python - Overriding special methods on an instance -

python - Overriding special methods on an instance -

i hope can reply has deep understanding of python :)

consider next code:

>>> class a(object): ... pass ... >>> def __repr__(self): ... homecoming "a" ... >>> types import methodtype >>> = a() >>> <__main__.a object @ 0x00ac6990> >>> repr(a) '<__main__.a object @ 0x00ac6990>' >>> setattr(a, "__repr__", methodtype(__repr__, a, a.__class__)) >>> <__main__.a object @ 0x00ac6990> >>> repr(a) '<__main__.a object @ 0x00ac6990>' >>>

notice how repr(a) not yield expected result of "a" ? want know why case , if there way accomplish this...

i contrast, next illustration works (maybe because we're not trying override special method?):

>>> class a(object): ... def foo(self): ... homecoming "foo" ... >>> def bar(self): ... homecoming "bar" ... >>> types import methodtype >>> = a() >>> a.foo() 'foo' >>> setattr(a, "foo", methodtype(bar, a, a.__class__)) >>> a.foo() 'bar' >>>

python doesn't phone call special methods, name surrounded __ on instance, on class, apparently improve performance. there's no way override __repr__() straight on instance , create work. instead, need so:

class a(object): def __repr__(self): homecoming self._repr() def _repr(self): homecoming object.__repr__(self)

now can override __repr__() on instance substituting _repr().

python metaprogramming