13
Aug

Using Python's namedtuple for mock objects in tests

I have become quite a fan of Python's built-in namedtuple collection lately. As others have already written, despite having been available in Python 2.x and 3.x for a long time now, namedtuple continues to be under-appreciated and under-utilised by many programmers.

# The ol'fashioned tuple way
fruits = [
    ('banana', 'medium', 'yellow'),
    ('watermelon', 'large', 'pink')]

for fruit in fruits:
    print('A {0} is coloured {1} and is {2} sized'.format(
        fruit[0], fruit[2], fruit[1]))

# The nicer namedtuple way
from collections import namedtuple

Fruit = namedtuple('Fruit', 'name size colour')

fruits = [
    Fruit(name='banana', size='medium', colour='yellow'),
    Fruit(name='watermelon', size='large', colour='pink')]

for fruit in fruits:
    print('A {0} is coloured {1} and is {2} sized'.format(
        fruit.name, fruit.colour, fruit.size))

namedtuples can be used in a few obvious situations in Python. I'd like to present a new and less obvious situation, that I haven't seen any examples of elsewhere: using a namedtuple instead of MagicMock or flexmock, for creating fake objects in unit tests.

namedtuple vs the competition

namedtuples have a number of advantages over regular tuples and dicts in Python. First and foremost, a namedtuple is (by defintion) more semantic than a tuple, because you can define and access elements by name rather than by index. A namedtuple is also more semantic than a dict, because its structure is strictly defined, so you can be guaranteed of exactly which elements are to be found in a given namedtuple instance. And, similarly, a namedtuple is often more useful than a custom class, because it gives more of a guarantee about its structure than a regular Python class does.

A namedtuple can craft an object similarly to the way that MagicMock or flexmock can. The namedtuple object is more limited, in terms of what attributes it can represent, and in terms of how it can be swapped in to work in a test environment. But it's also simpler, and that makes it easier to define and easier to debug.

Compared with all the alternatives listed here (dict, a custom class, MagicMock, and flexmock – all except tuple), namedtuple has the advantage of being immutable. This is generally not such an important feature, for the purposes of mocking and running tests, but nevertheless, immutability always provides advantages – such as elimination of side-effects via parameters, and more thread-safe code.

Really, for me, the biggest "quick win" that you get from using namedtuple over any of its alternatives, is the lovely built-in string representation that the former provides. Chuck any namedtuple in a debug statement or a logging call, and you'll see everything you need (all the fields and their values) and nothing you don't (other internal attributes), right there on the screen.

# Printing a tuple
f1 = ('banana', 'medium', 'yellow')

# Shows all attributes ordered nicely, but no field names
print(f1)
# ('banana', 'medium', 'yellow')


# Printing a dict
f1 = {'name': 'banana', 'size': 'medium', 'colour': 'yellow'}

# Shows all attributes with field names, but ordering is wrong
print(f1)
# {'colour': 'yellow', 'size': 'medium', 'name': 'banana'}


# Printing a custom class instance
class Fruit(object):
    """It's a fruit, yo"""

f1 = Fruit()
f1.name = 'banana'
f1.size = 'medium'
f1.colour = 'yellow'

# Shows nothing useful by default! (Needs a __repr__() method for that)
print(f1)
# <__main__.Fruit object at 0x7f1d55400e48>

# But, to be fair, can print its attributes as a dict quite easily
print(f1.__dict__)
# {'size': 'medium', 'name': 'banana', 'colour': 'yellow'}


# Printing a MagicMock
from mock import MagicMock

class Fruit(object):
    name = None
    size = None
    colour = None

f1 = MagicMock(spec=Fruit)
f1.name = 'banana'
f1.size = 'medium'
f1.colour = 'yellow'

# Shows nothing useful by default! (and f1.__dict__ is full of a tonne
# of internal cruft, with the fields we care about buried somewhere
# amongst it all)
print(f1)
# <MagicMock spec='Fruit' id='140682346494552'>


# Printing a flexmock
from flexmock import flexmock

f1 = flexmock(name='banana', size='medium', colour='yellow')

# Shows nothing useful by default!
print(f1)
# <flexmock.MockClass object at 0x7f691ecefda0>

# But, to be fair, printing f1.__dict__ shows minimal cruft
print(f1.__dict__)
# {
#     'name': 'banana',
#     '_object': <flexmock.MockClass object at 0x7f691ecefda0>,
#     'colour': 'yellow', 'size': 'medium'}


# Printing a namedtuple
from collections import namedtuple

Fruit = namedtuple('Fruit', 'name size colour')
f1 = Fruit(name='banana', size='medium', colour='yellow')

# Shows exactly what we need: what it is, and what all of its
# attributes' values are. Sweeeet.
print(f1)
# Fruit(name='banana', size='medium', colour='yellow')

As the above examples show, without any special configuration, namedtuple's string configuration Just Works™.

namedtuple and fake objects

Let's say you have a simple function that you need to test. The function gets passed in a superhero, which it expects is a SQLAlchemy model instance. It queries all the items of clothing that the superhero uses, and it returns a list of clothing names. The function might look something like this:

# myproject/superhero.py


def get_clothing_names_for_superhero(superhero):
    """List the clothing for the specified superhero"""
    clothing_names = []

    clothing_list = superhero.clothing_items.all()

    for clothing_item in clothing_list:
        clothing_names.append(clothing_item.name)

    return clothing_names

Since this function does all its database querying via the superhero object that's passed in as a parameter, there's no need to mock anything via funky mock.patch magic or similar. You can simply follow Python's preferred pattern of duck typing, and pass in something – anything – that looks like a superhero (and, unless he takes his cape off, nobody need be any the wiser).

You could write a test for that function, using namedtuple-based fake objects, like so:

# myproject/superhero_test.py


from collections import namedtuple

from myproject.superhero import get_clothing_names_for_superhero


FakeSuperhero = namedtuple('FakeSuperhero', 'clothing_items name')
FakeClothingItem = namedtuple('FakeClothingItem', 'name')
FakeModelQuery = namedtuple('FakeModelQuery', 'all first')


def get_fake_superhero_and_clothing():
    """Get a fake superhero and clothing for test purposes"""
    superhero = FakeSuperhero(
        name='Batman',
        clothing_items=FakeModelQuery(
            first=lambda: None,
            all=lambda: [
                FakeClothingItem(name='cape'),
                FakeClothingItem(name='mask'),
                FakeClothingItem(name='boots')]))

    return superhero


def test_get_clothing_for_superhero():
    """Test listing the clothing for a superhero"""
    superhero = get_fake_superhero_and_clothing()

    clothing_names = set(get_clothing_names_for_superhero(superhero))

    # Verify that list of clothing names is as expected
    assert clothing_names == {'cape', 'mask', 'boots'}

The same setup could be achieved using one of the alternatives to namedtuple. In particular, a FakeSuperhero custom class would have done the trick. Using MagicMock or flexmock would have been fine too, although they're really overkill in this situation. In my opinion, for a case like this, using namedtuple is really the simplest and the most painless way to test the logic of the code in question.

In summary

I believe that namedtuple is a great choice for fake test objects, when it fits the bill, and I don't know why it isn't used or recommended for this in general. It's a choice that has some limitations: most notably, you can't have any attribute that starts with an underscore (the "_" character) in a namedtuple. It's also not particularly nice (although it's perfectly valid) to chuck functions into namedtuple fields, especially lambda functions.

Personally, I have used namedtuples in this way quite a bit recently, however I'm still ambivalent about it being the best approach. If you find yourself starting to craft very complicated FakeFoo namedtuples, then perhaps that's a sign that you're doing it wrong. As with everything, I think that this is an approach that can really be of value, if it's used with a degree of moderation. At the least, I hope you consider adding it to your tool belt.

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Comments

14
Aug
2017
Test property

it's slow man )

14
Aug
2017
Henrick

What do you think about `attrs`?

https://attrs.readthedocs.i...

16
Aug
2017
Andrew Dalke

I want to use namedtuple more generally, as a way to generate simple constant classes. What I don't like, and what keeps me from using it, is that the instances also include indexing behavior. superhero[1] returns the name, even though I don't want my objects to support index lookup.

Yet I know downstream users will likely use that indexing API, even if it's not documented. In the future I might want to replace it with a standard class, but to avoid breaking compatibility that class will need to implement __getitem__ and __len__ as if it were a tuple.

For example, some might do things like "for (clothing_items, name) in superhero_list", which is shorter than using attribute lookup for the same.

And once people start using unpacking like this, I can't change the attributes. If I add "fake_identity" to the FakeSuperhero namedtuple attribute list then I'll break code which expects to unpack into only two elements.

So there's a tradeoff. Either I have to worry about backwards compatibility issues in the future, or I take a few minutes to write the class definition, including __slots__ and __repr__.

Of course, for testing this isn't a problem as the test suite is the only user of the API, and you control it entirely. I agree that this is a good example of appropriate use.

21
Aug
2017

I recently ran into the limitations of namedtuple. Immutability can come handy but can be a cause of headaches. Here's my blogpost about it: https://medium.com/@MTYS_FD...