[Python-Dev] Optional delta argument for assertAlmostEqual

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, 

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,



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