Expand warnings output for ResourceWarning

[fschulze]

It is difficult to debug ResourceWarning from the existing output.

In Python3.6 a source field was added to the warnings.WarningMessage objects.

Since pytest now requires Python 3.7 I propose the following addition to the existing warning_record_to_str:

def warning_record_to_str(warning_message: warnings.WarningMessage) -> str:
    """Convert a warnings.WarningMessage to a string."""
    warn_msg = warning_message.message
    msg = warnings.formatwarning(
        str(warn_msg),
        warning_message.category,
        warning_message.filename,
        warning_message.lineno,
        warning_message.line,
    )
    tb = tracemalloc.get_object_traceback(warning_message.source)
    if tb is not None:
        msg = "%sSource of unclosed object:\n%s" % (msg, '\n'.join(tb.format()))
    elif issubclass(warning_message.category, ResourceWarning):
        # tracemalloc is inactive
        # TODO needs to be adjusted, see below in ticket
        msg = "%sGet a traceback for where the unclosed object was allocated by ..." % msg
    return msg

The output looks like this as an example:

  /.../devpi/devel/lib/python3.8/enum.py:315: ResourceWarning: unclosed <socket.socket [closed] fd=53, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6>
    return cls.__new__(cls, value)
  Source of unclosed object:
    File "/.../devpi/devel/lib/python3.8/site-packages/urllib3/connection.py", line 205
      conn = self._new_conn()
    File "/.../devpi/devel/lib/python3.8/site-packages/urllib3/connection.py", line 174
      conn = connection.create_connection(
    File "/.../devpi/devel/lib/python3.8/site-packages/urllib3/util/connection.py", line 77
      sock = socket.socket(af, socktype, proto)

This requires that tracemalloc is active. So the open question is, when should tracemalloc.start() be called and how to pass the size of the stack trace to it, as the default of 1 is often too small (I needed 20 to find the source of an unclosed requests Response). I propose three possible solutions:

1. Document running pytest like this: python -X tracemalloc=20 -m pytest ...
2. Add a command line option to enable tracemalloc with the limit as parameter.
3. Enable tracing when warnings for ResourceWarning are set to once or always and always use a stack trace limit of 3.

[bluetech]

I think this would be a very nice feature. 👍 from me.

AFAIK tracemalloc has a large memory overhead so enabling it by default is not an option, and neither is option 3 really.

Can the logic be something like this?

if warning_message.source is not None:
    # If tracemalloc is active, and has captured the object's allocation, show where it was allocated.
    tb = tracemalloc.get_object_traceback(warning_message.source)
    if tb is not None:
        msg += "\nObject allocated at (most recent call first):\n" + (nice tb formatting here)
    else:
        Suggestion to enable tracemalloc (if it's not enabled), or increase the number of frames (if it is enabled)
        Is it helpful at all to do anything with `warning_message.source` here (like show its repr),
        or assume it's already covered by the warning message itself?

as for what to write in the suggestion, option 1 pytest -> python -X tracemalloc=20 -m pytest seems a bit awkward. An alternative is to suggest PYTHONTRACEMALLOC=20 pytest. Or we can just add the command line argument (option 2). Not sure which is better. Maybe avoid the cmdline argument at first, and only add if if people encounter difficulties without it.

[fschulze]

@bluetech I like the environment variable approach, I didn't think of that.

Your if statements will also work in case any other warning has a source field. So +1.

I don't think we need to do anything with warning_message.source. It is already represented and there is nothing generic possible with it, except maybe printing vars(warning_message.source), which only works if it has a __dict__. I think the usefulness is further restricted, because it is already garbage collected, but I'm not sure right now.

[nicoddemus]

Sounds good to me too, thanks folks!

[fschulze]

🎉