unittest.mock does not understand dataclasses

[parejkoj]

BPO	36580
Nosy	@ericvsmith, @cjw296, @voidspace, @mariocj89, @tirkarthi, @parejkoj

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-09-09.12:58:05.544>
created_at = <Date 2019-04-09.21:08:33.793>
labels = ['type-bug', '3.8', 'library', '3.7', 'tests', 'invalid']
title = 'unittest.mock does not understand dataclasses'
updated_at = <Date 2019-09-09.12:58:05.543>
user = 'https://github.com/parejkoj'

bugs.python.org fields:

activity = <Date 2019-09-09.12:58:05.543>
actor = 'xtreak'
assignee = 'none'
closed = True
closed_date = <Date 2019-09-09.12:58:05.544>
closer = 'xtreak'
components = ['Library (Lib)', 'Tests']
creation = <Date 2019-04-09.21:08:33.793>
creator = 'John Parejko2'
dependencies = []
files = []
hgrepos = []
issue_num = 36580
keywords = []
message_count = 6.0
messages = ['339808', '339811', '339812', '339816', '339930', '351447']
nosy_count = 6.0
nosy_names = ['eric.smith', 'cjw296', 'michael.foord', 'mariocj89', 'xtreak', 'John Parejko2']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36580'
versions = ['Python 3.7', 'Python 3.8']

[parejkoj]

The new dataclasses.dataclass is very useful for describing the properties of a class, but it appears that Mocks of such decorated classes do not catch the members that are defined in the dataclass. I believe the root cause of this is the fact that unittest.mock.Mock generates the attributes of its spec object via dir, and the non-defaulted dataclass attributes do not appear in dir.

Given the utility in building classes with dataclass, it would be very useful if Mocks could see the class attributes of the dataclass.

Example code:

import dataclasses
import unittest.mock

@dataclasses.dataclass
class Foo:
    name: str
    baz: float
    bar: int = 12

FooMock = unittest.mock.Mock(Foo)
fooMock = FooMock()  # should fail: Foo.__init__ takes two arguments
# I would expect these to be True, but they are False
'name' in dir(fooMock)
'baz' in dir(fooMock)
'bar' in dir(fooMock)

[ericvsmith]

I'm not sure why dataclasses would be different here:

>>> import dataclasses
>>> import unittest.mock
>>> @dataclasses.dataclass
... class Foo:
...     name: str
...     baz: float
...     bar: int = 12
...
>>> import inspect
>>> inspect.signature(Foo)
<Signature (name: str, baz: float, bar: int = 12) -> None>
>>>

Foo is just a normal class with a normal __init__.

This is no different than if you don't use dataclasses:

>>> class Bar:
...     def __init__(self, name: str, baz: float, bar: int = 12) -> None:
...         pass
...
>>> Bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 2 required positional arguments: 'name' and 'baz'
>>> inspect.signature(Bar)
<Signature (name: str, baz: float, bar: int = 12) -> None>
>>> BarMock = unittest.mock.Mock(Bar)
>>> barMock = BarMock()

[tirkarthi]

mock.Mock doesn't do signature validation by default for constructor and methods. This is expected. create_autospec [0] should be used to make sure the signature is validated.'

import dataclasses
import unittest.mock

@dataclasses.dataclass
class Foo:
    name: str
    baz: float
    bar: int = 12

FooMock = unittest.mock.create_autospec(Foo)
fooMock = FooMock()  # Will fail now since it's specced

➜  cpython git:(master) ./python.exe ../backups/bpo36580.py
Traceback (most recent call last):
  File "../backups/bpo36580.py", line 11, in <module>
    fooMock = FooMock()  # should fail: Foo.__init__ takes two arguments
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", line 984, in __call__
    _mock_self._mock_check_sig(*args, **kwargs)
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", line 103, in checksig
    sig.bind(*args, **kwargs)
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/inspect.py", line 3021, in bind
    return args[0]._bind(args[1:], kwargs)
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/inspect.py", line 2936, in _bind
    raise TypeError(msg) from None
TypeError: missing a required argument: 'name'

On the other hand 'name' in dir(FooMock) doesn't have the attributes (name and baz) present I suppose they are constructed dynamically when an object is created from Foo since they are present in dir(Foo()) and mock is not able to detect them? mock.create_autospec does an initial pass of dir(Foo) to copy the attributes [1] and perhaps it's not able to copy name and bar while baz is copied. Below are for FooMock = create_autospec(Foo) . So 'name' in dir(Foo) is False for dataclasses. Is this a known behavior?

dir(Foo)

['__annotations__', '__class__', '__dataclass_fields__', '__dataclass_params__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'bar']

dir(Foo(1, 2))

['__annotations__', '__class__', '__dataclass_fields__', '__dataclass_params__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'bar', 'baz', 'name']

dir(create_autospec(Foo))

['__annotations__', '__class__', '__dataclass_fields__', '__dataclass_params__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'assert_any_call', 'assert_called', 'assert_called_once', 'assert_called_once_with', 'assert_called_with', 'assert_has_calls', 'assert_not_called', 'attach_mock', 'bar', 'call_args', 'call_args_list', 'call_count', 'called', 'configure_mock', 'method_calls', 'mock_add_spec', 'mock_calls', 'reset_mock', 'return_value', 'side_effect']

print('name' in dir(fooMock)) # False
print('baz' in dir(fooMock)) # False
print('bar' in dir(fooMock)) # True

[0] https://docs.python.org/3/library/unittest.mock.html#unittest.mock.create_autospec
[1]

cpython/Lib/unittest/mock.py

Line 2263 in 0e10766

for entry in dir(spec):

[tirkarthi]

To add to this mock.Mock also copies dir(spec) but creating an instance from mock doesn't copy it where it's not a problem with create_autospec. Mock with spec does only attribute validation whereas create_autospec does signature validation. There is another open issue to make mock use spec passed as if it's autospecced bpo-30587 where this could be used as a data point to change API. I am adding mock devs for confirmation.

[tirkarthi]

Below is a even more simpler reproducer without dataclasses. 'name' is not listed as a class attribute in dir(Person) since it's not defined with a value but 'age' is with zero. Python seems to not to list something not defined with a value in declaration as a class attribute in dir(). Hence 'name' is not copied when Person is used as spec. spec only does attribute access validation. autospeccing [0] can be used for signature validation. The fields for dataclasses are defined in __dataclass_fields__ but I am not sure of special casing copying __dataclass_fields__ fields along with dir for dataclasses when normal Python doesn't list them as class attributes. If needed I would like dir(dataclass) to be changed to include __dataclass_fields__. I would propose closing as not a bug.

# ../backups/dataclass_dir.py

from unittest.mock import Mock

class Person:
    name: str
    age: int = 0

    def foo(self):
        pass

person_mock = Mock(spec=Person)
print(dir(Person))
print(dir(person_mock))
person_mock.foo
print("age" in dir(person_mock))
print("name" in dir(person_mock))

$ cpython git:(master) ./python.exe ../backups/dataclass_dir.py
['__annotations__', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'age', 'foo']

['__annotations__', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'age', 'assert_any_call', 'assert_called', 'assert_called_once', 'assert_called_once_with', 'assert_called_with', 'assert_has_calls', 'assert_not_called', 'attach_mock', 'call_args', 'call_args_list', 'call_count', 'called', 'configure_mock', 'foo', 'method_calls', 'mock_add_spec', 'mock_calls', 'reset_mock', 'return_value', 'side_effect']
True
False

[0] https://docs.python.org/3/library/unittest.mock.html#autospeccing

[tirkarthi]

Closing this as not a bug since autospeccing with create_autospec can be used and spec only does attribute access validation.

Thanks