Python Variable

Variable and Value

• A variable is a memory location where a programmer can store a value. Example : roll_no, amount, name etc.
• Value is either string, numeric etc. Example : "Sara", 120, 25.36
• Variables are created when first assigned.
• Variables must be assigned before being referenced.
• The value stored in a variable can be accessed or updated later.
• No declaration required
• The type (string, int, float etc.) of the variable is determined by Python
• The interpreter allocates memory on the basis of the data type of a variable.

Python Variable Name Rules

• Must begin with a letter (a - z, A - B) or underscore (_)
• Other characters can be letters, numbers or _
• Case Sensitive
• Can be any (reasonable) length
• There are some reserved words which you cannot use as a variable name because Python uses them for other things.

Good Variable Name

• Choose meaningful name instead of short name. roll_no is better than rn.
• Maintain the length of a variable name. Roll_no_of_a-student is too long?
• Be consistent; roll_no or RollNo
• Begin a variable name with an underscore(_) character for a special case.

Python Assignment Statements

The assignment statement creates new variables and gives them values. Basic assignment statement in Python is :

Syntax:

<variable> = <expr>

Where the equal sign (=) is used to assign value (right side) to a variable name (left side). See the following statements :

>>> Item_name = "Computer" #A String
>>> Item_qty = 10 #An Integer
>>> Item_value = 1000.23 #A floating point
>>> print(Item_name)
Computer
>>> print(Item_qty)
10
>>> print(Item_value)
1000.23
>>> 

One thing is important, assignment statement read right to left only.

Example:

a = 12 is correct, but 12 = a does not make sense to Python, which creates a syntax error. Check it in Python Shell.

>>> a = 12
>>> 12 = a
SyntaxError: can't assign to literal
>>> 

Multiple Assignment

The basic assignment statement works for a single variable and a single expression. You can also assign a single value to more than one variables simultaneously.

Syntax:

var1=var2=var3...varn= = <expr>

Example:

x = y = z = 1 

Now check the individual value in Python Shell.

>>> x = y = z = 1
>>> print(x)
1
>>> print(y)
1
>>> print(z)
1
>>> 

Here is an another assignment statement where the variables assign many values at the same time.

Syntax:

<var>, <var>, ..., <var> = <expr>, <expr>, ..., <expr>

Example:

x, y, z = 1, 2, "abcd"

In the above example x, y and z simultaneously get the new values 1, 2 and "abcd".

>>> x,y,z = 1,2,"abcd"
>>> print(x)
1
>>> print(y)
2
>>> print(z)
abcd 

You can reuse variable names by simply assigning a new value to them :

>>> x = 100
>>> print(x)
100
>>> x = "Python"
>>> print(x)
Python
>>>  

Swap variables

Python swap values in a single line and this applies to all objects in python.

Syntax:

var1, var2 = var2, var1

Example:

>>> x = 10
>>> y = 20
>>> print(x)
10
>>> print(y)
20
>>> x, y = y, x
>>> print(x)
20
>>> print(y)
10
>>>

Local and global variables in Python

In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.

Example:

var1 = "Python"
def func1():
    var1 = "PHP"
    print("In side func1() var1 = ",var1)

def func2():
    print("In side func2() var1 = ",var1)
func1()
func2()

Output:

In side func1() var1 =  PHP
In side func2() var1 =  Python

You can use a global variable in other functions by declaring it as global keyword :

Example:

def func1():
    global var1
    var1 = "PHP"
    print("In side func1() var1 = ",var1)

def func2():
    print("In side func2() var1 = ",var1)
func1()
func2()

Output:

In side func1() var1 =  PHP
In side func2() var1 =  PHP