There's also bisect_left and bisect_right which each do different things. The bisect_left function puts the needle on the right while the bisect_right function puts the needle on the left.

I've thought a bit about these scenarios as the author of Sorted Containers but no user has ever raised an issue about it. Personally, I think the needle should go on the left as it offers the caller greater opportunity to control and customize the __eq__ method that is used.

But personal preferences aside, I think the "ship has sailed" and the differences are not worth fixing. I agree with Guido that developers should fix __eq__ to use NotImplemented as it was intended.


On Sat, Jul 20, 2019 at 8:33 AM Guido van Rossum <> wrote:
In an ideal world, needle is on the right. Let's replace needle with a constant: which of the following looks more natural?

  for x in sequence:
      if x == 5: return True


  for x in sequence:
      if 5 == x: return True

For me, 'x == 5' wins with a huge margin. (There is a subculture of C coders who have trained themselves to write '5 == x' because they're afraid of accidentally typing 'x = 5', but that doesn't apply to Python.)

Should we unify the stdlib? I'm not sure -- it feels like a sufficiently obscure area that we won't get much benefit out of it (people should fix their __eq__ implementation to properly return NotImplemented) and changing it would surely cause some mysterious breakage in some code we cannot control.


On Sat, Jul 20, 2019 at 7:31 AM Serhiy Storchaka <> wrote:
Usually the order of operands of the == operator does not matter. bool(a
== b) should return the same as bool(b == a). Correct __eq__ should look

     def __eq__(self, other):
         if not know how to compare with other:
             return NotImplemented
         return the result of comparison

But we work with non-perfect code written by non-perfect people.
__eq__() can return False instead of NotImplemented for comparison with
different type (it is not the worst case, in worst case it raises
AttributeError or TypeError). So the order of operands can matter.

See as an example of a real world issue.

The typical implementation of the __contains__ method looks like:

     def __contains__(self, needle):
         for item in self:
             if item == needle:  # or needle == item
                 return True
         return False

The question is where the needle should be: at the right or at the left
side of ==?

In __contains__ implementations in list, tuple and general iterators
(see PySequence_Contains) the needle is at the right side. But in
count(), index() and remove() it is at the left side. In array it is
effectively always at the left side since its __eq__ is not invoked.

The question is whether we should unify implementations and always use
the needle at some particular side and what this side should be.
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--Guido van Rossum (
Pronouns: he/him/his (why is my pronoun here?)
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