[issue41777] When using `python -bb`, `struct.calcsize` raises a warning when used with str argument after being used with bytes (might be a larger problem with dicts)

[Edson Tadeu M. Manoel]

New submission from Edson Tadeu M. Manoel <e.ta...@gmail.com>:

Here is the inconsistent behavior, when running with `python -bb` (or just 
`python -b`), caused by an internal cache:

    >>> import struct
    >>> struct.calcsize(b'!d')  # cache for '!d' uses bytes
    8
    >>> struct.calcsize('!d')  # so there's a warning when trying to use str
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    BytesWarning: Comparison between bytes and string
    >>> struct.calcsize('>d')  # cache for '>d' uses str
    8
    >>> struct.calcsize(b'>d')  # so now the warning is inverted, it shows up 
when trying to use bytes
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    BytesWarning: Comparison between bytes and string
    >>> struct.calcsize('>d')  # no problem when using str
    8

Note that this might be caused by a possible larger problem when dealing with 
keys of different string types in dicts under `python -b` (or `python -bb`):

    $ python
    Python 3.8.5 (tags/v3.8.5:580fbb0, Jul 20 2020, 15:57:54) [MSC v.1924 64 
bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> d={}
    >>> d['a']=1
    >>> d[b'a']=2
    >>> d['a']
    1
    >>> d[b'a']
    2


    $ python -bb
    Python 3.8.5 (tags/v3.8.5:580fbb0, Jul 20 2020, 15:57:54) [MSC v.1924 64 
bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> d={}
    >>> d['a']=1
    >>> d[b'a']=2
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>

I'm not sure if this warning is intentional, since in Python 3 there seems to 
be no special reason for dicts to try to compare 'a' with b'a' (other than 
possible implementation details).


Note: this is from an issue found here: 
https://github.com/pytest-dev/pytest-xdist/issues/596

----------
components: Interpreter Core, Library (Lib)
messages: 376836
nosy: tadeu
priority: normal
severity: normal
status: open
title: When using `python -bb`, `struct.calcsize` raises a warning when used 
with str argument after being used with bytes (might be a larger problem with 
dicts)
type: behavior
versions: Python 3.8

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<https://bugs.python.org/issue41777>
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