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| -rw-r--r-- | Doc/reference/datamodel.rst | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 18ebe76..de649bb 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1147,6 +1147,21 @@ and ``x`` is an instance of this class, then ``x[i]`` is equivalent [#]_ to ``x.__getitem__(i)``. Except where mentioned, attempts to execute an operation raise an exception when no appropriate method is defined. +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 "<stdin>", line 1, in <module> + 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 |