New submission from Steven D'Aprano <steve+pyt...@pearwood.info>:
I'm reluctant to call this a bug, as small int interning/caching is an
implementation detail and there are no hard guarantees made.
But on the other hand, it seems that the intention is that small ints such as
0, 1 and 2 should be cached. Here are some examples where they are not.
Intentional or a bug?
>>> x = 1
>>> y = pow(2, 31, 2**31-1)
>>> y == x
True
>>> y is x
False
>>> x = 2
>>> y = pow(2, 31, 2**31-2)
>>> y == x
True
>>> y is x
False
It also affects values which are presumably constant-folded at compile time:
>>> x = 1
>>> y = 2**31 % (2**31 - 1)
>>> z = 2**31 % (2**31 - 1)
>>> x == y == z
True
>>> x is y
False
>>> y is z
False
>>> x is z
False
But if you run the code in exec, the value is interned:
>>> code = """
... x = 1
... y = 2**31 % (2**31-1)
... """
>>> dis(code)
2 0 LOAD_CONST 0 (1)
2 STORE_NAME 0 (x)
3 4 LOAD_CONST 0 (1)
6 STORE_NAME 1 (y)
8 LOAD_CONST 1 (None)
10 RETURN_VALUE
>>> exec(code)
>>> x is y
True
Also affects zero:
>>> x = 0
>>> y = 2**29 % (2**29)
>>> x is y
True
>>> y = 2**30 % (2**30)
>>> x is y
False
First noted here:
https://discuss.python.org/t/cached-integer-id-on-high-calculations/14128/1
>>> sys.version
'3.10.0 (default, Oct 28 2021, 20:43:43) [GCC 8.3.1 20190223 (Red Hat 8.3.1-2)]'
----------
components: Interpreter Core
messages: 414762
nosy: steven.daprano
priority: normal
severity: normal
status: open
title: Caching/interning of small ints sometimes fails
type: behavior
versions: Python 3.10
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46961>
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