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thanks for the example and you are right :
class Foo_emptylist(object):
 def __contains__(self,item): return []
class Foo_emptydict(object):
 def __contains__(self,item): return {}
class Foo_emptystring(object):
 def __contains__(self,item): return ''
for foo in
[Foo_false(),Foo_None(),Foo_emptylist(),Foo_emptydict(),Foo_emptystring(),Foo_neg(),Foo_true(),Foo_42()]:
 print("3 in foo:" + str(3 in foo))
 print("foo.__contains__(3)" + str(foo.__contains__(3)))
3 in foo:False
foo.__contains__(3)False
3 in foo:False
foo.__contains__(3)None
3 in foo:False
foo.__contains__(3)[]
3 in foo:False
foo.__contains__(3){}
3 in foo:False
foo.__contains__(3)
3 in foo:True
foo.__contains__(3)-42
3 in foo:True
foo.__contains__(3)True
3 in foo:True
foo.__contains__(3)42
So the proposition should be this then?
For user-defined classes which define a __contains__() method, the in
operator will apply bool() on the __contains__() method. In other words, "x
in y" is equivalent to "bool(y.__contains__(x))" and will return False if
bool(y.__contains__(x)) is equivalent to false.
Éric Lafontaine | Membre du Projet VUE, Groupe Contrôle
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2017年02月07日 21:10 GMT-05:00 R. David Murray <report@bugs.python.org>:
>
> R. David Murray added the comment:
>
> >>> bool(())
> False
> >>> bool([])
> False
> >>> bool('')
> False
>
> What you want to say is that 'in' coerces the result returned by
> __contains__ to a boolean value.
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue16011>
> _______________________________________
>