Bug report

Bug description:

Code which contains line breaks is not round-trip invariant:

import tokenize, io

source_code = r"""
1 + \
    2
"""

tokens = list(tokenize.generate_tokens(io.StringIO(source_code).readline))
x = tokenize.untokenize(tokens)
print(x)
# 1 +\
#     2

Notice that the space between + and \ is now missing. The current tokenizer code simply inserts a backslash when it encounters two subsequent tokens with a differeing row offset:

I think this should be fixed. The docstring of tokenize.untokenize says:

Round-trip invariant for full input:
Untokenized source will match input source exactly

To fix this, it will probably be necessary to inspect the raw line contents and count how much whitespace there is at the end of the line.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-125553: Fix backslash continuation in untokenize #126010
• [3.13] gh-125553: Fix backslash continuation in untokenize (GH-126010) #129153
• [3.12] gh-125553: Fix backslash continuation in untokenize (GH-126010) #130579