David Handy wrote:
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.

Yes, I believe that's the case. See Martelli's recent 'Black Magic' PyCon presentation for some details of what's going on, but IIRC, the essence is that a bound method is produced by calling __get__ on the function, with the class/instance as the argument


The problem is, this is non-intuitive (to me) and prevented me from
doing something I thought was useful.

Agreed that: >>> class C(object): ... def f(self):pass ... >>> C.f is C.f False

is surprising

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.

Another workaround is to compare method.im_func which does pass the identity test ...

If I complain about this, would I get any sympathy? ;)

Sympathy, sure ;-)


Michael

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