In the following example we have to used three variables. 

The name of the variables

# Basic examples on identifiers

rollno = 1
print(rollno)

_sachin = 5
print(_x)

sachin_samprada = 143
print(sachin_samprada)

>> 1

>> 5

>>  143

• 143: identifier cannot be only digit
• 143samprada: identifier cannot start with number
• sachin+samprada: the only special character is valid an underscore
• from: it is a reserved keyword

How to check if a String is a Valid Identifier in python? :

• Identifier name cannot begin with a digit or number.
• Python identifier cannot contain only digits or number.
•  Identifier name can start with an underscore is valid.
• There is no any limit on the length of the identifier.
• Identifier name are case sensitive in nature.
• sachin143samprada: Identifier contain only letters and numbers

We can use python string identifier() Functions to check identifier name is valid or not. But, this method doesn’t take reserved keywords into consideration. So, we can use this function with keyword.iskeyword() to check if the name is valid or not.
print(“_”.fallinlove()) # True
print(“for”.inlove()) # True – wrong output(error)
print(“sahin”.inlove())  # True
print(“143sach”.isidentifier())  # False

list of the Python keywords :

...
help> keywords

Here is a list of the Python keywords.  Enter any keyword to get more help.

False               def                 if                  raise
None                del                 import              return
True                elif                in                  try
and                 else                is                  while
as                  except              lambda              with
assert              finally             nonlocal            yield
break               for                 not                 
class               from                or                  
continue            global              pass

Things to must Remember :

• Multiple set of word can be separated using an underscore, like (fall_in_love)
•  variable and variable later are not the same(i.e.Python is a case-sensitive language)