Q.Which of the following identifier names are invalid and why?
i. Serial_no.
ii. 1st_Room
iii. Hundred$
iv. Total Marks
v. Total_Marks
vi. total-Marks
vii. _Percentage
viii. True

When you open a bank account, the bank does not let you choose absolutely any name for it: there are rules about what is acceptable on the form. A program faces the same situation constantly, because programmers must invent names for the things they create — variables that hold marks, functions that compute totals, and so on. These invented names are called identifiers, and every programming language publishes strict rules about which names it will accept. In Python those rules are short, but they are enforced without mercy: break one and the program refuses to run.

A valid Python identifier must satisfy all of the following:

• It may contain only letters (A to Z, a to z), digits (0 to 9), and the underscore character _. No spaces, and no special symbols such as @, #, -, or !.
• It must not begin with a digit. A digit may appear anywhere except the first position — so marks1 is fine, but 1marks is rejected.
• It must not be one of Python's keywords — the words the language reserves for its own grammar, such as if, else, for, while, True, False, and, or, not, def, class, import. A keyword already has a job and cannot moonlight as your variable name.
• There is no limit on length, though sensible names are rarely very long.

One further fact governs everything: Python is case-sensitive. The identifiers total, Total, and TOTAL are three different names, unrelated to each other as far as Python is concerned. Many baffling beginner bugs are nothing more than a capital letter quietly disagreeing with a small one.

Testing a few candidates against the rules:

roll_no = 17      # valid: letters, underscore, digits, starts with a letter
_temp = 5         # valid: starting with an underscore is allowed
2moro = "soon"    # INVALID: begins with a digit
my-total = 90     # INVALID: hyphen is not permitted
class = "XI-B"    # INVALID: class is a reserved keyword

Why do these rules exist at all? They are what lets Python read your code unambiguously. If names could contain spaces or start with digits or reuse keywords, the language could not tell where a name ends and an instruction or a number begins. The rules carve out a safe space in which your names can never be confused with Python's own vocabulary or with data.

Validity, though, is only the legal minimum. Good practice goes further: choose names that describe what they hold — student_name rather than sn, total_marks rather than t. Python accepts both; the human who reads your program six months later will thank you for only one of them.