[New-bugs-announce] [issue28835] Change in behavior when overriding warnings.showwarning and with catch_warnings(record=True)

[Thomas Robitaille]

New submission from Thomas Robitaille:

In Python 3.5, the following code:

    import warnings

    def deal_with_warning(*args, **kwargs):
        print("warning emitted")

    with warnings.catch_warnings(record=True):
        warnings.showwarning = deal_with_warning
        warnings.warn("This is a warning")

results in "warning emitted" being printed to the terminal.

In Python 3.6 however (at least in 3.6b1), nothing is printed, meaning that ``deal_with_warning`` is not getting called. I bisected the CPython history and tracked it down to the changes in this issue:

https://bugs.python.org/issue26568

However it doesn't look like this was an intentional change in behavior, since it says in the description of that issue:

"For backward compatibility, warnings.showmsg() calls warnings.showwarning() if warnings.showwarning() was replaced. Same for warnings.formatmsg(): call warnings.formatwarning() if replaced."

So I believe this is a bug? (since backward-compatibility is not preserved). If not, should the change in behavior be mentioned in the changelog?

----------
messages: 282056
nosy: Thomas.Robitaille
priority: normal
severity: normal
status: open
title: Change in behavior when overriding warnings.showwarning and with catch_warnings(record=True)
versions: Python 3.6

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28835>
_______________________________________