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Variables and Keywords

A variable can be used to store a certain value or object. In Python, all numbers (and everything else, including functions) are objects. A variable is created through assignment. Their type is assigned dynamically.

use type(object) to know the type of the variable

Eg: type(x)     type(y)           type(z)            type(l)
<type 'str'>      <type 'int'>      <type 'float'>   <type 'long'>
When we create a program, we often like to store values so that it can be used later. We
use objects to capture data, which then can be manipulated by computer to provide
information. By now we know that object/ variable is a name which refers to a value. It has a type and also store some value.

Variable can be mutable or immutable. A mutable variable is one whose value may change in place, whereas in an immutable variable change of value will not happen in place.

Programmers choose the names of the variable that are meaningful. A variable name:
1. Can be of any size
2. Have allowed characters, which are a-z, A-Z, 0-9 and underscore (_)
3. should begin with an alphabet or underscore
4. should not be a keyword

It is a good practice to follow these identifier naming conventions:

1. Variable name should be meaningful and short
2. Generally, they are written in lower case letters


They are the words used by Python interpreter to recognize the structure of program.
As these words have specific meaning for interpreter, they cannot be used for any other
There are 29  keywords in Python 2.7 

and           def      exec     if          not       return
assert        del      finally  import  or         try
break        elif      for        in         pass     while
class         else     from      is         print    yield
continue   except global   lambda raise

the following are some invalid variable names
>>> 76trombones = ’big parade’
SyntaxError: invalid syntax
Note: variable name cannot start with a digit.
>>> more$ = 1000000
SyntaxError: invalid syntax
Note:variable name cannot contain $
>>> class = ’Computer Science 101’
SyntaxError: invalid syntax
Note:keyword cannot be used as variable name. 


Y Variables are created when they are first assigned a value.
Y Variables type is determined by the value which is assigned.
Y Variables must be assigned a value before using them in expression,
Y Variables refer to an object and are never declared ahead of time.


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