Mypy: Type inference and hasattr

[JukkaL]

I just encountered code like this:

if hasattr(x, 'initialize'):
     x.initialize()

The type of x was an ABC but it doesn't include initialize. This can be easily worked around by rewriting it like this:

if hasattr(x, 'initialize'):
     cast(Any, x).initialize()

However, mypy could do better, plausibly. For example:

• If x has a union type, infer only union item types with attribute initialize after the hasattr check. So if type of x is Union[str, X] and X has initialize, infer type of x to be X in the if body.
• Allow specifying "potentially undefined" attributes in types. Accessing these requires a hasattr check (a little like Optional[...] requiring something like an is not None check). Not sure what the syntax for this would be like. It would be nice to support these in ABCs as well.

[timabbott]

I ran into an instance of this in the Zulip test runner code, where we add an attribute to a test method via a decorator to track the fact it's expected to be slow, and then check it like this:

def enforce_timely_test_completion(test_method, test_name, delay):                                                                                                     
    # type: (Callable[[], None], str, float) -> None                                                                                                                   
    if hasattr(test_method, 'expected_run_time'):                                                                                                                      
        # Allow for tests to run 50% slower than normal due                                                                                                            
        # to random variations.                                                                                                                                        
        max_delay = 1.5 * test_method.expected_run_time

See https://github.com/zulip/zulip/blob/master/zerver/lib/test_runner.py for the full code.

[gvanrossum]

In general i despise hasattr() checks; it's usually much better to add a
class level initialization to None and check for that.

On Saturday, June 4, 2016, Tim Abbott [email protected] wrote:

I ran into an instance of this in the Zulip test runner code, where we add
an attribute to a test method via a decorator to track the fact it's
expected to be slow, and then check it like this:

def enforce_timely_test_completion(test_method, test_name, delay):
# type: (Callable[[], None], str, float) -> None
if hasattr(test_method, 'expected_run_time'):
# Allow for tests to run 50% slower than normal due
# to random variations.
max_delay = 1.5 * test_method.expected_run_time

See https://github.com/zulip/zulip/blob/master/zerver/lib/test_runner.py
for the full code.

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[timabbott]

Just to be clear, these are attributes monkey-added to a function, not a class, so I don't think that solution would work here.

[gvanrossum]

So there's no place to declare their type anyway. Maybe using three-arg getattr is better?

[jhance]

If x has a union type, infer only union item types with attribute initialize after the hasattr check. So if type of x is Union[str, X] and X has initialize, infer type of x to be X in the if body.

Likely has false positives if one of the elements on the union is an ABC

[ikelos]

I was wondering whether the protocol/structural subtyping work could be used on a hasattr call to in some way to coerce the type into one that accepts the attribute within that scope?

[isidentical]

This also affects using features from future python versions (I'm aware that we can do sys.version_info compare but IMHO this case shouldn't give errors)

import ast
if hasattr(ast, "unparse"):
    print(ast.unparse)
else:
    print("no")