I had a program fail on me today because the following didn't work as I
expected:
>>> class C:
... def f(self):
... pass
...
>>> c = C()
>>> m = c.f
>>> m is c.f
False
I would have expected that if I set a variable equal to a bound method, that
variable, for all intents and purposes, *is* that bound method, especially
since I hadn't changed or deleted anything on the class or its instance.
The same thing happens when attempting to compare an unbound method with a
variable name bound to that unbound method:
>>> M = C.f
>>> M is C.f
False
If I were to guess what is going on here, I would say that the expression
c.f invokes a descriptor that manufactures a brand new "method object" each
time. The problem is, this is non-intuitive (to me) and prevented me from
doing something I thought was useful.
My use case is deferring operations till later, by placing tuples of a
method and its arguments in a list to be processed at some future time, but
doing some special-case processing only for certain methods:
deferred = []
...
deferred.append((c.f, ('abc', 123)))
...
for method, params in deferred:
method(*params)
if method is c.f:
# handle a special case
But I can't do that special-case handling this way, I have to resort to some
other means to identify a method, "method is c.f" is always False. Which
seems strange to me.
This behavior is in Python 2.4, 2.3, and 2.2.
The workaround really awkward:
SPECIAL_METHOD = c.f
...
deferred.append((SPECIAL_METHOD, ('abc', 123)))
...
if method is SPECIAL_METHOD:
# handle special case
If I forget at any time to use the name SPECIAL_METHOD, but resort to it's
"real" name of c.f, then the comparison fails.
So, is this a bug or expected?
If I complain about this, would I get any sympathy? ;)
regards,
David H.
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