A name in python program is called as Identifier. The name may be a variable name, or a function name, or a class name etc.

Example:

x=10

here x is an identifier

def f1():

pass

Here the function name f1 is an identifier

class Simple(Exception): # class name Simple is identifier

Rules & Regulations to define Python Identifiers:

1. The following symbols are allowed in python identifier;
   Alphabet symobols(both upper case and lower case)
   A-Z
   a-z
   digits(0-9)
   underscore(_)
   
   ` Example:number(valid identifier)
         Number#(invalid identifier)


2. Identifier should not start with digits.
   Sum13(valid identifier)
   123sum(invalid identifier)


3. 123sum(invalid identifier)
   sum=100
   SUM=120
   Here both sum and SUM are valid identifier.


4. Keywords never consider as identifier
   
   Sum=0(valid)
   if=100(invalid identifier)


5. There is no length limit of an identifier.
   Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx=10
   Here it is a valid identifier, but it is not recommended, because the readability of the code is reduced.

Note:

The underscore symbol(_) starts with any identifier, then that is private.

If two underscore symbol(__) starts with any identifier then it is strongly private.