Intro to Mocking - DjangoCon 2015

Intro To Mocking:

Why Unit Testing Doesn’t Have to Be So Hard!

Daniel Davis

@Ooblioob

• Software Developer for 8 years

• Senior Consultant: Excella Consulting in

Washington, DC

• Fun Fact:

– I ran the “Jingle All The Way” 5k dressed as a

Giant Gingerbread man

A Little About Me…

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Survey Time!

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• Struggled with unit testing until I learned

about Mocking

Learning To Test

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Me Learning about Mocking…

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Image credit: http://stylecaster.com/excited-gif/

Many of us are in the same boat…

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• “I wish someone would just give a talk on

this…”

• So if that’s you…

We all want to write better tests…

Let’s Talk About Unit Tests…

Unit – 70%

Integration

– 20%

UI

10%

Testing Types

• “They’re good when the problem is easy”

– A rabbit hole of testing

• “I spend too much time writing lots of code

to test, so I give up”

• “There’s just some stuff you can’t unit test”

Great Unit Testing Myths

Mocking makes unit testing easier!

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What Are Mocks?

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Test Double

Dummy

ObjectTest Stub Test Spy

Mock

Object

Fake

Object

Commonly referred to as “Mocks”

• Stubs

– Provide a canned response to method calls

• Spy

– Real objects that behave like normal except

when a specific condition is met

• Mocks

– Verifies behavior (calls) to a method

What Are Mocks?

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Blah Blah Blah professor Dan…

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• Eliminates dependencies

– Isolated Unit Tests

Problems Mocks Solve

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foo(x) bar(x)

• Tests methods that have no return value

Problems Mocks Solve

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How do we know that bar(x) has been called?

• Tests error handling

Problems Mocks Solve

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• Eliminate dependency on database calls

– Speed up testing!

• Reduce test complexity

– Don’t have to write complex logic to handle

behavior of methods not under test

• Don’t have to wait to implement other

methods

Other Reasons Mocks Are Important

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Ok, I’m sold…Show me how to actually do this…

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• Mock (MagicMock)– Most robust, popular

– Built-in as of Python 3.3!

• flexmock– Based on Ruby’s flexmock

• mox– Similar to Java’s EasyMock

• Mocker

• dingus– “record then assert” mocking library

• fudge– Similar to Mockito

• MiniMock– Simple mocking with DocTest

What Are The Python Options?

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Sample Problem

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Problem: Tinder Competitor

True love is an isolated

container…Image Credits:

- http://www.forbes.com/sites/jjcolao/2013/04/08/tinder-for-business-dating-app-looks-to-conquer-other-matchmaking-verticals/

- https://blog.docker.com

• Create a method to return a new, random

victim candidate

– Must not show the same person

– Must not show someone the user has already

“swiped” on

Problem: “Docker” dating app

Easy enough…

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“Surely no one could

have seen EVERYONE

in the database!!!”

- The Internget_next_person() get_random_person()

Write a Unit Test…

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It works!!!

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Noooooooooo!!!!

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Easy enough…

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What if knew the result of get_random_person()???

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PatchingModule.attribute

Mock method

It works EVERY SINGLE TIME!!!

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Variations on a theme

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Variations on a theme

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Variations on a theme

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Variations on a theme

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OMG, ContextManagers! WHAT???

But what if we call it multiple times???

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• What if I want to test the while loop?

Different results on multiple calls

Uh…umm…

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UMM…

Use side_effect

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• Use patching / mocks to bring certainty to

method calls

• Eliminates dependencies on other code

– Even unfinished code!!!

• Lots of ways to do it, pick your favorite

Recap: What did we learn?

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Mocking to Verify Behavior

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Problem: Matching in “Docker”

• When a user swipes right…

• If the other user “likes” them:

– Send them both a message with contact info

• If the other user “dislikes” them:

– Let the user down gently…

• If the other user hasn’t evaluated yet:

– Display the “give it time” message

Problem: “Docker” matches

Implementation

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How do we test this??? No return values!!!

Behavior Verification

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What about checking parameters???

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Verifying Parameters

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Shouldn’t we check the other

methods too?

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We’d need to have multiple mocks to do that!!!

Multiple Mocks

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Patch Multiple

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Testing Multiple Calls…

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Multiple calls

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Whew!!!Almost Done!The day is almost over!

You can sleep later!

Image Credits:

- http://www.clickhole.com/article/8-kittens-god-forbid-could-be-used-prop-wobbly-tab-1364

- http://stuffpoint.com/dogs/image/208632/adorable-corgi-puppies-running-ddddd-picture/

Mocking Exceptions

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Sample Problem: Docker Payments with Stripe

Stripe Payment Code

Stripe Testing…ugh!

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• Requires us to call the API

• Unclear what those numbers mean

• Must create the stripe_token

– Not a dictionary of CC fields

– Encrypted

That’s ridiculous…

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There’s got to be a better way…

…have you tried Mocking???

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Mocking an Exception

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Wrap Up:Key Take-Aways

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• Mocking makes writing unit tests simpler

– Eliminates dependencies

– Verifies behavior

– Tests error handling

• You just need some practice!

Remember This!

• http://mock.readthedocs.org/en/latest/

• Pip Install Mock (Python 2)

• Create a simple class, then write tests!

Try It On Your Own

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Let’s Go Write Some Tests!!!

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Questions?

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GitHub: https://github.com/Ooblioob

Twitter: @Ooblioob