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BDD with Python and Behave

— python, behave, bdd — 8 min read

Introduction

Behavior Driven Development (BDD) is an agile software development technique that mainly encourages collaboration between developers, non-technical or business participants in a software project. In short, both technical and non-technical individuals have a role to play towards the overall project. It has tests developed in plain text with the implementation logic in Python.

By the end of this tutorial, you should be able to write basic behavioral tests using Behave.

Prerequisites

Before starting, please make sure you have installed the following:

Setting Up Your Environment

This tutorial will walk you through writing tests for and coding a feature of a Simple Calculator. To get started, create a root directory where your code will go, and then create the following directories and blank files:

$ tree
.
|____features
| |____calculator.feature
| |____steps
|   |____steps.py
|____calculator.py

Here’s a brief explanation of the files:

  • calculator.feature: The written out tests for the calculator.
  • steps.py: The code that runs the tests in calculator.feature.
  • calculator.py: The implementation code for the calculator.

Writing Your First Test

Behavioral tests are much similar to TDD methodology. We will start with the tests first.

Writing the Scenario

Open calculator.feature and add the following first line:

Feature: Test Calculator Functionality

This line describes the features of the application. For our project, Calculator, we might not have many features but in large scale application we would have many features. Next, we will add a test. The first test would be very simple - Add two numbers.

Feature: Test Calculator Functionality
  Scenario: Add two numbers

Before we write more, we need to understand the three phases of a basic Behave test: Given, When, and Then.

Given initializes a state, When describes an action, and Then states the expected outcome. For this test, our state is having the two numbers, the action is adding them, and the expected outcome is that expecting the result to be sum of the two numbers. Here’s how this is translated into a Behave test:

Feature: Test Calculator Functionality=
  Scenario: Addition
      Given I have the numbers 10 and 5
      When I add them
      Then I expect the result to be 15

Notice that the three phases read like a normal English sentence. You should strive for this when writing behavioral tests because they are easily readable by anyone working in the code base (need not be a developer).

Now to see how Behave works, simply open a terminal in the root directory of your code and run the behave command and you should see the output.

$ behave
Feature: Test Calculator Functionality # features/calculator.feature:2


  Scenario: Addition                  # features/calculator.feature:4
    Given I have the numbers 10 and 5 # None
    When I add them                   # None
    Then I expect the result to be 15 # None




Failing scenarios:
  features/calculator.feature:4  Addition


0 features passed, 1 failed, 0 skipped
0 scenarios passed, 1 failed, 0 skipped
0 steps passed, 0 failed, 0 skipped, 3 undefined
Took 0m0.000s


...

The key part here is that we have one failing scenario (and therefore a failing feature) that we need to fix. Below that, Behave suggests how to implement steps. You can think of a step as a task for Behave to execute. Each phase (“given”, “when”, and “then”) are all implemented as steps.

Writing the Steps

The steps that Behave runs are written in Python and they are the link between the descriptive tests in .feature files and the actual application code. Go ahead and open steps.py and add the following imports:

from behave import given, when, then
from calculator import Calculator

Behave steps use annotations that match the names of the phases. This is the first step as described in the scenario:

@given(u'I have the numbers {num1} and {num2}')
def step_impl(context, num1, num2):
    print(u'STEP: Given I have the numbers {} and {}'.format(num1, num2))
    context.num1 = int(num1)
    context.num2 = int(num2)

It’s important to notice that the text inside of the annotation matches the scenario text exactly. If it doesn’t match, the test cannot run.

The context object is passed from step to step, and it is where we can store information to be used by other steps. Since this step is a “given”, we need to initialize our state. We do that by storing our numbers in num1 & num2 variables and attaching them to the context. If you run behave again, you’ll see the test fails, because the “when” and “then” steps are not implemented. You can run behave command after each step to see how the tests are working.

Here are the next steps to add to steps.py:

@when(u'I add them')
def step_impl(context, opr):
    print(u'STEP: When I add them')
    context.result = Calculator().add(
        context.num1,
        context.num2
    )

Again, the annotation text matches the text in the scenario exactly. In the “when” step, we have access to the two numbers using context and we call the Calculator class to call the add method with the numbers.

Finally, in the “then” step, we still have access to the numbers and their sum, and we assert that the result is equal to the expected sum.

@then(u'I expect the result to be {result}')
def step_impl(context, result):
    print(u'STEP: Then I expect the result to be {}'.format(result))
    assert context.result == int(result),
    'Expected {}, got {}'.format(result, context.result)

We are done with the tests now, let's switch to calculator.py and define the add method:

#!/usr/bin/env python3


class Calculator:
    def __init__(self, caching=True):
        """ init """
        pass


    def add(self, xxx, yyy):
        """ addition """
        return xxx + yyy

Everything looks good, let's go ahead and run the behave command again and you should see that the test passes:

$ behave
Feature: Test Calculator Functionality # features/calculator.feature:2


  Scenario: Addition                  # features/calculator.feature:4
    Given I have the numbers 10 and 5 # features/steps/steps.py:5 0.000s
    When I add them                   # features/steps/steps.py:12 0.000s
    Then I expect the result to be 15 # features/steps/steps.py:21 0.000s


1 feature passed, 0 failed, 0 skipped
1 scenario passed, 0 failed, 0 skipped
3 steps passed, 0 failed, 0 skipped, 0 undefined
Took 0m0.001s

Extending the Calculator

Now that we have addition, let's add multiplication too.

As mentioned above, we will add the tests first in calculator.feature file:

Scenario: Multiplication
      Given I have the numbers 10 and 5
      When I mult them
      Then I expect the result to be 50

Next, we are add the implementation in the steps.py. Also, see that the Given and Then looks similar to that of the Addition scenario, so we don't need to define them again.

@when(u'I mult them')
def step_impl(context, opr):
    print(u'STEP: When I mult them')
    context.result = Calculator().mult(
        context.num1,
        context.num2
    )

And lastly, implement the mult method in the calculator.py file:

#!/usr/bin/env python3


class Calculator:
    ...


    def mult(self, xxx, yyy):
        """ multiply """
        return xxx * yyy

When you run the behave command again, you can see that all the tests pass (1 Feature, 2 Scenarios and 6 Steps).

Optimizing the tests and code

You can see that the steps implementation have a lot in common. So, we can try to optimize it by parsing the arithmetic operation also as a variable in the steps.py:

@when(u'I {opr} them')
def step_impl(context, opr):
    print(u'STEP: When I add them')
    context.result = Calculator().operator(
        opr,
        context.num1,
        context.num2
    )

and the calculator.py should look something like:

class Calculator(object):
    def __init__(self, caching=True):
        """ init """
        pass


    def add(self, xxx, yyy):
        """ addition """
        return xxx + yyy


    def mult(self, xxx, yyy):
        """ multiply """
        return xxx * yyy


    def operator(self, opr, xxx, yyy):
        if opr == 'add':
            return self.add(xxx, yyy)
        elif opr == 'mult':
            return self.mult(xxx, yyy)

Environmental Controls

Similar to setUp and tearDown in unit tests, the environment.py module can define code to run before and after certain events during your testing. We can use this to initialize the Calculator class since we need it in every scenario.

Create environment.py file in the features/ folder and add the following code:

from calculator import Calculator




def before_all(context):
    context.calculator = Calculator()




def after_all(context):
    del context.calculator

This would initialize the Calculator before everything and stores it in the context so that we can use it in any step. With this, we can change the code in steps.py as:

@when(u'I {opr} them')
def step_impl(context, opr):
    print(u'STEP: When I add them')
    context.result = context.calculator.operator(
        opr,
        context.num1,
        context.num2
    )

Conclusion

This tutorial walked you through setting up a new project with the Behave library and using test-driven development to build a calculator based off of behavioral tests.

If you would like to get experience writing more tests with this project, try implementing the division, modulus and other arithmetic and advanced operations.

Resources

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