to wide source positions for exceptions during pattern matching (3.11.0rc2) #96999

15r10nk · 2022-09-21T21:16:13Z

The source range in the traceback is to wide in some cases, if an exception is thrown during matching of a pattern.

The Ranges

example for attribute lookup:

class A:
    def __getattr__(self, name):
        assert name[0] == "a"
        return 5


match A():
    case A(apple=5, banana=7):
        print("hi")

output (Python 3.11.0rc2+):

Traceback (most recent call last):
  File "/home/frank/projects/cpython/match_test.py", line 8, in <module>
    case A(apple=5, banana=7):
         ^^^^^^^^^^^^^^^^^^^^
  File "/home/frank/projects/cpython/match_test.py", line 3, in __getattr__
    assert name[0] == "a"
AssertionError

Annotating only the source positions where the attribute check failed (banana=7) would make the traceback easier to read.

The same problem exists for positional matching:

class A:
    __match_args__ = ("apple", "banana")

    def __getattr__(self, name):
        assert name == "apple"


match A():
    case A(5, 7):
        print("hi")

output (Python 3.11.0rc2+):

Traceback (most recent call last):
  File "/home/frank/projects/cpython/match_test2.py", line 9, in <module>
    case A(5, 7):
         ^^^^^^^
  File "/home/frank/projects/cpython/match_test2.py", line 5, in __getattr__
    assert name == "apple"
AssertionError

Equality checks are handled better:

class NeverEqual:
    def __eq__(self, other):
        assert other==5


class A:
    def __init__(self):
        self.a = NeverEqual()
        self.b = 3


match A():
    case A(a=5|3, b=3):
        print("hi")

output (Python 3.11.0rc2+):

Traceback (most recent call last):
  File "/home/frank/projects/cpython/match_test3.py", line 13, in <module>
    case A(a=5|3, b=3):
               ^
  File "/home/frank/projects/cpython/match_test3.py", line 3, in __eq__
    assert other==5
AssertionError

Instance checks are represented in the same way:

class MetaB(type):
    def __instancecheck__(self, instance):
        assert False, "do not use me"


class B(metaclass=MetaB):
    pass


class A:
    def __init__(self):
        self.a = 5
        self.b = 3


match A():
    case A(a=5, b=2|B()):
        print("hi")

output (Python 3.11.0rc2+):

Traceback (most recent call last):
  File "/home/frank/projects/cpython/match_test4.py", line 17, in <module>
    case A(a=5, b=2|B()):
                    ^^^
  File "/home/frank/projects/cpython/match_test4.py", line 3, in __instancecheck__
    assert False, "do not use me"
AssertionError: do not use me

Problem for the User

Consistent source ranges would improve the readability of the tracebacks.

Is it possible to highlight always only the failing Pattern (with attribute name if given) for failing attribute access?

example:

case A(a=5):
       ^^^
case A(a=B()):
       ^^^^^
case A(5):
       ^
case A(5|B()):
       ^^^^^

Instance checks and equality tests are already handled well:

case A(a=5):
         ^
case A(a=B()):
         ^^^
case A(5):
       ^
case A(B()):
       ^^^
case A(5|B())
         ^^^

Problem for libraries

Libraries are using the source ranges for all sorts of functionality.
Executing for example is using this ranges (for the new 3.11 support) to map from instructions back to ast-nodes. Useful ranges are here required to map to useful ast-nodes.

The text was updated successfully, but these errors were encountered:

15r10nk · 2022-09-21T21:53:33Z

I just saw that the arguments in the MatchClass ast-node have no node for the argument name with the value.

case A(a=5):
       ^^^ # there is no ast-node

This would make it really difficult for cpython and libraries to work with such ranges.

I don't know what the best solution might be.

brandtbucher · 2022-09-21T23:27:57Z

Yeah, this could certainly be tightened up a bit. It definitely falls under the general theme of #93691, so not a super high priority at the moment.

15r10nk added the type-bug An unexpected behavior, bug, or error label Sep 21, 2022

AlexWaygood assigned brandtbucher Sep 21, 2022

to wide source positions for exceptions during pattern matching (3.11.0rc2) #96999

to wide source positions for exceptions during pattern matching (3.11.0rc2) #96999

15r10nk commented Sep 21, 2022

15r10nk commented Sep 21, 2022 •

edited

brandtbucher commented Sep 21, 2022

to wide source positions for exceptions during pattern matching (3.11.0rc2) #96999

to wide source positions for exceptions during pattern matching (3.11.0rc2) #96999

Comments

15r10nk commented Sep 21, 2022

The Ranges

Problem for the User

Problem for libraries

15r10nk commented Sep 21, 2022 • edited

brandtbucher commented Sep 21, 2022

15r10nk commented Sep 21, 2022 •

edited