How to get more helpful error messages when comparing two instances?

Question

I'm comparing two class instances with each other. Below is my sample test code:

from unittest import TestCase

class Dog:
    age: int
    name: str

    def __eq__(self, other):
        if not isinstance(other, Dog):
            return False

        return self.age == other.age and self.name == other.name


class MyTests(TestCase):
    def test_compare(self):    
    d1 = Dog()
    d1.age = 1
    d1.name = 'dog1'

    d2 = Dog()
    d2.age = 2
    d2.name = 'dog2'

    self.assertEqual(d1, d2)

This generates an assertion error:

AssertionError: <test.Dog object at 0x0000020444FCA520> != <test.Dog object at 0x0000020444F97D60>

Is there a way to compare two instances directly and get a more helpful error message, such as the field that caused the assertion to fail? The only way I've found is to compare fields directly like below. Is there a less tedious way?

self.assertEqual(d1.age, d2.age)
self.assertEqual(d1.name, d2.name)

@manveti No, I don't think this is what I'm looking for, but thank you for sharing regardless! — Floating Sunfish, Nov 19 '20 at 02:10
I think you'll find that you'll get better messages if you implement str and repr, maybe using dataclasses. — Brian McCutchon, Nov 19 '20 at 02:15
@BrianMcCutchon After seeing the code in action in Jack Taylor's answer below, I think it is related to what I'm looking for after all! — Floating Sunfish, Nov 19 '20 at 02:18
@manveti Upon seeing Jack Taylor's answer below it does answer my question after all! Thank you for the link! — Floating Sunfish, Nov 19 '20 at 02:18
Also, using [pytest](https://docs.pytest.org/en/latest/) will generally give you more helpful error messages than the built-in unittest module. — Jack Taylor, Nov 19 '20 at 02:56

Jack Taylor · Accepted Answer · 2020-11-19T02:18:09.940

You can define a __repr__ method in the class. This is used to create string representations of objects for debugging purposes.

class Dog:
    age: int
    name: str

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

    def __eq__(self, other):
        if not isinstance(other, Dog):
            return False

        return self.age == other.age and self.name == other.name

    def __repr__(self):
        return f"{self.__class__.__name__}({repr(self.age)}, {repr(self.name)})"

print(Dog(5, "Fido"))  # Prints "Dog(5, 'Fido')"

Floating Sunfish · Answer 2 · 2020-11-19T02:30:06.937

Another way I just found is to create a helper function to compare the two instances.

There's no longer a need to implement __eq__.

It's less tedious because I only have to write the function once:

def test_compare(self):        
    d1 = Dog()
    d1.age = 1
    d1.name = 'dog1'

    d2 = Dog()
    d2.age = 2
    d2.name = 'dog2'

    self.__assertDogsAreEqual(d1, d2)

def __assertDogsAreEqual(self, dog1: Dog, dog2: Dog):
    self.assertEqual(dog1.age, dog2.age)
    self.assertEqual(dog1.name, dog2.name)

It now gives a more helpful error message:

Traceback (most recent call last):
  File "test.py", line 119, in test_temp
    self.__assertDogsAreEqual(d1, d2)
  File "test.py", line 122, in __assertDogsAreEqual
    self.assertEqual(dog1.age, dog2.age)
AssertionError: 1 != 2

I might accept this as the answer, but I'm still curious if there's a better way.

This does look like the most practical approach for now, though.

How to get more helpful error messages when comparing two instances?

2 Answers2