Add Define Sequence Of Tests With Parametrize Decorator as a Python TIL

python/define-sequence-of-tests-with-parametrize-decorator.md

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+# Define Sequence Of Tests With Parametrize Decorator
+
+I have a function that I want to test across a bunch of different inputs. That
+way I can make sure the logic of that function handles all the different
+scenarios I have in mind.
+
+While working on [`py-vmt`](https://github.com/jbranchaud/py-vmt), I started by
+writing a big single test function with a sequence of variable assignments and
+`assert` statements. Here's my starting point:
+
+```python
+def test_format_time_delta_everything():
+    # less than a minute
+    thirty_seconds = timedelta(seconds=30)
+    assert "30s" == format_time_delta(thirty_seconds)
+
+    # one minute exactly
+    one_minute = timedelta(seconds=60)
+    assert "1m" == format_time_delta(one_minute)
+
+    # more than a minute
+    assert "1m30s" == format_time_delta(one_minute + thirty_seconds)
+
+    # bunch of minutes and seconds
+    delta = timedelta(minutes=24, seconds=8)
+    assert "24m8s" == format_time_delta(delta)
+
+    # one hour exactly
+    one_hour = timedelta(hours=1)
+    assert "1h" == format_time_delta(one_hour)
+
+    # more than one hour
+    assert "1h24m" == format_time_delta(one_hour + delta)
+```
+
+I knew I would eventually need to break it up into individual test functions,
+but I couldn't bare to start there because it seemed quite repetitive.
+
+There is another way to approach this without all the duplication. Pytest comes
+with [a "parametrize" decorator](https://docs.pytest.org/en/stable/example/parametrize.html). This is
+used to define a set of test data (and expected values) that will get passed
+one-by-one to the test function as parameters.
+
+```python
+@pytest.mark.parametrize("input,expected", [
+    (timedelta(seconds=30), "30s"),
+    (timedelta(seconds=60), "1m"),
+    (timedelta(seconds=90), "1m30s"),
+    (timedelta(minutes=24, seconds=8), "24m8s"),
+    (timedelta(hours=1), "1h"),
+    (timedelta(hours=1, minutes=24, seconds=8), "1h24m"),
+])
+def test_format_time_delta(input, expected):
+    assert format_time_delta(input) == expected
+```
+
+I ditch all of the duplication this way. I define a list of tuples that
+represent my input values and expected values. Then the body of the test can be
+minimal. And I get a separate test execution for each parameter tuple making it
+easier to see fine-grained pass/fail results.