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List:       python-bugs-list
Subject:    [issue37728] 'is' behaving differently in script and interactive shel
From:       Eryk Sun <report () bugs ! python ! org>
Date:       2019-07-31 9:12:48
Message-ID: 1564564368.91.0.145672641602.issue37728 () roundup ! psfhosted ! org
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Eryk Sun <eryksun@gmail.com> added the comment:

The CPython interpreter doesn't centrally cache and reuse float objects, and even if \
it did that would be an implementation detail, like what it does with small int \
objects and strings. 

What you're seeing at the module level is a compilation side effect. The compiler is \
free to reuse immutable constants in a code object. For example:

    >>> code = compile('af=1.1\nbf=1.1', '', 'exec')
    >>> dis.dis(code)
      1           0 LOAD_CONST               0 (1.1)
                  2 STORE_NAME               0 (af)

      2           4 LOAD_CONST               0 (1.1)
                  6 STORE_NAME               1 (bf)
                  8 LOAD_CONST               1 (None)
                 10 RETURN_VALUE

In the above case, both af and bf are initialized with the same `1.1` constant float \
object, which is at index 0 of the code object's tuple of constants: 

    >>> code.co_consts
    (1.1, None)

But if we run `af=1.1` and `bf=1.1` as separate statements in the REPL shell, the \
statements are compiled separately, and af and bf won't necessarily reference the \
same float object. 

Don't expect two immutable objects that have the same value to be the same object -- \
unless it's a singleton such as `None` and `Ellipsis`:

    >>> type(None)() is None
    True
    >>> type(Ellipsis)() is Ellipsis
    True

Or an n-way extension of this case, such as bool in Python 3:

    >>> bool(0) is False
    True
    >>> bool(1) is True
    True

The consequences of immutability on object identity are discussed briefly in the \
final paragraph of "3.1 Objects, values, and types" [1]. Offhand, I don't know where \
the specific implementation details of CPython builtin immutable types and compiler \
behavior are discussed, if it's discussed at all.

[1] https://docs.python.org/3/reference/datamodel.html#objects-values-and-types

----------
nosy: +eryksun
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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<https://bugs.python.org/issue37728>
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