Lettuce is an extremely useful and charming tool for BDD (Behavior Driven Development). It can execute plain-text functional descriptions as automated tests for Python projects, just as Cucumber does for Ruby.

Lettuce makes the development and testing process really easy, scalable, readable and - what is best - it allows someone who doesn’t program to describe the behavior of a certain system, without imagining those descriptions will automatically test the system during its development.

Make sure you’ve got Python installed and then run from the terminal:

```
user@machine:~$ [sudo] pip install lettuce
```

Let’s choose a problem to lettuce:
**Given a number, what is its factorial?**

Note

The factorial of a positive integer n, denoted by n!, is the product of all positive integers less than or equal to n. The factorial of 0 is 1

Build the directory tree bellow such as the files `zero.feature` and `steps.py` are empty.

```
/home/user/projects/mymath
| tests
| features
- zero.feature
- steps.py
```

Lets begin to describe and solve our problem...

Start describing the expected behaviour of factorial in `zero.feature` using English:

```
Feature: Compute factorial
In order to play with Lettuce
As beginners
We'll implement factorial
Scenario: Factorial of 0
Given I have the number 0
When I compute its factorial
Then I see the number 1
```

Note

zero.feature must be inside features directory and its extension must be .feature. However, you’re free to choose its name.

Now let’s define the steps of the scenario, so Lettuce can
understand the behaviour description. Create the `steps.py` file which will contain
python code describing the steps.

Python:

```
from lettuce import *
@step('I have the number (\d+)')
def have_the_number(step, number):
world.number = int(number)
@step('I compute its factorial')
def compute_its_factorial(step):
world.number = factorial(world.number)
@step('I see the number (\d+)')
def check_number(step, expected):
expected = int(expected)
assert world.number == expected, \
"Got %d" % world.number
def factorial(number):
return -1
```

Note

`steps.py` must be inside features directory, but the names doesn’t
need to be `steps.py` it can be any python file with a `.py` extension.
Lettuce will look for python files recursively within features
dir.

Ideally, factorial will be defined somewhere else. However, as this is just a first example, we’ll implement it inside steps.py, so you get the idea of how to use Lettuce.

**Notice that, until now, we haven’t defined the factorial function (it’s returning -1).**

Go to the tests directory and run from the terminal:

```
user@machine:~/projects/mymath/tests$ lettuce
```

As you haven’t implemented factorial, it is no surprise the behavior won’t be reached:

Our only scenario failed :( Let’s solve it...

Well, by definition, we know that the factorial of 0 is 1. As our only feature is this... we could force factorial to return 1.

```
from lettuce import *
@step('I have the number (\d+)')
def have_the_number(step, number):
world.number = int(number)
@step('I compute its factorial')
def compute_its_factorial(step):
world.number = factorial(world.number)
@step('I see the number (\d+)')
def check_number(step, expected):
expected = int(expected)
assert world.number == expected, \
"Got %d" % world.number
def factorial(number):
return 1
```

Again, run from the terminal:

```
user@machine:~/projects/mymath/tests$ lettuce
```

And you’ll be happy to see your factorial implementation passed all the behaviours expected:

Great! :)

However, one test is not enough for checking the quality of our solution... So let’s lettuce it again!

Let’s provide more tests so our problem is better described, and so we provide a more accurate implementation of factorial:

Let’s provide two new scenarios, for numbers 1 and 2:

```
Feature: Compute factorial
In order to play with Lettuce
As beginners
We'll implement factorial
Scenario: Factorial of 0
Given I have the number 0
When I compute its factorial
Then I see the number 1
Scenario: Factorial of 1
Given I have the number 1
When I compute its factorial
Then I see the number 1
Scenario: Factorial of 2
Given I have the number 2
When I compute its factorial
Then I see the number 2
```

As we haven’t changed the definition, no need to make changes on this step.

```
user@machine:~/projects/mymath/tests$ lettuce
```

When running Lettuce we realize that our previous implementation of factorial works fine both for 0 and for 1, but not for 2 - it fails. :(

Let’s provide a solution so we get the right factorial for all scenarios, specially for number 2:

```
from lettuce import *
@step('I have the number (\d+)')
def have_the_number(step, number):
world.number = int(number)
@step('I compute its factorial')
def compute_its_factorial(step):
world.number = factorial(world.number)
@step('I see the number (\d+)')
def check_number(step, expected):
expected = int(expected)
assert world.number == expected, \
"Got %d" % world.number
def factorial(number):
number = int(number)
if (number == 0) or (number == 1):
return 1
else:
return number
```

```
user@machine:~/projects/mymath/tests$ lettuce
```

Great! Three scenarios described and they are alright!

Let’s provide more tests so our problem is better described and we get new errors so we’ll be able to solve them.

```
Feature: Compute factorial
In order to play with Lettuce
As beginners
We'll implement factorial
Scenario: Factorial of 0
Given I have the number 0
When I compute its factorial
Then I see the number 1
Scenario: Factorial of 1
Given I have the number 1
When I compute its factorial
Then I see the number 1
Scenario: Factorial of 2
Given I have the number 2
When I compute its factorial
Then I see the number 2
Scenario: Factorial of 3
Given I have the number 3
When I compute its factorial
Then I see the number 6
Scenario: Factorial of 4
Given I have the number 4
When I compute its factorial
Then I see the number 24
```

As we haven’t changed the definition, no need to make changes on this step.

```
user@machine:~/projects/mymath/tests$ lettuce
```

```
from lettuce import *
@step('I have the number (\d+)')
def have_the_number(step, number):
world.number = int(number)
@step('I compute its factorial')
def compute_its_factorial(step):
world.number = factorial(world.number)
@step('I see the number (\d+)')
def check_number(step, expected):
expected = int(expected)
assert world.number == expected, \
"Got %d" % world.number
def factorial(number):
number = int(number)
if (number == 0) or (number == 1):
return 1
else:
return number*factorial(number-1)
```

```
user@machine:~/projects/mymath/tests$ lettuce
```

All steps should be repeated as long as you can keep doing them - the quality of your software depends on these.

Available for versions > 0.2.19

To take a step sentence from function name or doc, just decorate it with “@step” without argument.

These two steps below, are identicals than the example above.

```
from lettuce import *
@step
def have_the_number(step, number):
'I have the number (\d+)'
world.number = int(number)
@step
def i_compute_its_factorial(step):
world.number = factorial(world.number)
```

```
from lettuce import world, steps
@steps
class FactorialSteps(object):
"""Methods in exclude or starting with _ will not be considered as step"""
exclude = ['set_number', 'get_number']
def __init__(self, environs):
self.environs = environs
def set_number(self, value):
self.environs.number = int(value)
def get_number(self):
return self.environs.number
def _assert_number_is(self, expected, msg="Got %d"):
number = self.get_number()
assert number == expected, msg % number
def have_the_number(self, step, number):
'''I have the number (\d+)'''
self.set_number(number)
def i_compute_its_factorial(self, step):
number = self.get_number()
self.set_number(factorial(number))
def check_number(self, step, expected):
'''I see the number (\d+)'''
self._assert_number_is(int(expected))
# Important!
# Steps are added only when you instanciate the "@steps" decorated class
# Internally decorator "@steps" build a closure with __init__
FactorialSteps(world)
def factorial(number):
number = int(number)
if (number == 0) or (number == 1):
return 1
else:
return number*factorial(number-1)
```

Have a nice lettuce...! ;)

Oct 01, 2014