Question

So I have a to string method in one of my classes:

def __str__(self):
    # some irrelevant code
    raise KeyError("aaa")

Then in my tests I do:

with self.assertRaises(KeyError) as cm:
    str(myobject)
self.assertEquals("%s" % cm.exception, "aaa")

I get this output:

self.assertEquals("%s" % cm.exception, "aaa")
AssertionError: "'aaa'" != 'aaa'

Any ideas what could be causing this?

Answer 1

When formatting a KeyError as a string, the representation of the offending key is included:

>>> d = {}
>>> d[2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 2
>>> d["a"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'a'

The representation of a string includes the surrounding single quotes, so you should use

self.assertEquals("%s" % cm.exception, "'aaa'")

or the more straight-foward

self.assertEquals(str(cm.exception), "'aaa'")