How to write unittest for variable assignment in python?

Question

This is in Python 2.7. I have a class called class A, and there are some attributes that I want to throw an exception when being set by the user:

myA = A()
myA.myattribute = 9   # this should throw an error

I want to write a unittest that ensures that this throws an error.

After creating a test class and inheriting unittest.TestCase, I tried to write a test like this:

myA = A()
self.assertRaises(AttributeError, eval('myA.myattribute = 9'))

But, this throws a syntax error. However, if I try eval('myA.myattribute = 9'), it throws the attribute error, as it should.

How do I write a unittest to test this correctly?

Thanks.

See my answer here for the proper unittest syntax.

Cees Timmerman
– Cees Timmerman

2025-03-22 16:02:45 +00:00
Commented Mar 22 at 16:02 — Cees Timmerman
– Cees Timmerman, Commented Mar 22 at 16:02

enderland · Accepted Answer · 2016-10-19 02:00:04Z

5

You can also use assertRaises as a context manager:

with self.assertRaises(AttributeError):
    myA.myattribute = 9

The documentation shows more examples for this if you are interested. The documentation for assertRaises has a lot more detail on this subject as well.

From that documentation:

If only the exception and possibly the msg arguments are given, return a context manager so that the code under test can be written inline rather than as a function:
with self.assertRaises(SomeException):
     do_something()

which is exactly what you are trying to do.

edited Oct 19, 2016 at 2:00

answered Oct 19, 2016 at 1:53

enderland

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Comments

chepner · Accepted Answer · 2016-10-19 11:26:27Z

2

self.assertRaises takes a callable (and optionally one or more arguments for that callable) as its argument; you are providing the value that results from calling the callable with its arguments. The correct test would be ~~self.assertRaises(AttributeError, eval, 'myA.myattribute = 9')~~

# Thanks to @mgilson for something that actually works while
# resembling the original attempt.
self.assertRaises(AttributeError, eval, 'myA.myattribute = 9', locals())

However, you should use assertRaises as a context manager, which allows you to write the much more natural

with self.assertRaises(AttributeError):
    myA.myattribute = 9

edited Oct 19, 2016 at 11:26

answered Oct 19, 2016 at 1:53

chepner

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2 Comments

mgilson Over a year ago

Would that first form work? eval will evaluate the expression in the current namespace -- which is the namespace in which eval is getting called which would not have myA defined. If you're going to use the functional form (which isn't advised), you probably should go with self.assertRaises(AttributeError, setattr, myA, 'myattribute', 9), or you'll have to pass the current function's locals through: self.assertRaises(AttributeError, eval, 'myA.myattribute = 9', locals())

chepner Over a year ago

Yeah, I didn't really give any thought to how you would correctly use eval. I should have just explained that assertRaises takes something that it can call, rather than the result of a expression, and left it at that.

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