[Python-Dev] Unittest PEP do's and don'ts (BDFL pronouncement) (original) (raw)

Robert Kern robert.kern at gmail.com
Thu Jul 17 01:45:45 CEST 2008

Guido van Rossum wrote:

On Wed, Jul 16, 2008 at 2:03 PM, Raymond Hettinger <python at rcn.com> wrote:

From: "Michael Foord" <fuzzyman at voidspace.org.uk>

assertIn / assertNotIn I use very regularly for collection membership - self.assert(func(x) in resultset) + self.assertIn(func(x), resultset)

Yawn. The gain is zero. Actually, it's negative because the second doesn't read as nicely as the pure python expression. I disagree. The reason why we have assertEquals(x, y) is that the error message can show the values of x and y, whereas assert x == y can't show those. Showing the values can be tremendously useful to debugging the failure. (Doing an intelligent comparison, e.g. a string or list "diff" instead of showing the two values, can be even more useful, and I'd be in favor of that rather than adding new methods like assertListsEqual.) (Titus asks if the assert statement could be adjusted to do better reporting. But that's not going to happen -- it would require a tremendous amount of compiler support that would have to be implemented in every Python implementation (last I counted there were at least five). In addition, python -O removes all asserts from your code -- that's why we use assertXxx functions in the first place.)

The assert statement itself does not have to be modified. nose and py.test are already capable of picking out the variables used in a failing assert expression and print them out. For example:

[~]$ cat test_nicefail.py def test_nicefail(): x = 12 assert x == 10

[~]$ nosetests --detailed-errors test_nicefail.py F

FAIL: test_nicefail.test_nicefail

Traceback (most recent call last): File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/nose-0.10.3-py2.5.egg/nose/case.py", line 182, in runTest self.test(*self.arg) File "/Users/rkern/test_nicefail.py", line 3, in test_nicefail assert x == 10 AssertionError: 12 = 12

assert 12 == 10

Ran 1 test in 0.044s

FAILED (failures=1)

[~]$ py.test test_nicefail.py ====================== test process starts ======================= executable: /Library/Frameworks/Python.framework/Versions/2.5/Resources/Python.app/Contents/MacOS/Python (2.5.1-final-0) using py lib: /Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/py

test_nicefail.py[1] F

___________________ entrypoint: test_nicefail ____________________

 def test_nicefail():
     x = 12

E assert x == 10

  assert 12 == 10

[/Users/rkern/test_nicefail.py:3]

============ tests finished: 1 failed in 0.09 seconds ============

-- Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco

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