Rules to define identifiers in Python:

1. The only allowed characters in Python are alphabet symbols(either lower case or upper case) digits(0 to 9) underscore symbol(_)

By mistake if we are using any other symbol like $ then we will get syntax error.
cash = 10 Correct
ca$h = 20 Incorrect

1. Identifier should not starts with digit
   123total Correct
   total123 Incorrect

2. Identifiers are case sensitive. Of course Python language is case sensitive language.

total=10
TOTAL=999
print(total) #10
print(TOTAL) #999

Identifier:

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.Identifier:

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