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pprint() gives exception when ran from pythonw #85718

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pprint() gives exception when ran from pythonw#85718
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3.11only security fixesOS-windowsstdlibStandard Library Python modules in the Lib/ directory

Description

@luizeldorado
luizeldorado
mannequin
opened on Aug 14, 2020
BPO 41546
Nosy @terryjreedy, @pfmoore, @tjguk, @ned-deily, @zware, @zooba, @vedgar, @iritkatriel, @luizeldorado
PRs
  • bpo-41546: make pprint (like print) not write to stdout when it is None #26810
    		•
    Files
  • pprint_error_showcase.pyw: Showcase of the problem
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
closed_at = <Date 2021-07-19.09:20:09.349>
created_at = <Date 2020-08-14.00:10:04.341>
labels = ['library', 'OS-windows', '3.11']
title = 'pprint() gives exception when ran from pythonw'
updated_at = <Date 2021-07-19.09:20:09.349>
user = 'https://github.com/luizeldorado'

    bugs.python.org fields:

    activity = <Date 2021-07-19.09:20:09.349>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-07-19.09:20:09.349>
closer = 'iritkatriel'
components = ['Library (Lib)', 'Windows']
creation = <Date 2020-08-14.00:10:04.341>
creator = 'luizeldorado'
dependencies = []
files = ['49388']
hgrepos = []
issue_num = 41546
keywords = ['patch']
message_count = 10.0
messages = ['375358', '375382', '375401', '375413', '375422', '375435', '375503', '375534', '375565', '397781']
nosy_count = 10.0
nosy_names = ['terry.reedy', 'paul.moore', 'tim.golden', 'ned.deily', 'zach.ware', 'steve.dower', 'veky', 'iritkatriel', 'luizeldorado', 'henriquesdj0']
pr_nums = ['26810']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue41546'
versions = ['Python 3.11']

    Activity

    1. luizeldorado commented on Aug 14, 2020

      @luizeldorado
      luizeldoradomannequin
      on Aug 14, 2020
      MannequinAuthor

      If you use the pprint function when running on windowed mode (either by using the .pyw extension or running the file with pythonw) a "AttributeError" exception occurs, saying that "'NoneType' object has no attribute 'write'".

      Looking at the source code (Lib/pprint.py), it happens because in this mode sys.stdout is set to None, therefore it has no write attribute.

      I think intended behavior is to simply ignore any console output when in this mode, since many programs have loads of print statements for debugging with a console window; it shouldn't crash because of a extension change.

      There's two solutions I can think of. One is to check if sys.stdout is None, not printing anything in case it is. Another is, when in windowed mode, setting sys.stdout to some dummy null file. This has the advantage of automatically fixing this in other parts of Python where this error might also occur.

      This error is particularly hard to find, since in windowed mode there's no console to show the exception - it simply closes. In the attached file, where I showcase the error, I have to log to a file to see the problem.

      I'm not sure how can this be unnoticed for so much time, it can't possibly be intended, or maybe it is and it's print() that actually has a problem, lol.

    2. luizeldorado
      added
      type-bugAn unexpected behavior, bug, or error
      3.8 (EOL)end of life
      stdlibStandard Library Python modules in the Lib/ directory
      on Aug 14, 2020

    3. ned-deily commented on Aug 14, 2020

      @ned-deily
      ned-deily
      on Aug 14, 2020
      Member

      Thanks for the report. Just to be clear, this is running on Windows? And, if so, which version?

    4. ned-deily
      added
      OS-windows
      on Aug 14, 2020

    5. luizeldorado commented on Aug 14, 2020

      @luizeldorado
      luizeldoradomannequin
      on Aug 14, 2020
      MannequinAuthor

      I'm on Windows 7.

    6. zooba commented on Aug 14, 2020

      @zooba
      zooba
      on Aug 14, 2020
      Member

      This is normal, if obscure, behaviour. Pythonw starts without a console, and so stdout is not connected to anything. As a result, you can't print (or pprint).

      You'll need to set sys.stdout to something or provide a file if you want to print output.

      If someone wants to contribute a specialised sys.stdout implementation that can raise a more helpful error message in this case, that would be helpful. But as it's a breaking change it would only go into 3.10.

    7. luizeldorado commented on Aug 14, 2020

      @luizeldorado
      luizeldoradomannequin
      on Aug 14, 2020
      MannequinAuthor

      If this is normal behavior, then why does print() not produce any error? Shouldn't it say that it can't print because there's no stdout? That's not very consistent, both functions have almost the same name yet produce very different results.

    8. terryjreedy commented on Aug 14, 2020

      @terryjreedy
      terryjreedy
      on Aug 14, 2020
      Member

      If you run in windowed mode, you are expected to either not use sys.stdout or to provide a working sys.stdout object. I am not sure if that is explicitly documented as I cannot find python.exe and pythonw.exe or .py or .pyw in the index and
      https://docs.python.org/3/using/windows.html#python-launcher-for-windows
      does not mention the pyw version.

      But this is not really a windowed mode issue.  In a Windows console:
>>> print('hello')
hello
>>> import sys; sys.stdout = None
>>> print('hello')
>>> sys.stdout = sys.__stdout__
>>> print('hello')
hello

      print and pprint act differently because pprint uses stream.write without try/except. Check the doc or code. So pprint users must run it with a stream with that method. Otherwise, failure is to be expected.

      If you try to print to None, print defaults to sys.stdout. The doc does not define what happens when the latter is None. On CPython, print passes. But maybe this is left undefined intentionally. It must either check that file is not None or catch the AttributeError.

      Shouldn't it say that it can't print because there's no stdout?
      How could it if there is no channel to speak? I guess Guido decided to silently pass rather than silently exit. Debug prints should not crash a program.

      So closing as 'Not a bug' would be one appropriate resolution for this issue.

    9. terryjreedy
      removed
      type-bugAn unexpected behavior, bug, or error
      on Aug 14, 2020

    10. 1 remaining item

    11. henriquesdj0 commented on Aug 16, 2020

      @henriquesdj0
      henriquesdj0mannequin
      on Aug 16, 2020
      Mannequin

      Debug prints should not crash a program.

      'Not a bug'

      wait

    12. vedgar commented on Aug 17, 2020

      @vedgar
      vedgarmannequin
      on Aug 17, 2020
      Mannequin

      The big part of the justification for making print a function in Py3 is that it can be painlessly replaced with other functions, such as (example given by BDFL) pprint.pprint. I think we should do what we can to make the replacement as painless as possible.

    13. zooba commented on Aug 17, 2020

      @zooba
      zooba
      on Aug 17, 2020
      Member

      I'm inclined to agree that it should pass silently in this case, as if it were printing with print() rather than .write().

      What better meaning is there for sys.stdout == None than "no output"?

    14. zooba
      added
      3.10 (EOL)end of life
      and removed
      3.8 (EOL)end of life
      on Aug 17, 2020
    15. iritkatriel
      added
      3.11only security fixes
      and removed
      3.10 (EOL)end of life
      on Jun 20, 2021

    16. iritkatriel commented on Jul 19, 2021

      @iritkatriel
      iritkatriel
      on Jul 19, 2021
      Member

      New changeset aab1899 by Irit Katriel in branch 'main':
      bpo-41546: make pprint (like print) not write to stdout when it is None (GH-26810)
      aab1899

    17. iritkatriel
      closed this as completedon Jul 19, 2021
    18. iritkatriel
      closed this as completedon Jul 19, 2021
    19. ezio-melotti
      transferred this issue fromon Apr 10, 2022
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