Booleans

You can find all the code for this chapter here

Truthy and Falsy in Python

In Python, there are a few values that are considered falsy, and everything else is truthy.

Let’s validate our hypothesis with some tests:

Note: In order to make our tests more dynamic, we’re going to parametrize them using pytest, more info here.

@pytest.mark.parametrize(
    "value",
    [
        True,
        1,
        "Hello",
        [1, 2, 3],
        (1, "x")
    ],
)
def test_is_truthy(self, value):
    expect(is_truthy(value)).to(be_true)

@pytest.mark.parametrize(
    "value",
    [
        False,
        0,
        "",
        [],
        (),
        None,
    ],
)
def test_is_falsy(self, value):
    expect(is_truthy(value)).to(be_false)

Let’s implement the is_truthy function just casting the value to a boolean:

def is_truthy(value):
    return bool(value)

We can say now that as we assumed:

True, 1, strings with values, lists and tuples with item are truthy.
False, 0, "", [], (), None are falsy.

Boolean operators

In Python instead of || or && operators, we have the and and or operators to combine boolean expressions.

Write the test first

Let’s say we have a function that checks if a user has permission to do something.

The user must be an admin and logged in:

def test_has_permission(self):
    expect(has_permission(is_admin=True, is_logged_in=False)).to(be_false)

Try and run the test

================= short test summary info ================= 
FAILED test_booleans.py::TestBooleans::test_has_permission
NameError: name 'has_permission' is not defined
================= 1 failed in 0.02s ============

Pretty explanatory, we need to define the has_permission function.

Write the minimal amount of code to make it run

The simplest implementation would be just create it without any logic:

def has_permission(is_admin, is_logged_in):
    pass

Re-run the tests:

E       expected: None to be false
================= short test summary info ================= 
FAILED test_booleans.py::TestBooleans::test_has_permission
AssertionError: 
================= 1 failed in 0.02s ============

Now the problem is that we’re not returning nothing, that’s good.

Step by step, let’s make the test pass.

Write enough code to make it pass

Right now, we only have one test, so we can just return False:

def has_permission(is_admin, is_logged_in):
    return False

And the test passes:

========================= 1 passed in 0.01s =========================

Refactor

Now we can add more cases to our test in order to make our function working as expected:

def test_has_permission(self):
    expect(has_permission(True, False)).to(be_false)
    expect(has_permission(True, True)).to(be_true)
    expect(has_permission(False, False)).to(be_false)

Re-run the tests in order to see them failing.

After that we can implement the has_permissions function:

def has_permission(is_admin, is_logged_in):
    return is_admin and is_logged_in

Comparing values

Regardig the comparison of values, we can use the == operator to compare two values as in many other languages, nothing relevante here actually.

So just let’s write a test to check if a number is even or not.

Note: We need to make muscle, so let’s repeat the process again (Red - Green - Refactor)

Write the test first

def test_is_even(self):
    expect(is_even(4)).to(be_true)

Try and run the test

================= short test summary info ================= 
FAILED test_booleans.py::TestBooleans::test_is_even
NameError: name 'is_even' is not defined
================= 1 failed in 0.02s =======================

Write the minimal amount of code to make it run

def is_even(n):
    pass

E       expected: None to be true
================= short test summary info ================= 
FAILED test_booleans.py::TestBooleans::test_is_even
AssertionError: 
================= 1 failed in 0.02s =======================

Write enough code to make it pass

def is_even(n):
    return True

=================== 1 passed in 0.01s ====================

Refactor

def test_is_even(self):
    expect(is_even(4)).to(be_true)
    expect(is_even(5)).to(be_false)

def is_even(n):
    return n % 2 == 0

Wrapping up

What we have covered:

Parametrize our tests in order to test multiple cases.
More practice of the TDD workflow.
What is truthy and falsy in Python.
The and and or operators.
Comparing values.

Hello World Numbers