Variables

    Two basic things related to the programs are values and types. Values are like 1, 5, 2.7, and 'Hello'. Here 1 and 5 is an integer (int), 2.7 is a floating point number (float) and 'Hello' is a string (str) .

     Variable is a name that refers to a value. In python type of the variable is not defined when we create a variable. Based on the value of the variable memory is allocated for it.

Assignment statement (=) used to assign values to a variable.

Example:

>>> message = 'Hello'

>>> message

'Hello'

>>> type(message)

<type 'str'>

>>>

Here message is a variable, we assign a value 'Hello' to it. If you are not sure what type a value has, the interpreter can tell you. In this example 'Hello' is a string (str). Based on this value memory is allocated for message.

Another examples are,

>>> a=4.5

>>> type(a)

<type 'float'>

>>> b='Alice'

>>> type(b)

<type 'str'>

>>> c=684

>>> type(c)

<type 'int'>

>>> a

4.5

>>> b

'Alice'

>>> c

684

>>>

We can also use the print statement to display the value of a variable.

>>> a='Bob'

>>> print a

Bob

>>>

We can assign value of one variable in to another variable by using assignment operator.

>>> a=45

>>> b=a

>>> a

45

>>> b

45

>>>

Variable names can contain both letters and numbers. But they have to begin with a letter. The underscore character (_) can appear in a name. It is often used in names with multiple words such as

front_end, back_end. If we given an illegal name, get a syntax error.

Example:

>>> 76message='Hello'                # Begin with integer

SyntaxError: invalid syntax

>>>

>>> class = 23                              # 'class' is an python keyword

SyntaxError: invalid syntax

>>>

>>> begin@ = 3                           # '@' is an illegal character

SyntaxError: invalid syntax

>>>

python has 31 keywords.

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

The interpreter uses keywords to recognize the structure of the program, and they cannot be used as variable names.