Track simple changes in values from the start to end of a context, and raise an
AssertionError if the final value was not as expected.
assert_changes() two functions: one which returns a snapshot of
the monitored value, and another which represents the expected change.
You can also pass in a function which compares the old and new versions,
True if the change is acceptable and
False otherwise. This allows
you to use operators other than simple equality for your tests.
pip install assert_changes
>>> value = 123 >>> with assert_changes(lambda: value, new=lambda x: x + 1): ... value = 124 >>> value = 123 >>> with assert_changes(lambda: value, new=lambda x: x + 1): ... value = 122 Traceback (most recent call last): ... AssertionError: Value changed from 123 to 123 (expected: 124)
>>> import operator >>> value = 123 >>> with assert_changes(lambda: value, cmp=operator.lt): ... value += 4 >>> value = 123 >>> with assert_changes(lambda: value, cmp=operator.gt): ... value += 4 Traceback (most recent call last): ... AssertionError: operator.gt(123, 127) not True
You can even use an assertion as your comparison function:
>>> value = 123 >>> with assert_changes(lambda: value, cmp=assert_greater_than): ... value += 4 Traceback (most recent call last): ... AssertionError: 123 not greater than 127
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