Is there a way to make len()
work with instance methods without modifying the class?
Example of my problem:
>>> class A(object):
... pass
...
>>> a = A()
>>> a.__len__ = lambda: 2
>>> a.__len__()
2
>>> len(a)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'A' has no len()
Note:
A
will have different __len__
methods attachedA
It is actually possible without modifying the class based on this answer by Alex Martelli:
class A(object):
def __init__(self, words):
self.words = words
def get_words(self):
return self.words
a = A("I am a")
b = A("I am b")
def make_meth(inst, _cls, meth, lm):
inst.__class__ = type(_cls.__name__, (_cls,), {meth: lm})
make_meth(a, A, "__len__", lambda self: 12)
make_meth(b, A, "__len__", lambda self: 44)
print(len(b))
print(len(a))
print(a.get_words())
print(b.get_words())
If we run the code:
In [15]: a = A("I am a")
In [16]: b = A("I am b")
In [17]: make_meth(a, A, "__len__", lambda self: 12)
In [18]: make_meth(b, A, "__len__", lambda self: 44)
In [19]: print(len(b))
44
In [20]: print(len(a))
12
In [21]: print(a.get_words())
I am a
In [22]: print(b.get_words())
I an b
As per the last part of the last part of the linked answer, you can add any methods on a per instance basis using inst.specialmethod
once you have used inst.__class__ = type(...
:
In [34]: b.__class__.__str__ = lambda self: "In b"
In [35]: print(str(a))
<__main__.A object at 0x7f37390ae128>
In [36]: print(str(b))
In b