New submission from Ken Jin <[email protected]>:
While writing the docs for PEP 585's GenericAlias, I noticed that the following
anti-pattern described in PEP 585 no longer throws an error when it should:
>>> l = list
>>> l[-1]
Whereas in versions of Python before 3.9, a "TypeError: 'type' object is not
subscriptable" would have been thrown.
Quoting PEP 585 again: "Say, if a user is mistakenly passing a list type
instead of a list object to a function, and that function is indexing the
received object, the code would no longer raise an error."
Although the context of that statement isn't the same. I fully agree with its
reasoning. This makes Python more confusing for beginners coming from other
languages too:
// in c, makes a 10 element array
int l[10];
# now in python, this is fine
l[10]
This may give beginners the false impression that an empty 10 element list has
been created.
I have created a PR and a test for this. The changed code blocks ints, floats,
bools and complex from indexing for type objects. str is allowed since
list['mytype'] seems analogous to typing.List[ForwardRef('mytype')]. Also, imo
beginners are unlikely to accidentally write that code while intending to index
a list.
Feedback is greatly appreciated, as I've not touched the C codebase in cpython
before. Thanks!
----------
components: Interpreter Core
messages: 378475
nosy: kj
priority: normal
severity: normal
status: open
title: GenericAlias accepts indexing/subscripting, causing confusion
type: behavior
versions: Python 3.10, Python 3.9
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<https://bugs.python.org/issue42010>
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