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

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