Mocking Strategies

Please don’t mock me after this

presentation…

Daniel Davis

• Daniel Davis

• Software Developer for 8 years

• Fun Fact:

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

Giant Gingerbread man

Who Am I?

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Really though, who are you?

• Came from Java world

• Python developer for 2 years

• DevOps

– Lots of work with automation and quality

• Passionate about quality!

– Doesn’t happen overnight…

Testing Journey

• Became a Certified Scrum Developer

(CSD) about 2 years ago

• Finally learned about craftsmanship and

writing better tests

– Basically black magic

• Learned in Java…

I want to be a better developer

• Switching to Python was jarring

• Kept wondering about writing unit tests

• Dealing with complicated frameworks like

Django

Finding Python

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• Asked colleagues

– Many had the same questions

• Decided to learn…

• Wanted to share with the community

Unit testing? Mocks?

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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 in the CUT

(class under test)

– 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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How do we generate this exception???

• 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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Valentine’s Day Edition

Problem: Tinder Competitor

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Get yo’ container on…

• 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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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!There will be beer soon!

Sweet, delicious beer!

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Mocking built-ins and Exceptions

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Simple JSON reader

How do we test something like this???

• Test parsing a valid file

• Test an IOError (i.e. file missing)

• Test a ValueError (i.e invalid json)

Testing JSON reader

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• Let’s just create a sample file!

How do I test open()

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Can you even mock a builtin?

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• open() returns a File object

• open(filename).read()

• So we really need to mock File.read()

– But it’s an instance!!! Oh no!

What? Why Not???

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Have you tried solving it with Mocks???

Mocks returning Mocks? WAT???

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What about error handling?

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What about error handling?

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What about ValueError?

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

• 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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