MagicMock With Spec

Thanks to @immoralist I’ve learned a new Python testing trick. I didn’t know about the “spec” argument for MagicMock. m(
Let’s see an example:

from mock import MagicMock

class SomeModel(object):
    foo = "f**"
    bar = "b**"

m = MagicMock(spec=SomeModel)

Here we create a mock object which mimics the interface of 


  as we would expect, returning mock values for things we access.

<MagicMock name='' id='4506756880'>
<MagicMock name='' id='4506754192'>

Let’s see what happens if we call something else:

>>> m.baz
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../env/lib/python2.7/site-packages/", line 658, in __getattr__
    raise AttributeError("Mock object has no attribute %r" % name)
AttributeError: Mock object has no attribute 'baz'

It will fail loudly while a mock object without a spec would have returned a mock value as it did in the previous example.

But the magic doesn’t end there. You can still set additional attributes/methods “by hand” and have them not fail even if they aren’t part of the original spec.

>>> m.baz = "bazzzzz"
>>> m.baz

Learning new things makes me happy. 😀

Tripping Over Property Setters in Python

In Python there is a simple way to make methods behave like properties using the @property decorator. But this only covers the getter side of things. What if you want to have a setter function for this “property”? Well there is a way. 🙂
Consider the following example:

import json

class SomeModel(object):
  _foo = '{"foo":["bar", "baz"]}'

  def foo(self):
    return json.loads(self._foo)

  def foo_setter(self, new_value):
    self._foo = json.dumps(new_value)

m = SomeModel()

Now you can use the 


  method like a property.

{u'foo': [u'bar', u'baz']}

This is a simple way to have a property contain a JSON string but access it as a Python dict, doing (de-)serialization on the fly.
So what if you want to set the value using a dict?

>>> = ["foo", "bar"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

This is can easily trip up even seasoned Python developers. I’ve read code that did exactly this and I (as a novice) had to find out why the code failed. m(
The solution is quite simple … but “non-obvious” (as in: I wouldn’t have thought of that without consulting the docs) 🙁

  def foo(self, new_value)
    self._foo = json.dumps(new_value)

Notice the method name? The setter and the getter methods have to have the same name!