From 65ea9bd87e294ec9eb80f2686bbb9d17276a14e4 Mon Sep 17 00:00:00 2001 From: Georg Brandl Date: Wed, 5 Sep 2007 13:36:27 +0000 Subject: Bug #1684991: explain __special__ lookup semantics. --- Doc/reference/datamodel.rst | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 9e847cc..29e6220 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 -- cgit v0.12