Q.Which of the following identifier names are invalid and why?

This is a Plain theory question — it asks you to classify each identifier as valid or invalid and give the rule-based reason.

Identifier Validity Rules (Python)

A valid Python identifier must:

• Start with a letter (a–z, A–Z) or an underscore (_). It cannot start with a digit.
• Contain only letters, digits, or underscores. No spaces, hyphens, or special symbols like $, ., - are allowed.
• Not be a reserved keyword (e.g., True, False, if, for).

Analysis of each identifier

a) Serial_no. — Invalid. The dot (.) is not allowed in an identifier. Only letters, digits, and underscores are permitted.

b) 1st_Room — Invalid. It starts with a digit (1). Identifiers must begin with a letter or underscore.

c) Hundred$ — Invalid. The dollar sign ($) is not a valid character. Only underscores are allowed as special characters.

d) Total Marks — Invalid. It contains a space. Spaces are not permitted; use an underscore instead.

e) Total_Marks — Valid. It starts with a capital letter, contains only letters and an underscore, and is not a keyword.

f) total-Marks — Invalid. The hyphen (-) is not allowed. Python interprets this as a subtraction operator, not part of a name.

g) _Percentage — Valid. It starts with an underscore, followed by letters. Underscore-prefixed names are perfectly legal (though often used for special purposes by convention).

h) True — Invalid. True is a reserved keyword in Python (a boolean literal). Keywords cannot be used as identifiers.

The Core Idea: Identifier Validity Rules

In Python (and most programming languages), an identifier is a name given to a variable, function, class, or other entity. The rules are not arbitrary — they exist so the compiler/interpreter can unambiguously parse your code. Python's rules are:

1. Start character: Must be a letter (a-z, A-Z) or an underscore (_). Cannot start with a digit.
2. Body characters: After the first character, can contain letters, digits (0-9), and underscores only. No spaces, hyphens, dollar signs, or special symbols.
3. Reserved keywords: Cannot use a Python keyword (like True, False, if, for, etc.) as an identifier.
4. Case-sensitive: Total_Marks and total_marks are different identifiers.

These rules are universal across Python versions. Let's apply them to each candidate.

Checking Each Identifier

a) Serial_no. — Invalid. The dot (.) is not allowed in an identifier. Dots are used for attribute access (e.g., obj.attribute), not inside names.

b) 1st_Room — Invalid. Starts with a digit (1). Python cannot parse this — it would see 1 as a number literal, then st_Room as something else, causing a syntax error.

c) Hundred$ — Invalid. The dollar sign ($) is not a valid character in Python identifiers. (Some other languages like JavaScript allow it, but Python does not.)

d) Total Marks — Invalid. Contains a space. Spaces are delimiters in Python — Total Marks would be parsed as two separate tokens.

e) Total_Marks — Valid. Starts with a capital letter, contains only letters and an underscore. Perfectly fine.

f) total-Marks — Invalid. The hyphen (-) is the subtraction operator in Python. Writing total-Marks would be interpreted as total minus Marks, not a single name.

g) _Percentage — Valid. Starts with an underscore (allowed), followed by letters. Underscore-prefixed names are conventionally used for "private" or internal-use identifiers, but they are syntactically valid.

h) True — Invalid. True is a reserved keyword in Python (a boolean literal). You cannot use it as a variable name.

Summary Table