#23 Check for mutable default arguments

[Simon Dahlbacka]

I got bitten by mutable default arguments, and wanted
pychecker to find those for me.

This is sort of a pseudo patch, since I don't know the
pychecker codebase (and I didn't seem to find a decent
diff utility at the moment)

Patch probably need to be reworked a bit (and probably
add a switch to turn the check on/off) to fit nicely
into pychecker. But I think that it would be a
beneficial check to add.

[Simon Dahlbacka]

[Simon Dahlbacka]

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Got myself together and prodused a somewhat more usable
patch. This one adds a --mutabledefaults option that
currently defaults to on.

The patch tests all the function arguments against
MUTABLE_TYPES, and issues a warning is a mutable argument is
found. Currently it does not bother to check the rest of the
arguments if one that is mutable was found.

[Neal Norwitz]

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This patch seems to implement similar functionality to the
following option: -j, --moddefvalue check if modify (call
method) on a parameter that has a default value [on]

I understand this goes through all arguments where the
current code tries to check default argument accesses. It
does this to try to eliminate spurious warnings. Can you
give an example where the current code doesn't work?

Thanks.

[Simon Dahlbacka]

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I can understand the need to avoid spurious warnings, (I
also currently have problems with too many spurious warnings
that leads to not using pychecker as often as I should)

That aside, the following testcase shows two cases where
current code do not find the modification of the default
value. In my experience, a mutable default value is more
often than not a bug (that also can be hard to pinpoint)

[Simon Dahlbacka]

[Neal Norwitz]

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Ok, I buy that argument (ie, the code). Though, I'm not
sure how to marry the existing option with this one or if
they should remain separate.

What problems are you having with spurious warnings? Are
you using the last release 0.8.14 or the CVS version? The
CVS version does have a lot of bug fixes. I hope to release
it real soon now, but there's been so many changes over a
long period, I think there will be some problems with it.