Rich Comparisons Gotcha

Robert Kern robert.kern at
Mon Dec 8 23:13:12 CET 2008

Terry Reedy wrote:
> Rasmus Fogh wrote:

>> much, though, just to get code like this to work as intended:
>>   alist.append(x)
>>   print ('x is present: ', x in alist)
> Even if rich comparisons as you propose, the above would *still* not 
> necessarily work.  Collection classes can define a __contains__ that 
> overrides the default and that can do anything, though True/False is 
> recommended.

No, it's actually required.

In [4]: class A(object):
     def __contains__(self, other):
         return 'foo'

In [7]: a = A()

In [8]: 1 in a
Out[8]: True

Okay, so it will coerce to True/False for you, but unlike rich comparisons, the 
return value must be interpretable as a boolean.

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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