Tal Tal - 6 months ago 13
Python Question

Why does '20000 is 20000' result in True?

is
in Python tests if 2 references point to the same object.
Numbers between -5 and 256 are cached internally so:

a = 10
b = 10
a is b # Results in True


How does this explain something like:

20000 is 20000 # Results in True


Both numbers are above 256.
Should not the 2 integers be 2 distinct objects?

Answer

The Python interpreter sees you are re-using a immutable object, so it doesn't bother to create two:

>>> import dis
>>> dis.dis(compile('20000 is 20000', '', 'exec'))
  1           0 LOAD_CONST               0 (20000)
              3 LOAD_CONST               0 (20000)
              6 COMPARE_OP               8 (is)
              9 POP_TOP
             10 LOAD_CONST               1 (None)
             13 RETURN_VALUE

Note the two LOAD_CONST opcodes, they both load the constant at index 0:

>>> compile('20000 is 20000', '', 'exec').co_consts
(20000, None)

In the interactive interpreter Python is limited to having to compile each (simple or compound) statement you enter separately, so it can't reuse these constants across different statements.

But within a function object it certainly would only create one such integer object, even if you used the same int literal more than once. The same applies to any code run at the module level (so outside of functions or class definitions); those all end up in the same code object constants too.