1) Alphabet Symbols (Either Upper case OR Lower case)

2) If Identifier is start with Underscore (_) then it indicates it is private.

3) Identifier should not start with Digits.

4) Identifiers are case sensitive.

5) We cannot use reserved words as identifiers Eg: def = 10 

6) There is no length limit for Python identifiers. But not recommended to use too lengthy identifiers.

7) Dollor ($) Symbol is not allowed in Python.