[Python-Dev] Re: PEP 622: Structural Pattern Matching -- followup

[Rob Cliffe via Python-Dev]

On 24/06/2020 20:38, Guido van Rossum wrote:

Everyone,

If you've commented and you're worried you haven't been heard, please 
add your issue *concisely* to this new thread. Note that the following 
issues are already open and will be responded to separately; please 
don't bother commenting on these until we've done so:


- Alternative spellings for '|'
- Whether to add an 'else' clause (and how to indent it)
- A different token for wildcards instead of '_'
- What to do about the footgun of 'case foo' vs. 'case .foo'

(Note that the last two could be combined, e.g. '?foo' or 'foo?' to 
mark a variable binding and '?' for a wildcard.)


(Prefatory remarks:  I am sure you get a lot of questions to which the 
answer is basically "Read the PEP".  I myself have been guilty in this 
regard.  But I fear this is inevitable when the PEP is so long and there 
is so much new stuff to absorb.  Apologies if this is yet another one.)


_First question_: Sometimes no action is needed after a case clause.  If 
the Django example had been written


if (
isinstance(value, (list, tuple)) and
len(value) > 1 and
isinstance(value[-1], (Promise, str))
):
*value, label = value
else:
label = key.replace('_', ' ').title()

the replacement code would/could be

match value:
case [*value, label := (Promise() | str())] if value:
pass
case _:
label = key.replace('_', ' ').title()

AFAICS the PEP does not *explicitly* state that the 'pass' line is necessary 
(is it?), i.e. that the block following `case` cannot (or can?) be empty.
The term `block` is not defined in the PEP, or in 
https://docs.python.org/3/reference/grammar.html.
But an empty block following a line ending in `:` would AFAIK be unprecedented 
in Python.  I think it is worth clarifiying this.

_Second question_: in the above example replacement, if `case _:` does not bind 
to `_`, does that mean that the following line will not work?
Is this one of the "two bugs" that Mark Shannon alluded to?  (I have read every 
message in the threads and I don't remember them being spelt out.)
And I'm curious what the other one is (is it binding to a variable `v`?).

Best wishes
Rob Cliffe

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[Python-Dev] Re: PEP 622: Structural Pattern Matching -- followup

[Tobias Kohn]

 Hi Rob,

 You are right: the grammar should probably read `suite` rather
than `block` (i.e. the `pass` is necessary).  Thanks for catching
this!

 As for the second question, I assume there might be a slight
oversight on your part.  The last line in the example replaces the
string `"_"` rather than the variable `_`.  The not-binding of `_`
thus has no influence on the last line.

 I think I will leave it for Mark himself to name the two bugs rather
than start a guessing game.  However, in an earlier version we had
left out the `if value` for the first case, accidentally translating
the `len(value) > 1` as a `len(value) >= 1` instead.

 Kind regards,
Tobias

Quoting Rob Cliffe via Python-Dev :


[...]

_First question_: Sometimes no action is needed after a case  
clause.  If the Django example had been written
if ( isinstance(value, (list, tuple)) and len(value) > 1 and  
isinstance(value[-1], (Promise, str)) ): *value, label =  
value else: label = key.replace('_', ' ').title()  the  
replacement code would/could be  match value: case [*value,  
label := (Promise() | str())] if value: pass case _:  
label = key.replace('_', ' ').title()  AFAICS the PEP does not  
*explicitly* state that the 'pass' line is necessary (is it?), i.e.  
that the block following `case` cannot (or can?) be empty. The term  
`block` is not defined in the PEP, or in  
https://docs.python.org/3/reference/grammar.html. But an empty block  
following a line ending in `:` would AFAIK be unprecedented in  
Python.  I think it is worth clarifiying this.  _Second question_:  
in the above example replacement, if `case _:` does not bind to `_`,  
does that mean that the following line will not work? Is this one of  
the "two bugs" that Mark Shannon alluded to?  (I have read every  
message in the threads and I don't remember them being spelt out.)  
And I'm curious what the other one is (is it binding to a variable  
`v`?).  Best wishes Rob Cliffe
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[Python-Dev] Re: PEP 622: Structural Pattern Matching -- followup

[Rob Cliffe via Python-Dev]

I think we are storing up trouble unless we
    1) Allow arbitrary expressions after `case`, interpreted *as now*
    2) Use *different* syntaxes, not legal in expressions, for
            alternative matching values (i.e. not `|` or `or`) (NB 
simply stacking with multiple `case` lines is one possibility)

            templates such as `Point(x, 0)`
            anything else particular to `match`
I am reminded of the special restrictions for decorator syntax, which 
were eventually removed.


On 24/06/2020 20:38, Guido van Rossum wrote:

Everyone,

If you've commented and you're worried you haven't been heard, please 
add your issue *concisely* to this new thread. Note that the following 
issues are already open and will be responded to separately; please 
don't bother commenting on these until we've done so:


- Alternative spellings for '|'
- Whether to add an 'else' clause (and how to indent it)
- A different token for wildcards instead of '_'
- What to do about the footgun of 'case foo' vs. 'case .foo'

(Note that the last two could be combined, e.g. '?foo' or 'foo?' to 
mark a variable binding and '?' for a wildcard.)


--
--Guido van Rossum (python.org/~guido )
/Pronouns: he/him //(why is my pronoun here?)/ 



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[Python-Dev] Request for code review

[Xin, Peixing]

Hi,

I have several PRs pending there to get review for months. Anyone can help to 
review and do a merge? Thanks.

https://github.com/python/cpython/pull/19447
https://github.com/python/cpython/pull/20254
https://github.com/python/cpython/pull/20256
https://github.com/python/cpython/pull/12670

bpo: https://bugs.python.org/issue31904

Thanks,
Peixing

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