Testing the new metaclass support for mypy in Py2

.gitignore

__pycache__/
*.py[cod]
*$py.class
pip-log.txt
pip-delete-this-directory.txt
.venv

README.md

mypy Type-checker Metaclass Tests

The mypy type-checker recently merged a pull request for metaclass support in mypy. The goal here is to test how far that support extends in Python 2, given mypy didn't even parse Python 2 metaclass declarations previously.

Test Cases

• py2_example1.py is testing cases where the metaclass is defined before its instance, then passing in by name to the __metaclass__ attribute
• py2_example2.py consists of my test cases from python/mypy#2413, modified for python 2
• py2_example3.py is testing metaclasses defined as a child class named __metaclass__
• py3_example1.py is the equivalent of py2/example1.py but with Python 3 metaclass declarations
• py3_example2.py consists of my test cases from python/mypy#2413
• six.py is testing metclasses defined like in py2/example1.py, but declared using six.with_metaclass

Running tests

If you're not familliar with Github Gist, you can use the URL of this gist to clone it as a git repository.

./setup.sh blah will set up a venv virtualenv at path blah (or .venv if no path is passed) and install mypy at the metaclass support commit. You will have to source into the virtualenv after that. (source .venv/bin/activate)

./run_mypy.sh will typecheck the example files using the current Python environment (make sure you're in a venv with mypy and Python 3).

Conclusions

It's pretty clear that metaclass detection is still not happening in mypy for Python 2 codebases, but if a metaclass is detected (ie in Python 3) then everything seems to work as expected.

Errors to do with the six.py test case are due to the fact that six.with_metaclass needs to be special-cased, and it hasn't been special-cased yet.

All in all, this is still fantastic work, and I'm very grateful for it. Still, it means that I'll have to look into updating the metaclass detection logic to support Python 2.

Update

Since the pull request, several issues on the mypy repository have been updated and/or opened, which will fix all the above issues I found (among others):

• mypy doesn't understand six.with_metaclass (#1764)
• Mypy doesn't recognize metaclass implementing getitem (#1771)
• Add Python 2 support for metaclass (#2823)
• Metaclass is not properly inherited when using multiple inheritance with int (#2824)
• Update the behaviour of the type function for metaclasses (#2826)

py2_example1.py

class MetaA(type):
    arg1 = 1 # type: float

    @property
    def arg2(cls):
        # type: () -> int
        return 2

    name = "metaclass A" # type: str

    def return_name_class(cls):
        return cls.name


class A(object):
    __metaclass__ = MetaA

    name = "class A"

    def __init__(self, name):
        self.name = name

    def return_name_instance(self):
        return self.name

assert A.return_name_class() == A.name

instance1 = A("instance1")
assert instance1.return_name_instance() == "instance1"

instance2 = A("instance2")
assert type(instance2).return_name_class() == A.name

reveal_type = lambda *x: None
reveal_type(instance_A)
reveal_type(type(instance_A))
reveal_type(A)
reveal_type(A.arg1)
reveal_type(A.arg2)

py2_example2.py

class MetaA(type):
    @property
    def prop(cls):
        return "Hi!"

class ConcreteA(object):
    __metaclass__ = MetaA
    pass

print(ConcreteA.prop)

class MetaB(type):
    attr = "attr"

    def fn(cls):
        return "fn"

class ConcreteB(object):
    __metaclass__ = MetaB
    pass

print(ConcreteB.fn())
print(ConcreteB.attr)

py2_example3.py

class A(object):
    class __metaclass__(type):
        arg1 = 1 # type: float

        @property
        def arg2(cls):
            # type: () -> int
            return 2

        name = "metaclass A" # type: str

        def return_name_class(cls):
            return cls.name

    name = "class A"

    def __init__(self, name):
        self.name = name

    def return_name_instance(self):
        return self.name

instance_A = A("instance A") # type; A


assert A.return_name_class() == A.name

instance1 = A("instance1")
assert instance1.return_name_instance() == "instance1"

instance2 = A("instance2")
assert type(instance2).return_name_class() == A.name

reveal_type = lambda *x: None
reveal_type(instance_A)
reveal_type(type(instance_A))
reveal_type(A)
reveal_type(A.arg1)
reveal_type(A.arg2)

py3_example1.py

class MetaA(type):
    arg1 = 1 # type: float

    @property
    def arg2(cls):
        # type: () -> int
        return 2

    name = "metaclass A" # type: str

    def return_name_class(cls):
        return cls.name


class A(object, metaclass=MetaA):
    name = "class A"

    def __init__(self, name):
        self.name = name

    def return_name_instance(self):
        return self.name

assert A.return_name_class() == A.name

instance1 = A("instance1")
assert instance1.return_name_instance() == "instance1"

instance2 = A("instance2")
assert type(instance2).return_name_class() == A.name

reveal_type = lambda *x: None
reveal_type(instance_A)
reveal_type(type(instance_A))
reveal_type(A)
reveal_type(A.arg1)
reveal_type(A.arg2)

py3_example2.py

class MetaA(type):
    @property
    def prop(cls):
        return "Hi!"

class ConcreteA(object, metaclass=MetaA):
    pass

print(ConcreteA.prop)

class MetaB(type):
    attr = "attr"

    def fn(cls):
        return "fn"

class ConcreteB(object, metaclass=MetaB):
    pass

print(ConcreteB.fn())
print(ConcreteB.attr)

requirements.txt

-e git://github.com/python/mypy.git@cab1e0d1a676f10aa947f35dd07d8fe77532b010#egg=mypy

run_mypy.sh

echo "Python 2 tests"
mypy --show-traceback --fast-parser -2 six.py
mypy --show-traceback --fast-parser -2 py2_example1.py
mypy --show-traceback --fast-parser -2 py2_example2.py
mypy --show-traceback --fast-parser -2 py2_example3.py
echo ""
echo "Python 3 tests"
mypy --show-traceback --fast-parser six.py
mypy --show-traceback --fast-parser py3_example1.py
mypy --show-traceback --fast-parser py3_example2.py

setup.sh

#!/usr/bin/env bash
VENV_NAME="${1:-.venv}"
echo "Setting up virtualenv as $VENV_NAME"
virtualenv $VENV_NAME --python=python3
source .venv_typecheck/bin/activate
pip install -r requirements.txt

six.py

from six import with_metaclass

class MetaA(type):
    arg1 = 1 # type: float

    @property
    def arg2(cls):
        # type: () -> int
        return 2

    name = "metaclass A" # type: str

    def return_name_class(cls):
        return cls.name


class A(with_metaclass(MetaA, object)):
    name = "class A"

    def __init__(self, name):
        self.name = name

    def return_name_instance(self):
        return self.name

instance_A = A("instance A") # type; A


assert A.return_name_class() == A.name

instance1 = A("instance1")
assert instance1.return_name_instance() == "instance1"

instance2 = A("instance2")
assert type(instance2).return_name_class() == A.name

reveal_type = lambda *x: None
reveal_type(instance_A)
reveal_type(type(instance_A))
reveal_type(A)
reveal_type(A.arg1)
reveal_type(A.arg2)