Mirror
/
cpython
mirror of https://github.com/python/cpython
4
1
Fork 0
Code Issues Releases Wiki Activity

Backport from Py3k: Bug #1684991: explain lookup semantics for __special__ methods (new-style classes only).

This commit is contained in:
Georg Brandl 2007-09-05 13:36:44 +00:00
parent d44a4e9719
commit 5768d577d3
1 changed files with 15 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
Doc/reference/datamodel.rst
View File

@ -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 ``x.__getitem__(i)``. Except where mentioned, attempts to execute an operation
raise an exception when no appropriate method is defined. 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 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 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 object being modelled. For example, some sequences may work well with retrieval