Python: Fail in three characters with 1/0
Here’s a code snippet I often type:
1/0
When this runs, Python raises ZeroDivisionError:
$ python -c '1/0'
Traceback (most recent call last):
File "<string>", line 1, in <module>
1/0
~^~
ZeroDivisionError: division by zero
I add such a “forced failure” to check if and when a line of code runs. It’s a quick-and-dirty way of answering questions like “is this function even called?” or “which tests run this block?”.
For example, I recently added 1/0 to a function that didn’t seem used and ran the test suite:
$ pytest
...
====================== short test summary info =======================
ERROR example/tests/test_widgets.py::RedWidgetTests::test_paint - ZeroDivisionError: division by zero
ERROR example/tests/test_views.py::FactoryViewTests::test_get - ZeroDivisionError: division by zero
ERROR example/tests/test_canteen.py::FoodTests::test_yumminess - ZeroDivisionError: division by zero
========================= 3 errors in 1.72s ==========================
The three errors’ stack traces showed me different code paths to the target function. I used this information when refactoring.
assert 0
If you want to be more “strict” about it, the shortest failing assert is:
assert 0
This is because 0 is falsey, so the assert will always fail. But I can’t be bothered to type this out during quick investigations.
KeyboardInterrupt for more power
The 1/0 technique doesn’t work if there is error handling that suppresses the exception, such as except Exception, or even the anti-pattern of a bare except. A more powerful but longer-to-type alternative is to raise KeyboardInterrupt:
raise KeyboardInterrupt()
This is the exception class that Python raises when interrupted with Control-C. It is very rarely suppressed.
Fin
I didn’t invent this technique, but I also can’t remember or find where I picked it up. Whoever told me about it, thank you!
To infinity and beyond,
—Adam
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Tags: python