What is variable

Variable is a name that is used to refer to memory location. Python variable is also known as an identifier and used to hold value.

It is helpful to think of variables as a container that holds data that can be changed later in the program.

In Python, we don't need to specify the type of variable because Python is a infer language and smart enough to get variable type.

Variable names can be a group of both the letters and digits, but they have to begin with a letter or an underscore.

It is recommended to use lowercase letters for the variable name. Rahul and rahul both are two different variables.

Declaring and assigning value to a variable

• The equal (=) operator is used to assign value to a variable.

Code:

number = 0
product_weight = 2.1
price_per_kilogram = 10
filename = 'file.txt'
trace = False
sentence = "this is an example"

print(number)
print(product_weight)
print(price_per_kilogram)
print(trace)
print(sentence)

Output:

0
2.1
10
False
this is an example

Changing the value of a variable

Code:

website = "google.com"
print(website)
 
# assigning a new value to website 
website = "atnyla.com"
print(website)

Output:

google.com 
atnyla.com

Assigning multiple values to multiple variables

variable1, variable2, variable3 =    5, 3.2, "Hello"           

print (variable1)
print (variable2)
print (variable3)

Output:

5
3.2
Hello

If we want to assign the same value to multiple variables at once, we can do this as:

x  = y = z  =  "Kolkata" 

print (x)
print (y)
print (z)

Output:

Kolkata
Kolkata
Kolkata

Rules and Naming Convention for Variables and constants

1. The first character of the variable must be an alphabet or underscore ( _ ).
2. All the characters except the first character may be an alphabet of lower-case(a-z), upper-case (A-Z), underscore, or digit (0-9).
3. Identifier name must not contain any white-space, or special character (!, @, #, %, ^, &, *).
4. Identifier name must not be similar to any keyword defined in the language.
5. Identifier names are case sensitive; for example, myname, and MyName is not the same.
6. Create a name that makes sense. For example, my_age = 25 makes more sense than m = 25.
7. Examples of valid identifiers: a123, _n, n_9, etc.
8. Examples of invalid identifiers: 1a, n%4, n 9, etc.

Constant

Assigning value to constant in Python

In Python, constants are usually declared and assigned in a module. Here, the module is a new file containing variables, functions, etc which is imported to the main file. Inside the module, constants are written in all capital letters and underscores separating the words.

Declaring and assigning value to a constant

constant.py

PI = 3.14
INTREST = 2.8  

main.py

import constant

print(constant.PI)
print(constant.INTREST)