Identifiers:

• A name in Python program is called identifier. 
• It can be class name or function name or module name or variable name. 
• a = 10 

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 √ 
• ca$h =20 

2. Identifier should not starts with digit 

• 123total
• total123 √ 

3. 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. 
7. Dollor ($) Symbol is not allowed in Python.