Print from background thread has broken line endings
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Description
Bug report
Bug description:
I noticed that in python 3.13 (in linux), messages that were previously logged nicely to the console are now behaving as if the lines end like a windows line feed character without carriage return -- the cursor moves down a row, but not back to the beginning of the line. The example I chose below looks tame, but it makes most real logging output very difficult to read.
This only seems to happen for lines printed from background threads:
import threading, time
def msg():
print('this is line 1')
print('this is line 2')
time.sleep(0.1) # first few lines work as expected, but after a brief time, the problem begins
print('this is line 3')
print('this is line 4')
thread = threading.Thread(target=msg)
thread.start()
Running this code in python 3.13.1, interactive mode looks like:
$ python -i print_test.py
this is line 1
this is line 2
>>> this is line 3
this is line 4
>>>
Non-interactive looks fine:
$ python print_test.py
this is line 1
this is line 2
this is line 3
this is line 4
And interactive in 3.12.8 looks fine:
$ python -i print_test.py
this is line 1
this is line 2
>>> this is line 3
this is line 4
CPython versions tested on:
3.13
Operating systems tested on:
Linux
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Research direction
Start by running the supplied script with interactive mode on Linux under Python 3.13, then compare its output with Python 3.12 and non-interactive mode. Investigate the interactive prompt and background-thread output handling; done means background-thread lines return the cursor to the start of each line without disrupting the prompt.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- cli
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 48/100