diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 9e847cc74e4..29e6220fe04 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1060,6 +1060,20 @@ raise an exception when no appropriate method is defined. .. XXX above translation is not correct for new-style classes! +Special methods are only guaranteed to work if defined in an object's class, not +in the object's instance dictionary. That explains why this won't work:: + + >>> class C: + ... pass + ... + >>> c = C() + >>> c.__len__ = lambda: 5 + >>> len(c) + Traceback (most recent call last): + File "", line 1, in + TypeError: object of type 'C' has no len() + + When implementing a class that emulates any built-in type, it is important that the emulation only be implemented to the degree that it makes sense for the object being modelled. For example, some sequences may work well with retrieval