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Comment by mbo | Hacker News Reader

What's the rationale for not introducing a new scope?

The only clause I can find in the PEP is

> A capture pattern always succeeds. It binds the subject value to the name using the scoping rules for name binding established for named expressions in PEP 572. (Summary: the name becomes a local variable in the closest containing function scope unless there's an applicable nonlocal or global statement.)

This seems... incredibly bad.

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It's consistent with how scoping works in loops and `with` clauses. Agreed that it's more problematic here, though.

That works the same as the rest of the language, doesn't it?

(That is, for, if, while, etc. also don't introduce new scopes.)

"Introducing a new scope" is not a concept that exists in Python, you only have function scope and module scope.

There are some exceptions to this - for example, the iteration variable(s) in a list comprehension are only in scope within the comprehension.

Thanks, I didn't realize that. That makes their choice really baffling.

The explanation is that (in Python 3) list comprehensions are really just syntactic sugar for generator expressions. In Python, "scope" is really a synonym for "dictionary attached to some object", so generators can have local variables (since they have their own internal scope), but purely syntactic constructs cannot.

Worth noting that this was _fixed_ in Python 3.

In Python 2.7 you get this:

    >>> my_list = [x for x in 'hello']
    >>> x
    'o'

and class scope

Having only function, module, class, generator, etc. scope in Python before this PEP might have made sense, but they really should have added pattern matching scope to keep things sane here.

So you can easily create a variable in a match case

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