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Invalid mapping patterns give confusing SyntaxErrors #88534
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Here are a few that I found. Not sure when they were introduced:
match ...:
case {**rest, "key": value}:
passmatch ...:
case {"first": first, **rest, "last": last}:
passmatch ...:
case {**_}:
passThese all give the following error while parsing the second line:
File "<stdin>", line 1
match ...:
^^^^^^^^^
SyntaxError: invalid syntax. Perhaps you forgot a comma?Perhaps we need something like invalid_mapping_pattern or invalid_mapping_pattern_double_star rules?
Probably, otherwise is going to hurt because the syntax errors that you describe trigger when two names are place together, which in general is a missing comma.
The error message also doesn't say: "you are missing a comma" it says that the most typical reason is a missing comma, which is important to distinguish :)
The backtracking with the soft keyword may make this very annoying, by the way.
Oh, wait, I think I misunderstood the problem. The problem is that the parser is backtracking and identifying match as a name.
Indeed, the soft keyword is a pain :(
I think it actually will be very useful to explain that these cases are invalid in the Syntax error, which will also solve this problem.
This on the other hand shows a bigger problem: any generic syntax error that happens inside "match" will probably end identifying the keyword as a name, even if we don't have specific errors, the parser will point to it and complain about "match".
Oh, turns out I already added machinery to solved this but I was missing a piece!
Wow, that was quite a roller-coaster ride. Thanks Pablo. :)
I found a similar one, by the way (not related to mapping patterns):
>>> match ...: ... case 42 as _: ... pass File "<stdin>", line 2 case 42 as _: ^ SyntaxError: expected ':'
Is this covered by your fix?
Is this covered by your fix?
No, that is not a backtracking error (is not going all the way to the "match" expression).
I will fix that one in another PR
Oh, this one is actually correct:
match ...:
case 42 as _:
passFile "<stdin>", line 2
case 42 as _:
^
SyntaxError: expected ':'That is literally expecting a ":" and that's the error. It has identified "case 42" correctly and it now expects a ":". Is just that the error marker is wrong because it has reached the "as and _" as part of the parsing so the error is there.
That is going to be a bit tricky to "fix"
Could we just try parsing "as _" and raise if so? That wouldn't conflict with any existing rules, and that way we could actually have a helpful error message.
Like "SyntaxError: can't capture into a wildcard (consider removing)".
Could we just try parsing "as _" and raise if so? That wouldn't conflict with any existing rules, and that way we could actually have a helpful error message.
No, the same happens with other targets such as:
File "/home/pablogsal/github/python/master/lel.py", line 2
case 42 as 1+1:
^
SyntaxError: expected ':'@pablogsal @brandtbucher The issue seems to be solved. We can close the issue.
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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