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_pyio.IOBase.readlines(hint) stops one line early when the total size read equals *hint* #161272
Description
Bug report
Bug description
IOBase.readlines(hint) is documented so that "no more lines will be read if the total
size (in bytes/characters) of all lines so far exceeds hint" (the _pyio docstring
says the same). _io implements exactly that: Modules/_io/iobase.c breaks on
line_length > hint - length, i.e. only once the new total is strictly greater than
hint. _pyio.IOBase.readlines() breaks on n >= hint, so it returns one line fewer
whenever the accumulated size is exactly hint:
import io, _pyio
with open("x", "wb") as f:
f.write(b"a\nbb\nccc\n")
for mod in (io, _pyio):
with mod.open("x", "rb") as f:
print(mod.__name__, f.readlines(2))io [b'a\n', b'bb\n']
_pyio [b'a\n']
With b"a\nbb\nccc\ndddd\n" the same happens for hint=5 (3 lines vs. 2) and hint=9
(4 lines vs. 3), and for text streams as well.
CPython versions tested on
3.11, 3.13 — the affected code in Lib/_pyio.py is unchanged on main.
Operating systems tested on
macOS, but the code is platform independent.
Activity
cmaloney commentedon Nov 3, 2026
Currently holding of on
_pyiochanges while python/steering-council#372 is decided.bedevere-app commentedon Nov 3, 2026
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tsayin4 commentedon Nov 3, 2026
Fair enough, I hadn't seen python/steering-council#372 when I filed. Close it if _pyio is going away, no objection.
In my words: _pyio breaks out of readlines() when the running total is
>= hint, iobase.c breaks when it's> hint. So for lines of 2, 3 and 4 bytes with hint=2, io gives me two lines and _pyio gives one. I found it by writinga\nbb\nccc\nand calling readlines() on both for hint 1, 2, 3, 5, 9, 0, -1 and None and diffing the lists, they only differ when the total lands exactly on hint. I used an LLM for the English.