How to correctly use pytest mocker to patch a function?

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@joe-elmoufak nailed the basics! But sometimes, you need more flexibility than a fixed return value. That’s where side_effect comes in.

:small_blue_diamond: Patching with a side_effect to Change Function Behavior Dynamically:

Instead of returning a fixed value, you can provide a custom function to control behavior:

def test_sum2(mocker):
    def crazy_sum(a, b):
        return b + b  # A different behavior, summing only 'b'

    mocker.patch(__name__ + ".sum", side_effect=crazy_sum)
    assert sum(2, 3) == 6  # This will pass because the new behavior sums 'b + b'

Now, pytest mocker replaces sum() with crazy_sum(), changing the behavior dynamically. Super useful for simulating complex logic!