A student wants to name a variable _total#sum to store the total sum of numbers. What is the main reason Python will reject this variable name?

Question

BlackTom AI · Accepted Answer

Python identifiers may only include letters (A–Z, a–z), digits (0–9) and the underscore (_); they must start with a letter or underscore and cannot contain other special characters such as #, @ or -.

1) Relevant knowledge points and concepts
- Identifier rules in Python: an identifier (variable name) must start with a letter or underscore. Subsequent characters may be letters, digits, or underscores. Special characters (for example '#', '!', '@', '-') are not allowed in identifiers because they have other syntactic meanings (e.g., # begins a comment).

2) Step-by-step reasoning applied to _total#sum
- Examine characters in order: '_' → allowed as first character.
- 't' → allowed (letter).
- 'o' → allowed.
- 't' → allowed.
- 'a' → allowed.
- 'l' → allowed.
- '#' → this is a special character; Python does not permit '#' inside an identifier.
- Because '#' appears, the whole identifier is invalid; parsing would treat '#' as starting a comment (or raise a SyntaxError if placed mid-expression).

3) Intermediate “calculation” steps (character check)
- Allowed set for first char: {letter, _} → '_' ∈ allowed.
- Allowed set for subsequent chars: {letter, digit, _} → 't','o','t','a','l' ∈ allowed, but '#' ∉ allowed → identifier invalid.

4) Why the correct answer is correct
- Option c states “Variable name cannot contain special characters.” That directly matches the reason: the '#' in _total#sum is a special character not permitted in Python identifiers, so Python will reject it.

5) Why the other options are incorrect
- a: Incorrect — underscores are allowed as the first character and are commonly used (e.g., _private).
- b: Incorrect — this option omits underscores; variable names may include underscores in addition to letters and digits.
- d: Incorrect — variable names can and typically do contain letters; this statement is false.

6) Conclusion
- Therefore the main reason Python will reject _total#sum is that it contains a special character; option c is correct.

A student wants to name a variable _total#sum to store the total sum of numbers. What is the main reason Python will reject this variable name?单项选择题

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