Faking whether an object is an Instance of a Class in Python

Question:

Suppose I have a class FakePerson which imitates all the attributes and functionality of a base class RealPerson without extending it. In Python 3, is it possible to fake isinstance() in order to recognise FakePerson as a RealPerson object by only modifying the FakePerson class. For example:

class RealPerson():
    def __init__(self, age):
        self.age = age

    def are_you_real(self):
        return 'Yes, I can confirm I am a real person'

    def do_something(self):
        return 'I did something'

    # Complicated functionality here

class FakePerson(): # Purposely don't extend RealPerson
    def __init__(self, hostage):
        self.hostage = hostage

    def __getattr__(self, name):
        return getattr(self.hostage, name)

    def do_something(self):
        return 'Ill pretend I did something'

    # I don't need complicated functionality since I am only pretending to be a real person.


a = FakePerson(RealPerson(30))
print(isinstance(a, RealPerson))

The context of this is suppose I have a class that imitates most / all of the functionality of a Pandas DataFrame row (a namedtuple object). If I have a list of rows list_of_rows, Pandas generates a DataFrame object by pandas.DataFrame(list_of_rows). However, since each element in list_of_rows is not a namedtuple and just a ‘fake’, the constructor can’t recognise these ‘fake’ row objects as real rows even if the fake object does fake all the underlying methods and attributes of the Pandas namedtuple.

Asked By: Bentley Carr

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Answers:

The isInstance() function is a python builtin who’s implementation explicitly looks for an object’s (direct, indirect or virtual) class or subclass. The ‘imitations’ you’re referring to is also known as duck typing. In your case, it looks like you do want to extend or subclass the DataFrame row. Though, you might get away with assigning the class attribute, but know that this may lead to undefined behavior as it is implementation-specific.

Answered By: TLane

You may need to subclass your RealPerson class.

class RealPerson:
    def __init__(self, age):
        self.age = age

    def are_you_real(self):
        return 'Yes, I can confirm I am a real person'

    def do_something(self):
        return 'I did something'

    # Complicated functionality here

class FakePerson: # Purposely don't extend RealPerson
    def __init__(self, hostage):
        self.hostage = hostage

    def __getattr__(self, name):
        return getattr(self.hostage, name)

    def do_something(self):
        return 'Ill pretend I did something'

    # I don't need complicated functionality since I am only pretending to be a real person.


class BetterFakePerson(RealPerson):
    pass

BetterFakePerson.__init__ = FakePerson.__init__
BetterFakePerson.__getattr__ = FakePerson.__getattr__
BetterFakePerson.do_something = FakePerson.do_something

a = FakePerson(RealPerson(30))
print(isinstance(a, RealPerson))

b = BetterFakePerson(RealPerson(30))
print(isinstance(b, RealPerson))

Hope this answer would not be too late for you LOL

Answered By: Autumnii

Define __class__

It seems that unittest.mock does this, judging by the following paragraph in the docs.

If spec is an object (rather than a list of strings) then __class__ returns the class of the spec object. This allows mocks to pass isinstance() tests.

There is bit more on that a few paragraphs onwards on the same page.

# Works, but needs ignore comment for pyre-check, if you use that

class FakePerson:

    __class__ = RealPerson  # pyre-ignore[15] Inconsistent override

    # ...

Define __instancecheck__? Nope!

The __instancecheck__ and __subclasscheck__ special methods are documented in the 3.3.4. Customizing instance and subclass checks section of the Datamodel chapter of the Python reference.

Reading that docs, one would think that the following is also a solution, however, it does not work. See https://stackoverflow.com/a/17736152/1047788 for more on that. One would have to implement this __instancecheck__ in RealPerson for it to work, which is not what we want.

# Valiant attempt, but does not work

def FakePerson:

    def __instancecheck__(self, instance):
        return issubclass(instance, RealPerson)

    # ...
Answered By: user7610