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A single-line comment in Python is a lexical construct discarded by the Python interpreter during the lexical analysis (tokenization) phase. It is initiated by the hash character (#) and instructs the tokenizer to ignore all subsequent characters up to the end of the physical line. The scope of the comment terminates strictly at the end of the physical line. Python does not support explicit line joining for comments; a backslash (\) inside a comment is evaluated as literal text, not as an escape character for the newline. Comments can be placed at the beginning of a line or inline, following whitespace, standalone expressions, complete statements, or incomplete statements (such as elements inside unclosed parentheses, brackets, or braces). 

# Full-line comment preceding code
counter = 0

counter += 1  # Inline comment following a complete statement

elements = [
    "alpha",  # Inline comment following an incomplete statement
    "beta"    # Backslashes here \
              # do not continue the comment to the next line
]

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Syntactical Rules and Exceptions

  • String Literals: If the # character appears within a string literal (enclosed in single, double, or triple quotes), the tokenizer evaluates it as a standard string character rather than a comment initiator.
delimiter = "#"
regex_pattern = r"[A-Z]#\d+"
  • Indentation: The tokenizer ignores the indentation level of comments. Lines containing only whitespace and a comment do not generate INDENT or DEDENT tokens and therefore do not affect the structural block of the surrounding code.
  • Encoding Declarations: If a comment on the first or second physical line of a script matches the regular expression coding[=:]\s*([-\w.]+), the Python interpreter processes it specifically to determine the source file’s character encoding.
# -*- coding: utf-8 -*-
  • Shebang (POSIX): A comment on the absolute first line starting with #! is parsed by Unix-like operating systems to determine the execution environment path. The Python tokenizer still treats this line as a standard ignored token.
#!/usr/bin/env python3
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