Günther Jena Günther Jena - 1 month ago 6
Python Question

'is' operator behaves unexpectedly with floats

I came across a confusing problem when unit testing a module. The module is actually casting values and I want to compare this values.

There is a difference in comparison with

==
and
is
(partly, I'm beware of the difference)

>>> 0.0 is 0.0
True # as expected
>>> float(0.0) is 0.0
True # as expected


As expected till now, but here is my "problem":

>>> float(0) is 0.0
False
>>> float(0) is float(0)
False


Why? At least the last one is really confusing to me. The internal representation of
float(0)
and
float(0.0)
should be equal. Comparison with
==
is working as expected.

Answer

This has to do with how is works. It checks for references instead of value. It returns True if either argument is assigned to the same object.

In this case, they are different instances; float(0) and float(0) have the same value ==, but are distinct entities as far as Python is concerned. CPython implementation also caches integers as singleton objects in this range -> [x | x ∈ ℤ ∧ -5 ≤ x ≤ 256 ]:

>>> 0.0 is 0.0
True
>>> float(0) is float(0)  # Not the same reference, unique instances.
False

In this example we can demonstrate the integer caching principle:

>>> a = 256
>>> b = 256
>>> a is b
True
>>> a = 257
>>> b = 257
>>> a is b
False

Now, if floats are passed to float(), the float literal is simply returned (short-circuited), as in the same reference is used, as there's no need to instantiate a new float from an existing float:

>>> 0.0 is 0.0
True
>>> float(0.0) is float(0.0)
True

This can be demonstrated further by using int() also:

>>> int(256.0) is int(256.0)  # Same reference, cached.
True
>>> int(257.0) is int(257.0)  # Different references are returned, not cached.
False
>>> 257 is 257  # Same reference.
True
>>> 257.0 is 257.0  # Same reference. As @Martijn Pieters pointed out.
True

However, the results of is are also dependant on the scope it is being executed in (beyond the span of this question/explanation), please refer to user: @Jim's fantastic explanation on code objects. Even python's doc includes a section on this behavior:

[7] Due to automatic garbage-collection, free lists, and the dynamic nature of descriptors, you may notice seemingly unusual behaviour in certain uses of the is operator, like those involving comparisons between instance methods, or constants. Check their documentation for more info.