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Thu Jun 26 17:14:36 CEST 2008 · https://mail.python.org/pipermail/python-dev/2008-June/080682.html

Jean-Paul Calderone wrote:
> The first assertion passes (by the way, I don't understand why w.message
> isn't the message passed to warn, but is instead an instance of 
> UserWarning)
> but the second assertion fails.

See Brett's comment elsewhere in this thread about the level of thought 
(or lack thereof) that went into the design of some of the test_support 
APIs (including catch_warning).

> A more subtle example might include two
> functions, the first of which is deprecated and called by the second, and
> one test for each of them.  Now the test for the warning will only pass if
> it runs before the other test; if they accidentally run in the other order,
> you won't see the warning, so as far as I can tell, you can't reliably 
> write
> a unit test for warnings using catch_warning.

Yeah, the Python regression tests actually cheat quite a bit when it 
comes to testing warnings - several of the tests clear the warning cache 
just in case the warning has already been triggered by other code.

I suspect what some of those tests *should* be doing (aside from those 
testing the warning module itself) is configuring the warning system to 
convert the relevant warnings to exceptions, then catching the resulting 
exception.

> The real problem with testing many uses of the warning system is that it
> doesn't expose enough public APIs for this to be possible.  You *have*
> to use APIs which are, apparently, private (such as warn_explicit).

Hmm, I think the bigger problem is that there is no documented way to 
save the warnings filter and restore it to a previous state - the 
'filters' attribute (which holds the actual list of filters) isn't 
documented and isn't included in __all__. This makes it hard to write an 
officially supported test that fiddles with the warning settings then 
puts them back the way they were.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
---------------------------------------------------------------
             http://www.boredomandlaziness.org

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[Python-Dev] Py3k DeprecationWarning in stdlib