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Runaway memory use when repeatedly printing empty string #151814
Description
Bug report
Bug description:
Printing the empty string in a loop seems to cause memory use of Python to grow without bound.
while True:
print('', end='')or
import sys
while True:
sys.stdout.write('')The latter apparently does not work in interactive mode.
Relatedly,
while True:
for _ in range(10**8):
print('', end='')
print()causes memory use to grow to about 1.5GB and then reset when the newline is printed.
CPython versions tested on:
3.14, 3.11, 3.13
Operating systems tested on:
Linux
Linked PRs
- gh-151814: Fix unbounded memory growth from repeated empty writes to
io.TextIOWrapper#151817 - [3.15] gh-151814: Fix unbounded memory growth from repeated empty writes to
io.TextIOWrapper(GH-151817) #152071 - [3.14] gh-151814: Fix unbounded memory growth from repeated empty writes to
io.TextIOWrapper(GH-151817) #152072 - [3.13] gh-151814: Fix unbounded memory growth from repeated empty writes to
io.TextIOWrapper(GH-151817) #152073 - gh-151814: Add test for TextIOWrapper.write unbounded memory growth from repeated writes #152244
Activity
picnixz commentedon Jun 20, 2026
I think it's simply because there is no flushing so I think it's still being buffered.
cc @cmaloney
zware commentedon Jun 20, 2026
Agreed that it seems to be a buffering issue, but since there's not actually anything to buffer it seems there may be a real bug here.
On the other hand this is a degenerate case, so I'm not sure how much effort it's worth to track down :)
StanFromIreland commentedon Jun 20, 2026
+1 for fixing, this is a real bug.
I took a look and it seems straightforward. Each
write('')appended an empty bytes object to theTextIOWrapper'spending_byteslist but added 0 topending_bytes_count(below,bytes_lenis 0 withb''). Since the buffer flush is triggered only whenpending_bytes_countreacheschunk_size, it never flushed but did accumulate the objects.cpython/Modules/_io/textio.c
Lines 1848 to 1853 in aec0aed
cmaloney commentedon Jun 20, 2026
👍 real bug should fix; I think this is missing a path for
bytes_len == 0to avoid appending to the list in that case. Currently in that case inwrite_throughmode we still flush which I think is important to keep.StanFromIreland commentedon Jun 20, 2026
Indeed, I already wrote a little patch: #151817 :-)
gh-151814: Fix unbounded memory growth from repeated empty writes to …
[3.15] gh-151814: Fix unbounded memory growth from repeated empty wri…
[3.14] gh-151814: Fix unbounded memory growth from repeated empty wri…
[3.13] gh-151814: Fix unbounded memory growth from repeated empty wri…