[Python-Dev] Optional delta argument for assertAlmostEqual (original) (raw)

Michael Foord fuzzyman at voidspace.org.uk
Sat Mar 27 01:59:21 CET 2010

Hello all,

A user has suggested an optional argument to unittest.TestCase.assertAlmostEqual for specifying a maximum difference between the expected and actual values, instead of using rounding.

This sounds great to me as the default implementation of assertAlmostEqual has never been useful to me (YMMV). In fact one of the first things I do on setting up a test suite is provide a TestCase that overrides assertAlmostEqual with an implementation that uses a delta rather than rounding.

The implementation would be effectively:

assert abs(actual - expected) < delta

This has the advantage that it allows things like:

self.assertAlmostEqual(timeStamp, expected, delta=datetime.timedelta(seconds=5))

The issue is this would make the signature of assertAlmostEqual (and its negative counterpart):

def assertAlmostEqual(self, first, second, places=7, msg=None, delta=None)

Note that delta comes after msg, which is different to other assert methods. To put delta before msg would be backwards incompatible with existing uses passing arguments positionally. In Python 3.2 we can make delta a keyword argument. Passing both places and delta would be an error (TypeError).

Anyway, unless there are strenuous objections I intend to do this.

All the best,

Michael

-- http://www.ironpythoninaction.com/ http://www.voidspace.org.uk/blog

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