Lysandros Nikolaou <[email protected]> added the comment:
I had a look at this. I have been testing with the input a{x}, which faces the
same problem.
It's actually because of an invalid_* rule. The call stack looks like this:
...
invalid_comprehension_rule(Parser * p)
(/home/lysnikolaou/repos/cpython/Parser/parser.c:15065)
genexp_rule(Parser * p) (/home/lysnikolaou/repos/cpython/Parser/parser.c:11381)
primary_raw(Parser * p) (/home/lysnikolaou/repos/cpython/Parser/parser.c:10361)
primary_rule(Parser * p) (/home/lysnikolaou/repos/cpython/Parser/parser.c:10285)
await_primary_rule(Parser * p)
(/home/lysnikolaou/repos/cpython/Parser/parser.c:10240)
...
The invalid_comprehension rule acecpts an LBRACE as the starting token and only
fails after it's parsed it, which means that the parser fails with three tokens
in the tokens array, the NAME which is valid, the LBRACE which is parsed for
the invalid_comprehension rule and the NAME thereafter, upon which the parser
fails and backs out of everything. Then, we look at the last token we've parsed
and that's where we're placing the caret.
Because of invalid_comprehension, we can even go as far as making the parser
show a completely different error, for example:
➜ cpython git:(master) ✗ cat a.py
a{*x for x in a}
➜ cpython git:(master) ✗ ./python a.py
File "/home/lysnikolaou/repos/cpython/a.py", line 1
a{*x for x in a}
^
SyntaxError: iterable unpacking cannot be used in comprehension
Or place the caret even further:
➜ cpython git:(master) ✗ cat a.py
a{*x + a + b + c}
➜ cpython git:(master) ✗ ./python a.py
File "/home/lysnikolaou/repos/cpython/a.py", line 1
a{*x + a + b + c}
^
SyntaxError: invalid syntax
There's a simple fix, which is adding an alternative to the primary rule, that
parses something along the lines of `primary set` and then call
RAISE_SYNTAX_ERROR_KNOWN_LOCATION there, but that would have to come before the
genexp alternative, which worries me because of the performance implications.
`primary` is a left recursive rule that gets called VERY often, probably more
than a few times even when parsing a single NAME.
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Python tracker <[email protected]>
<https://bugs.python.org/issue41659>
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