[Python-Dev] Py3k DeprecationWarning in stdlib (original) (raw)

Andrew Bennetts andrew-pythondev at puzzling.org
Wed Jun 25 15:08:57 CEST 2008

Nick Coghlan wrote: [...]

I forgot this had already been added to the Python regression test machinery, so it will just be a matter of updating the relevant tests to use it:

That's a nice surprise! I'm glad the standard library is growing facilities like this.

I think it could be improved a little, though:

http://docs.python.org/dev/library/test.html#module-test.testsupport

test.testsupport.catchwarning(record=True)ΒΆ Return a context manager that guards the warnings filter from being permanently changed and records the data of the last warning that has been issued. The record argument specifies whether any raised warnings are captured by the object returned by warnings.catchwarning() or allowed to propagate as normal.

The description doesn't really make the record=False case clear. This context manager is doing two different jobs: 1) restore the filters list and showwarning function to their original state when done, and 2) optionally (if record=True) record the last warning in the "as" target. That feels a bit weird.

I think a clearer way to provide that functionality would be with two separate context managers: one that copies and finally restores the filters list and showwarnning function called protect_warnings_module, and then catch_warnings to record the warnings. The catch_warnings context manager could reuse the protect_warnings_module one. "with protect_warnings_module:" seems easier to understand (and document) than "with catch_warning(record=False)".

Should I file a bug for this?

The context manager is typically used like this:

with catchwarning() as w: warnings.warn("foo") assert str(w.message) == "foo"

Maybe this is a YAGNI, but what if an expression triggers multiple warnings? Perhaps the WarningMessage object ought to keep a list rather than just the last warning.

Another thought: if the warnings module had a public, documented way to manipulate the filter list (e.g. warnings.get_all_filters() and warnings.set_all_filters(...)), then people could build these sorts of test facilities for themselves if the test_support one turns out to be too limiting or otherwise a poor fit.

-Andrew.

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