We are trying to monkey-patch a third-party library that mixes new and
old-style classes with multiple inheritance. In so doing we have
uncovered some unexpected behaviour:
<quote>
class Foo:
pass
class Bar(object):
pass
class Baz(Foo,Bar):
pass
# Monkey-patch Foo to add a special method
def my_nonzero(self):
print "my_nonzero called"
return False
Foo.__nonzero__ = my_nonzero
b = Baz()
print "doing the test on Baz(Foo,Bar). Should return false"
if b:
print "true"
else:
print "false"
</quote>
Produces this output:
doing the test on Baz(Foo,Bar). Should return false
true
With some experimentation it is clear that this behaviour only occurs
when you combine new+old-style multiple inheritance, monkey-patching
and special methods. If Foo and Bar are either old or new-style it
works. calling b.__nonzero__() directly works. Defining __nonzero__
within Foo works.
I know this level of messing with python internals is a bit risky but
I'm wondering why the above code doesn't work.
Any ideas?
Cheers,
Stephen.
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