as above which passes two values using HTML FORM and submit button. We are going to use same CGI script hello_get.py to handle this imput.

<form action="/cgi-bin/hello_get.py" method="post">
First Name: <input type="text" name="first_name"><br />
Last Name: <input type="text" name="last_name" />

<input type="submit" value="Submit" />
</form>

Here is the actual output of the above form. You enter First and Last Name and then click submit button to see the result.

First Name:
Last Name:

Passing Checkbox Data to CGI Program

Checkboxes are used when more than one option is required to be selected.

Here is example HTML code for a form with two checkboxes:

<form action="/cgi-bin/checkbox.cgi" method="POST" target="_blank">
<input type="checkbox" name="maths" value="on" /> Maths
<input type="checkbox" name="physics" value="on" /> Physics
<input type="submit" value="Select Subject" />
</form>

The result of this code is the following form:

Maths PhysicsBelow is checkbox.cgi script to handle input given by web browser for checkbox button.

#!/usr/bin/python

# Import modules for CGI handling 
import cgi, cgitb 

# Create instance of FieldStorage 
form = cgi.FieldStorage() 

# Get data from fields
if form.getvalue('maths'):
   math_flag = "ON"
else:
   math_flag = "OFF"

if form.getvalue('physics'):
   physics_flag = "ON"
else:
   physics_flag = "OFF"

print "Content-type:text/html\r\n\r\n"
print "<html>"
print "<head>"
print "<title>Checkbox - Third CGI Program</title>"
print "</head>"
print "<body>"
print "<h2> CheckBox Maths is : %s</h2>" % math_flag
print "<h2> CheckBox Physics is : %s</h2>" % physics_flag
print "</body>"
print "</html>"

Passing Radio Button Data to CGI Program

Radio Buttons are used when only one option is required to be selected.

Here is example HTML code for a form with two radio buttons:

<form action="/cgi-bin/radiobutton.py" method="post" target="_blank">
<input type="radio" name="subject" value="maths" /> Maths
<input type="radio" name="subject" value="physics" /> Physics
<input type="submit" value="Select Subject" />
</form>

The result of this code is the following form:

Maths PhysicsBelow is radiobutton.py script to handle input given by web browser for radio button:

#!/usr/bin/python

# Import modules for CGI handling 
import cgi, cgitb 

# Create instance of FieldStorage 
form = cgi.FieldStorage() 

# Get data from fields
if form.getvalue('subject'):
   subject = form.getvalue('subject')
else:
   subject = "Not set"

print "Content-type:text/html\r\n\r\n"
print "<html>"
print "<head>"
print "<title>Radio - Fourth CGI Program</title>"
print "</head>"
print "<body>"
print "<h2> Selected Subject is %s</h2>" % subject
print "</body>"
print "</html>"

Passing Text Area Data to CGI Program

TEXTAREA element is used when multiline text has to be passed to the CGI Program.

Here is example HTML code for a form with a TEXTAREA box:

<form action="/cgi-bin/textarea.py" method="post" target="_blank">
<textarea name="textcontent" cols="40" rows="4">
Type your text here...
</textarea>
<input type="submit" value="Submit" />
</form>

The result of this code is the following form:

Type your text here…Below is textarea.cgi script to handle input given by web browser:

#!/usr/bin/python

# Import modules for CGI handling 
import cgi, cgitb 

# Create instance of FieldStorage 
form = cgi.FieldStorage() 

# Get data from fields
if form.getvalue('textcontent'):
   text_content = form.getvalue('textcontent')
else:
   text_content = "Not entered"

print "Content-type:text/html\r\n\r\n"
print "<html>"
print "<head>";
print "<title>Text Area - Fifth CGI Program</title>"
print "</head>"
print "<body>"
print "<h2> Entered Text Content is %s</h2>" % text_content
print "</body>"

Passing Drop Down Box Data to CGI Program

Drop Down Box is used when we have many options available but only one or two will be selected.

Here is example HTML code for a form with one drop down box:

<form action="/cgi-bin/dropdown.py" method="post" target="_blank">
<select name="dropdown">
<option value="Maths" selected>Maths</option>
<option value="Physics">Physics</option>
</select>
<input type="submit" value="Submit"/>
</form>

The result of this code is the following form:

Below is dropdown.py script to handle input given by web browser.

#!/usr/bin/python

# Import modules for CGI handling 
import cgi, cgitb 

# Create instance of FieldStorage 
form = cgi.FieldStorage() 

# Get data from fields
if form.getvalue('dropdown'):
   subject = form.getvalue('dropdown')
else:
   subject = "Not entered"

print "Content-type:text/html\r\n\r\n"
print "<html>"
print "<head>"
print "<title>Dropdown Box - Sixth CGI Program</title>"
print "</head>"
print "<body>"
print "<h2> Selected Subject is %s</h2>" % subject
print "</body>"
print "</html>"

Using Cookies in CGI

HTTP protocol is a stateless protocol. But for a commercial website, it is required to maintain session information among different pages. For example, one user registration ends after completing many pages. But how to maintain user’s session information across all the web pages.

In many situations, using cookies is the most efficient method of remembering and tracking preferences, purchases, commissions, and other information required for better visitor experience or site statistics.

How It Works?

Your server sends some data to the visitor’s browser in the form of a cookie. The browser may accept the cookie. If it does, it is stored as a plain text record on the visitor’s hard drive. Now, when the visitor arrives at another page on your site, the cookie is available for retrieval. Once retrieved, your server knows/remembers what was stored.

Cookies are a plain text data record of 5 variable-length fields:

  • Expires : The date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser.
  • Domain : The domain name of your site.
  • Path : The path to the directory or web page that sets the cookie. This may be blank if you want to retrieve the cookie from any directory or page.
  • Secure : If this field contains the word “secure”, then the cookie may only be retrieved with a secure server. If this field is blank, no such restriction exists.
  • Name=Value : Cookies are set and retrieved in the form of key and value pairs.

Setting up Cookies

It is very easy to send cookies to browser. These cookies will be sent along with HTTP Header before to Content-type field. Assuming you want to set UserID and Password as cookies. So cookies setting will be done as follows:

#!/usr/bin/python

print "Set-Cookie:UserID=XYZ;\r\n"
print "Set-Cookie:Password=XYZ123;\r\n"
print "Set-Cookie:Expires=Tuesday, 31-Dec-2007 23:12:40 GMT";\r\n"
print "Set-Cookie:Domain=www.tutorialspoint.com;\r\n"
print "Set-Cookie:Path=/perl;\n"
print "Content-type:text/html\r\n\r\n"
...........Rest of the HTML Content....

From this example, you must have understood how to set cookies. We use Set-Cookie HTTP header to set cookies.

Here, it is optional to set cookies attributes like Expires, Domain and Path. It is notable that cookies are set before sending magic line “Content-type:text/html\r\n\r\n.

Retrieving Cookies

It is very easy to retrieve all the set cookies. Cookies are stored in CGI environment variable HTTP_COOKIE and they will have following form:

key1=value1;key2=value2;key3=value3....

Here is an example of how to retrieve cookies.

#!/usr/bin/python

# Import modules for CGI handling 
from os import environ
import cgi, cgitb

if environ.has_key('HTTP_COOKIE'):
   for cookie in map(strip, split(environ['HTTP_COOKIE'], ';')):
      (key, value ) = split(cookie, '=');
      if key == "UserID":
         user_id = value

      if key == "Password":
         password = value

print "User ID  = %s" % user_id
print "Password = %s" % password

This will produce the following result for the cookies set by above script:

User ID = XYZ
Password = XYZ123

File Upload Example:

To upload a file, the HTML form must have the enctype attribute set to multipart/form-data. The input tag with the file type will create a “Browse” button.

<html>
<body>
   <form enctype="multipart/form-data" 
                     action="save_file.py" method="post">
   <p>File: <input type="file" name="filename" /></p>
   <p><input type="submit" value="Upload" /></p>
   </form>
</body>
</html>

The result of this code is the following form:

File:

Above example has been disabled intentionally to save people uploading file on our server, but you can try above code with your server.

Here is the script save_file.py to handle file upload:

#!/usr/bin/python

import cgi, os
import cgitb; cgitb.enable()

form = cgi.FieldStorage()

# Get filename here.
fileitem = form['filename']

# Test if the file was uploaded
if fileitem.filename:
   # strip leading path from file name to avoid 
   # directory traversal attacks
   fn = os.path.basename(fileitem.filename)
   open('/tmp/' + fn, 'wb').write(fileitem.file.read())

   message = 'The file "' + fn + '" was uploaded successfully'
   
else:
   message = 'No file was uploaded'
   
print """\
Content-Type: text/html\n
<html>
<body>
   <p>%s</p>
</body>
</html>
""" % (message,)

If you are running above script on Unix/Linux, then you would have to take care of replacing file separator as follows, otherwise on your windows machine above open() statement should work fine.

fn = os.path.basename(fileitem.filename.replace("\\", "/" ))

How To Raise a “File Download” Dialog Box ?

Sometimes, it is desired that you want to give option where a user will click a link and it will pop up a “File Download” dialogue box to the user instead of displaying actual content. This is very easy and will be achieved through HTTP header. This HTTP header will be different from the header mentioned in previous section.

For example,if you want make a FileName file downloadable from a given link, then its syntax will be as follows:

#!/usr/bin/python

# HTTP Header
print "Content-Type:application/octet-stream; name=\"FileName\"\r\n";
print "Content-Disposition: attachment; filename=\"FileName\"\r\n\n";

# Actual File Content will go hear.
fo = open("foo.txt", "rb")

str = fo.read();
print str

# Close opend file
fo.close()

Python MySQL Database Access

The Python standard for database interfaces is the Python DB-API. Most Python database interfaces adhere to this standard.

You can choose the right database for your application. Python Database API supports a wide range of database servers:

  • GadFly
  • mSQL
  • MySQL
  • PostgreSQL
  • Microsoft SQL Server 2000
  • Informix
  • Interbase
  • Oracle
  • Sybase

Here is the list of available Python database interfaces: Python Database Interfaces and APIs .You must download a separate DB API module for each database you need to access. For example, if you need to access an Oracle database as well as a MySQL database, you must download both the Oracle and the MySQL database modules.

The DB API provides a minimal standard for working with databases using Python structures and syntax wherever possible. This API includes the following:

  • Importing the API module.
  • Acquiring a connection with the database.
  • Issuing SQL statements and stored procedures.
  • Closing the connection

We would learn all the concepts using MySQL, so let’s talk about MySQLdb module only.

What is MySQLdb?

MySQLdb is an interface for connecting to a MySQL database server from Python. It implements the Python Database API v2.0 and is built on top of the MySQL C API.

How do I install the MySQLdb?

Before proceeding, you make sure you have MySQLdb installed on your machine. Just type the following in your Python script and execute it:

#!/usr/bin/python

import MySQLdb

If it produces the following result, then it means MySQLdb module is not installed:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    import MySQLdb
ImportError: No module named MySQLdb

To install MySQLdb module, download it from MySQLdb Download page and proceed as follows:

$ gunzip MySQL-python-1.2.2.tar.gz
$ tar -xvf MySQL-python-1.2.2.tar
$ cd MySQL-python-1.2.2
$ python setup.py build
$ python setup.py install

Note: Make sure you have root privilege to install above module.

Database Connection:

Before connecting to a MySQL database, make sure of the followings:

  • You have created a database TESTDB.
  • You have created a table EMPLOYEE in TESTDB.
  • This table is having fields FIRST_NAME, LAST_NAME, AGE, SEX and INCOME.
  • User ID “testuser” and password “test123” are set to access TESTDB.
  • Python module MySQLdb is installed properly on your machine.
  • You have gone through MySQL tutorial to understand MySQL Basics.

Example:

Following is the example of connecting with MySQL database “TESTDB”

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using cursor() method
cursor = db.cursor()

# execute SQL query using execute() method.
cursor.execute("SELECT VERSION()")

# Fetch a single row using fetchone() method.
data = cursor.fetchone()

print "Database version : %s " % data

# disconnect from server
db.close()

While running this script, it is producing the following result in my Linux machine.

Database version : 5.0.45

If a connection is established with the datasource, then a Connection Object is returned and saved into db for further use, otherwise db is set to None. Next, db object is used to create a cursor object, which in turn is used to execute SQL queries. Finally, before coming out, it ensures that database connection is closed and resources are released.

Creating Database Table:

Once a database connection is established, we are ready to create tables or records into the database tables using execute method of the created cursor.

Example:

First, let’s create Database table EMPLOYEE:

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using cursor() method
cursor = db.cursor()

# Drop table if it already exist using execute() method.
cursor.execute("DROP TABLE IF EXISTS EMPLOYEE")

# Create table as per requirement
sql = """CREATE TABLE EMPLOYEE (
         FIRST_NAME  CHAR(20) NOT NULL,
         LAST_NAME  CHAR(20),
         AGE INT,  
         SEX CHAR(1),
         INCOME FLOAT )"""

cursor.execute(sql)

# disconnect from server
db.close()

INSERT Operation:

INSERT operation is required when you want to create your records into a database table.

Example:

Following is the example, which executes SQL INSERT statement to create a record into EMPLOYEE table:

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using cursor() method
cursor = db.cursor()

# Prepare SQL query to INSERT a record into the database.
sql = """INSERT INTO EMPLOYEE(FIRST_NAME,
         LAST_NAME, AGE, SEX, INCOME)
         VALUES ('Mac', 'Mohan', 20, 'M', 2000)"""
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

# disconnect from server
db.close()

Above example can be written as follows to create SQL queries dynamically:

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using cursor() method
cursor = db.cursor()

# Prepare SQL query to INSERT a record into the database.
sql = "INSERT INTO EMPLOYEE(FIRST_NAME, \
       LAST_NAME, AGE, SEX, INCOME) \
       VALUES ('%s', '%s', '%d', '%c', '%d' )" % \
       ('Mac', 'Mohan', 20, 'M', 2000)
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

# disconnect from server
db.close()

Example:

Following code segment is another form of execution where you can pass parameters directly:

..................................
user_id = "test123"
password = "password"

con.execute('insert into Login values("%s", "%s")' % \
             (user_id, password))
..................................

READ Operation:

READ Operation on any databasse means to fetch some useful information from the database.

Once our database connection is established, we are ready to make a query into this database. We can use either fetchone() method to fetch single record or fetchall() method to fetech multiple values from a database table.

  • fetchone(): This method fetches the next row of a query result set. A result set is an object that is returned when a cursor object is used to query a table.
  • fetchall(): This method fetches all the rows in a result set. If some rows have already been extracted from the result set, the fetchall() method retrieves the remaining rows from the result set.
  • rowcount: This is a read-only attribute and returns the number of rows that were affected by an execute() method.

Example:

Following is the procedure to query all the records from EMPLOYEE table having salary more than 1000:

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using cursor() method
cursor = db.cursor()

# Prepare SQL query to INSERT a record into the database.
sql = "SELECT * FROM EMPLOYEE \
       WHERE INCOME > '%d'" % (1000)
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Fetch all the rows in a list of lists.
   results = cursor.fetchall()
   for row in results:
      fname = row[0]
      lname = row[1]
      age = row[2]
      sex = row[3]
      income = row[4]
      # Now print fetched result
      print "fname=%s,lname=%s,age=%d,sex=%s,income=%d" % \
             (fname, lname, age, sex, income )
except:
   print "Error: unable to fecth data"

# disconnect from server
db.close()

This will produce the following result:

fname=Mac, lname=Mohan, age=20, sex=M, income=2000

Update Operation:

UPDATE Operation on any databasse means to update one or more records, which are already available in the database. Following is the procedure to update all the records having SEX as ‘M’. Here, we will increase AGE of all the males by one year.

Example:

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using cursor() method
cursor = db.cursor()

# Prepare SQL query to UPDATE required records
sql = "UPDATE EMPLOYEE SET AGE = AGE + 1
                          WHERE SEX = '%c'" % ('M')
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

# disconnect from server
db.close()

DELETE Operation:

DELETE operation is required when you want to delete some records from your database. Following is the procedure to delete all the records from EMPLOYEE where AGE is more than 20:

Example:

#!/usr/bin/python

import MySQLdb

# Open database connection
db = MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using cursor() method
cursor = db.cursor()

# Prepare SQL query to DELETE required records
sql = "DELETE FROM EMPLOYEE WHERE AGE > '%d'" % (20)
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

# disconnect from server
db.close()

Performing Transactions:

Transactions are a mechanism that ensures data consistency. Transactions should have the following four properties:

  • Atomicity: Either a transaction completes or nothing happens at all.
  • Consistency: A transaction must start in a consistent state and leave the system in a consistent state.
  • Isolation: Intermediate results of a transaction are not visible outside the current transaction.
  • Durability: Once a transaction was committed, the effects are persistent, even after a system failure.

The Python DB API 2.0 provides two methods to either commit or rollback a transaction.

Example:

You already have seen how we have implemented transations. Here is again similar example:

# Prepare SQL query to DELETE required records
sql = "DELETE FROM EMPLOYEE WHERE AGE > '%d'" % (20)
try:
   # Execute the SQL command
   cursor.execute(sql)
   # Commit your changes in the database
   db.commit()
except:
   # Rollback in case there is any error
   db.rollback()

COMMIT Operation:

Commit is the operation, which gives a green signal to database to finalize the changes, and after this operation, no change can be reverted back.

Here is a simple example to call commit method.

 db.commit()

ROLLBACK Operation:

If you are not satisfied with one or more of the changes and you want to revert back those changes completely, then use rollback() method.

Here is a simple example to call rollback() method.

 db.rollback()

Disconnecting Database:

To disconnect Database connection, use close() method.

 db.close()

If the connection to a database is closed by the user with the close() method, any outstanding transactions are rolled back by the DB. However, instead of depending on any of DB lower level implementation details, your application would be better off calling commit or rollback explicitly.

Handling Errors:

There are many sources of errors. A few examples are a syntax error in an executed SQL statement, a connection failure, or calling the fetch method for an already canceled or finished statement handle.

The DB API defines a number of errors that must exist in each database module. The following table lists these exceptions.

Exception Description
Warning Used for non-fatal issues. Must subclass StandardError.
Error Base class for errors. Must subclass StandardError.
InterfaceError Used for errors in the database module, not the database itself. Must subclass Error.
DatabaseError Used for errors in the database. Must subclass Error.
DataError Subclass of DatabaseError that refers to errors in the data.
OperationalError Subclass of DatabaseError that refers to errors such as the loss of a connection to the database. These errors are generally outside of the control of the Python scripter.
IntegrityError Subclass of DatabaseError for situations that would damage the relational integrity, such as uniqueness constraints or foreign keys.
InternalError Subclass of DatabaseError that refers to errors internal to the database module, such as a cursor no longer being active.
ProgrammingError Subclass of DatabaseError that refers to errors such as a bad table name and other things that can safely be blamed on you.
NotSupportedError Subclass of DatabaseError that refers to trying to call unsupported functionality.

Your Python scripts should handle these errors, but before using any of the above exceptions, make sure your MySQLdb has support for that exception. You can get more information about them by reading the DB API 2.0 specification.

Python Network Programming

Python provides two levels of access to network services. At a low level, you can access the basic socket support in the underlying operating system, which allows you to implement clients and servers for both connection-oriented and connectionless protocols.

Python also has libraries that provide higher-level access to specific application-level network protocols, such as FTP, HTTP, and so on.

This tutorial gives you understanding on most famous concept in Networking – Socket Programming

What is Sockets?

Sockets are the endpoints of a bidirectional communications channel. Sockets may communicate within a process, between processes on the same machine, or between processes on different continents.

Sockets may be implemented over a number of different channel types: Unix domain sockets, TCP, UDP, and so on. The socket library provides specific classes for handling the common transports as well as a generic interface for handling the rest.

Sockets have their own vocabulary:

Term Description
domain The family of protocols that will be used as the transport mechanism. These values are constants such as AF_INET, PF_INET, PF_UNIX, PF_X25, and so on.
type The type of communications between the two endpoints, typically SOCK_STREAM for connection-oriented protocols and SOCK_DGRAM for connectionless protocols.
protocol Typically zero, this may be used to identify a variant of a protocol within a domain and type.
hostname The identifier of a network interface:

  • A string, which can be a host name, a dotted-quad address, or an IPV6 address in colon (and possibly dot) notation
  • A string “<broadcast>”, which specifies an INADDR_BROADCAST address.
  • A zero-length string, which specifies INADDR_ANY, or
  • An Integer, interpreted as a binary address in host byte order.
port Each server listens for clients calling on one or more ports. A port may be a Fixnum port number, a string containing a port number, or the name of a service.

The socket Module:

To create a socket, you must use the socket.socket() function available in socket module, which has the general syntax:

s = socket.socket (socket_family, socket_type, protocol=0)

Here is the description of the parameters:

  • socket_family: This is either AF_UNIX or AF_INET, as explained earlier.
  • socket_type: This is either SOCK_STREAM or SOCK_DGRAM.
  • protocol: This is usually left out, defaulting to 0.

Once you have socket object, then you can use required functions to create your client or server program. Following is the list of functions required:

Server Socket Methods:

Method Description
s.bind() This method binds address (hostname, port number pair) to socket.
s.listen() This method sets up and start TCP listener.
s.accept() This passively accept TCP client connection, waiting until connection arrives (blocking).

Client Socket Methods:

Method Description
s.connect() This method actively initiates TCP server connection.

General Socket Methods:

Method Description
s.recv() This method receives TCP message
s.send() This method transmits TCP message
s.recvfrom() This method receives UDP message
s.sendto() This method transmits UDP message
s.close() This method closes socket
socket.gethostname() Returns the hostname.

A Simple Server:

To write Internet servers, we use the socket function available in socket module to create a socket object. A socket object is then used to call other functions to setup a socket server.

Now call bind(hostname, port function to specify a port for your service on the given host.

Next, call the accept method of the returned object. This method waits until a client connects to the port you specified, and then returns a connection object that represents the connection to that client.

#!/usr/bin/python           # This is server.py file

import socket               # Import socket module

s = socket.socket()         # Create a socket object
host = socket.gethostname() # Get local machine name
port = 12345                # Reserve a port for your service.
s.bind((host, port))        # Bind to the port

s.listen(5)                 # Now wait for client connection.
while True:
   c, addr = s.accept()     # Establish connection with client.
   print 'Got connection from', addr
   c.send('Thank you for connecting')
   c.close()                # Close the connection

A Simple Client:

Now we will write a very simple client program which will open a connection to a given port 12345 and given host. This is very simple to create a socket client using Python’s socket module function.

The socket.connect(hosname, port ) opens a TCP connection to hostname on the port. Once you have a socket open, you can read from it like any IO object. When done, remember to close it, as you would close a file.

The following code is a very simple client that connects to a given host and port, reads any available data from the socket, and then exits:

#!/usr/bin/python           # This is client.py file

import socket               # Import socket module

s = socket.socket()         # Create a socket object
host = socket.gethostname() # Get local machine name
port = 12345                # Reserve a port for your service.

s.connect((host, port))
print s.recv(1024)
s.close                     # Close the socket when done

Now run this server.py in background and then run above client.py to see the result.

# Following would start a server in background.
$ python server.py & 

# Once server is started run client as follows:

$ python client.py

This would produce following result:

Got connection from ('127.0.0.1', 48437)
Thank you for connecting

Python Internet modules

A list of some important modules which could be used in Python Network/Internet programming.

Protocol Common function Port No Python module
HTTP Web pages 80 httplib, urllib, xmlrpclib
NNTP Usenet news 119 nntplib
FTP File transfers 20 ftplib, urllib
SMTP Sending email 25 smtplib
POP3 Fetching email 110 poplib
IMAP4 Fetching email 143 imaplib
Telnet Command lines 23 telnetlib
Gopher Document transfers 70 gopherlib, urllib

Please check all the libraries mentioned above to work with FTP, SMTP, POP, and IMAP protocols.

Python Regular Expression

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