[issue5837] support.EnvironmentVarGuard broken

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

It looks like the unset method should check whether the variable
is set before trying to unset it.

Perhaps this also needs to happen in the __exit__ method?

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue5837>
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