Pretty print why two objects are not equal

Question

When using pytest, I get nice pretty printing when two objects are not equivalent:

Expected :Foo(id='red', other_thing='green')
Actual   :Foo(id='red', other_thing='blue')
<Click to see difference>

def test_baz():
    
        oneFoo = Foo(id="red", other_thing="blue")
        twoFoo = Foo(id="red", other_thing="green")
    
>       assert oneFoo == twoFoo
E       AssertionError: assert Foo(id='red', other_thing='blue') == Foo(id='red', other_thing='green')
E         
E         Full diff:
E         - Foo(id='red', other_thing='green')
E         ?                            ^^ --
E         + Foo(id='red', other_thing='blue')
E         ?                            ^^^

baz.py:22: AssertionError

If I use an assert directly in my code, I just get an AssertionError and a stacktrace.

I am writing some integration tests right now that are NOT driven by pytest but would like to pretty print when two items (specifically Pydantic dataclasses) are not equal.

Answer 1

This works pretty well:

from difflib import Differ

import structlog
from colorama import Fore
from pydantic import BaseModel

logger = structlog.getLogger(__name__)

def check_equals(expected: BaseModel, actual: BaseModel) -> None:
    if not expected == actual:

        logger.error("Objects are not equal", expected=expected, actual=actual)

        diff_output = Differ().compare(
            _get_lines(expected),
            _get_lines(actual)
        )

        color_diff_output = _color_diff(diff_output)

        print("\n".join(color_diff_output))

        raise ValueError("Objects not equal")

def _get_lines(item: BaseModel) -> str:
    item_json = item.model_dump_json(indent=4)
    lines = item_json.splitlines()
    return lines

def _color_diff(diff):
    """
    Found this nice function here: https://chezsoi.org/lucas/blog/colored-diff-output-with-python.html
    """
    for line in diff:
        if line.startswith('+'):
            yield Fore.GREEN + line + Fore.RESET
        elif line.startswith('-'):
            yield Fore.RED + line + Fore.RESET
        elif line.startswith('^'):
            yield Fore.BLUE + line + Fore.RESET
        else:
            yield line

Prints:

When you run this code:

from pydantic import BaseModel

from util.integration_test_validation.check_equals import check_equals

class Bar(BaseModel):
    beaches: int
    oranges: int

class Foo(BaseModel):
    id: str
    other_thing: str
    count: int
    bar: Bar



oneFoo = Foo(id="red", other_thing="blue", count=3, bar=Bar(beaches=5, oranges=7))
twoFoo = Foo(id="red", other_thing="green", count=3, bar=Bar(beaches=5, oranges=7))

check_equals(oneFoo, twoFoo)