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A comment on Pattern matching accepted for Python. Parent comment

Re:Pattern matching accepted for Python by CogitoCogito on Tuesday February 09, 2021 @07:34PM (#26084456)
I don't know if I even know what "being consistent" would mean given the example here:

https://news.ycombinator.com/item?id=26082225

We don't shadow variables when they are re-used in comprehensions for example:

    >>> i = 4
    >>> [i for i in range(10)]
    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    >>> i
    4
Given they're already accepting the oddity of having "case 404:" and "case variable:" mean very different things, I think they should have just gone the whole way and _not_ mutated outside variables. There seems to be little consistent with this addition to the language.
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Re:Pattern matching accepted for Python by nemetroid on Wednesday February 10, 2021 @05:08AM (#26087393)
> We don't shadow variables when they are re-used in comprehensions for example:

I think "for example" makes this sound more general than it is. List comprehensions are one of the rare few constructs that introduce a new scope. The match block is more syntactically similar to if/while/etc. blocks, and so it makes sense that it would follow similar scoping rules.

This is not to say that I agree with the design as a whole. I think they should have went with:

  case obj.some_value:
      ...
  case some_value:  # behaves the same way as the previous
      ...
  case _ as variable:
      ...
  case (variable := _):  # an alternative to the previous syntax
      ...
I.e. I think the complete situation is messy, but it's not the variable scoping rules' fault.
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Re:Pattern matching accepted for Python by chriswarbo on Wednesday February 10, 2021 @12:27PM (#26092091)
> The match block is more syntactically similar to if/while/etc. blocks, and so it makes sense that it would follow similar scoping rules.

I would argue that a stack of 'case's are a 'lambda' (and in particular a single 'case x: ...' is equivalent), and hence they should have function scoping like 'lambda' does.

'match' is a distraction; it's just a function call written backwards, i.e. `match x: f` is equivalent to `f(x)` (in this case Python happens to require `f` to be implemented using the stack-of-'case's form, but that seems incidental; similar to how 'raise' and 'except' are a general form of control flow, despite Python requiring values to be wrapped in something descended from 'Exception')

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Re:Pattern matching accepted for Python by CogitoCogito on Wednesday February 10, 2021 @10:53AM (#26090813)
> I think "for example" makes this sound more general than it is. List comprehensions are one of the rare few constructs that introduce a new scope.

The rarity of the construct doesn't matter honestly. Are you saying that list comprehensions should _not_ have introduced a new scope because it was unusual?

> The match block is more syntactically similar to if/while/etc. blocks, and so it makes sense that it would follow similar scoping rules.

Syntactically yes, but semantically it is very different and the semantics must be taken into account as well. The example given at the top of this thread (case 404: and case variable:) is enough to convince me that having variable scoping is a brain-dead obvious requirement.

> I think the complete situation is messy, but it's not the variable scoping rules' fault.

I agree with that statement. I think that improving the design/semantics would be more effective than just adding some more scoping rules in. In fact, I don't think this PEP should have been accepted in this current form at all. But given the current design, block-level scoping is appropriate. Given another design like maybe those you mention might not require that, but I think focusing on the fact that python doesn't have block-level scoping makes no sense. The python language before this PEP is not the same as the one after this PEP. The new one should have block-level scoping in this instance.

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Re:Pattern matching accepted for Python by pfalcon on Wednesday February 10, 2021 @05:19AM (#26087457)
> I think they should have just gone the whole way and _not_ mutated outside variables. There seems to be little consistent with this addition to the language.

Interested in block-level scoping in Python? Please post on the python-ideas mailing list. Thanks.

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