[Python-Semantic-gaps] Python compatibility gaps in Codon `with` semantics and syntax

[BI71317]

Summary

Codon's current with implementation diverges from Python in several ways.

This issue tracks three compatibility gaps.

1. as binds the class object, not __enter__()'s return value

In Python:

class Manager:
    def __enter__(self):
        return (1, 2)

    def __exit__(self, exc_type, exc, tb):
        return None

with Manager() as pair:
    assert pair == (1, 2)

pair receives the return value of __enter__() method.

In current Codon, Identifier pair is not assigned tuple (1, 2). Instead, an instance of class Manager itself is assigned. So below code is more likely to work:

class Manager:
    def __enter__(self):
        return None

    def __exit__(self):
        return None

    def total(self):
        return 3

with Manager() as pair:  
    assert pair.total() == 3

# pair must be “None” Object. 
# But Actually, pair.total() works.

print("PASS classes_and_with")

This causes a clear Python compatibility gap.

2. with ... as ... only accepts a single identifier target

Python allows assignment targets such as tuple unpacking:

Current Codon grammar only accepts an identifier after as, so forms like these are rejected:

with Manager() as (x, y):
with Manager() as obj.attr:
with Manager() as arr[i]:

So the code below is not working.

class Manager:
    def __enter__(self):
        return (1, 2)

    def __exit__(self):
        return None

with Manager() as (x, y):
    assert x + y == 3

print("PASS with_tuple_unpack_target_python_compat")

The restriction appears in codon/parser/peg/grammar.peg , and the current WithStmt AST representation also only preserves a single identifier-like binding target.

This means the limitation is not only in parsing, but also in AST representation and lowering.

3. __exit__ does not receive Python-style exception information

Python context managers receive:

__exit__(exc_type, exc, tb)

and may suppress exceptions by returning a truthy value.

Current Codon lowering calls __exit__() without exception arguments and does not model Python's suppression behavior. This means Python-style context manager cleanup/error handling semantics are not preserved.

But this likely requires more than a local with lowering change, as it touches the exception model, traceback representation, and with error-path lowering.

This item seems closely related to issue #758.

Why these matter

As you can see in interop pattern of #758, These differences make Codon's with syntax look Python-compatible on the surface while behaving differently in meaningful cases. That can lead to subtle surprises when porting Python code or reasoning about context manager behavior.