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Learn Python basics | Python basics You Should learn before starting python

 Learn Python basics | Python basics You Should learn before starting python 


Intro basics

First Python Program
Interactive Mode Programming
Invoking the interpreter without passing a script file as a parameter brings up the following prompt −
$ python
Python 2.4.3 (#1, Nov 11 2010, 13:34:43)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
Type the following text at the Python prompt and press the Enter:
>>> print "Hello, Python!"
If you are running new version of Python, then you would need to use print statement with parenthesis as in print ("Hello, Python!");. However in Python version 2.4.3, this produces the following result:
Hello, Python!

Script Mode Programming

Invoking the interpreter with a script parameter begins execution of the script and continues until the script is finished. When the script is finished, the interpreter is no longer active. Let us write a simple Python program in a script. Python files have extension .py. Type the following source code in a test.py file:
print "Hello, Python!"
We assume that you have Python interpreter set in PATH variable. Now, try to run this program as follows −
$ python test.py
This produces the following result:
Hello, Python!
Let us try another way to execute a Python script. Here is the modified test.py file −
#!/usr/bin/python

print "Hello, Python!"
We assume that you have Python interpreter available in /usr/bin directory. Now, try to run this program as follows −
$ chmod +x test.py     # This is to make file executable
$./test.py
This produces the following result −
Hello, Python!

Python Identifiers

A Python identifier is a name used to identify a variable, function, class, module or other object. An identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores and digits (0 to 9). Python does not allow punctuation characters such as @, $, and % within identifiers. Python is a case sensitive programming language. Thus, Manpower and manpower are two different identifiers in Python. Here are naming conventions for Python identifiers −

  • Class names start with an uppercase letter. All other identifiers start with a lowercase letter.

  • Starting an identifier with a single leading underscore indicates that the identifier is private.

  • Starting an identifier with two leading underscores indicates a strongly private identifier.

  • If the identifier also ends with two trailing underscores, the identifier is a language-defined special name.

Reserved Words

AndexecNot
Assertfinallyor
Breakforpass
Classfromprint
Continueglobalraise
defifreturn
delimporttry
elifinwhile
elseiswith
exceptlambdayield

Lines and Indentation

Python provides no braces to indicate blocks of code for class and function definitions or flow control. Blocks of code are denoted by line indentation, which is rigidly enforced. The number of spaces in the indentation is variable, but all statements within the block must be indented the same amount. For example −
if True:
    print "True"
else:
  print "False"
However, the following block generates an error −
if True:
    print "Answer"
    print "True"
else:
    print "Answer"
  print "False"
Thus, in Python all the continuous lines indented with same number of spaces would form a block. The following example has various statement blocks −Note: Do not try to understand the logic at this point of time. Just make sure you understood various blocks even if they are without braces.

Multi-Line Statements

Statements in Python typically end with a new line. Python does, however, allow the use of the line continuation character (\) to denote that the line should continue. For example −
total = item_one + \
        item_two + \
        item_three
Statements contained within the [], {}, or () brackets do not need to use the line continuation character. For example −
days = ['Monday', 'Tuesday', 'Wednesday',
        'Thursday', 'Friday']

Quotation in Python

Python accepts single ('), double (") and triple (''' or """) quotes to denote string literals, as long as the same type of quote starts and ends the string. The triple quotes are used to span the string across multiple lines. For example, all the following are legal −
word = 'word'
sentence = "This is a sentence."
paragraph = """This is a paragraph. It is
made up of multiple lines and sentences."""

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Comments in Python

A hash sign (#) that is not inside a string literal begins a comment. All characters after the # and up to the end of the physical line are part of the comment and the Python interpreter ignores them.
#!/usr/bin/python

# First comment
print "Hello, Python!" # second comment
You can type a comment on the same line after a statement or expression −
name = "Madisetti" # This is again comment
You can comment multiple lines as follows −
# This is a comment.
# This is a comment, too.
# This is a comment, too.
# I said that already.

Multiple Statements on a Single Line

The semicolon ( ; ) allows multiple statements on the single line given that neither statement starts a new code block. Here is a sample snip using the semicolon −
import sys; x = 'foo'; sys.stdout.write(x + '\n')

Multiple Statement Groups as Suites

A group of individual statements, which make a single code block are called suites in Python. Compound or complex statements, such as if, while, def, and class require a header line and a suite. Header lines begin the statement (with the keyword) and terminate with a colon ( : ) and are followed by one or more lines which make up the suite. For example −
if expression : 
   suite
elif expression : 
   suite 
else : 
   suite

Waiting for the User

The following line of the program displays the prompt, the statement saying “Press the enter key to exit”, and waits for the user to take action −
#!/usr/bin/python

input = raw_input("\n\nPress the enter

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