CODEWITHSUNDEEP
pythonpython-3.xunit-testing
Haytam
Haytam

Reputation: 4733

Weird behavior of assertRaises (TestCase)

I've been trying/learning about unit testing works in Python and I came across this weird behavior, am I doing something wrong?

Class

class Entity:

    def __init__(self, n):
        if not isinstance(n, int):
            raise TypeError('n must be a number.')
        self.n = n

Test

from src.entity import Entity

class TestEntity(TestCase):

    def test__init__types(self):
        self.assertRaises(TypeError, Entity.__init__, "2")
        self.assertRaises(TypeError, Entity.__init__, 2)

Shouldn't the test fail in the second assertRaises since 2 is a number thus the TypeError isn't raised? instead it says OK.

Upvotes: 0

Views: 308

Answers (1)

ForceBru
ForceBru

Reputation: 44858

Entity.__init__(self, n) takes two arguments: self and n. You're supplying only one, and getting a TypeError.

Observe:

>>> class A:
...     def __init__(self, n):
...         print(n)
...         
>>> A.__init__('s')
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: __init__() missing 1 required positional argument: 'n'

To test the constructor, you could do:

with self.assertRaises(TypeError):
    Entity("2")

Now, why does this happen and where could one get that self argument from? The thing is that __init__ is supplied with this argument automatically be A.__new__, so you could call A.__new__ yourself, which would result in this weird looking code:

self.assertRaises(TypeError, A.__new__(A).__init__, "2")

Upvotes: 1

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