02 PyTest Get started

Install pytest¶

pytest requires: Python 3.7+ or PyPy3.

• Run the following command in your command line:
• Check that you installed the correct version:

In [2]:

# !pip install -U pytest
!pytest --version

# !pip install -U pytest !pytest --version

pytest 8.1.1

Create your first test¶

1. Create a new file called test_sample.py, containing a function, and a test:

In [3]:

# content of test_sample.py
def func(x):
    return x + 1


def test_answer():
    assert func(3) == 5

# content of test_sample.py def func(x): return x + 1 def test_answer(): assert func(3) == 5

2. pytest .\test_sample.py

Run multiple tests¶

• pytest will run all files of the form test_*.py or *_test.py in the current directory and its subdirectories.
  • More generally, it follows standard test discovery rules.

Assert that a certain exception is raised¶

• Use the raises helper to assert that some code raises an exception:

• Execute the test function with quiet reporting mode:

  • The -q/--quiet flag keeps the output brief in this and following examples.
  • pytest -q test_sysexit.py

  

In [4]:

# content of test_sysexit.py
import pytest


def f():
    raise SystemExit(1)


def test_mytest():
    with pytest.raises(SystemExit):
        f()

# content of test_sysexit.py import pytest def f(): raise SystemExit(1) def test_mytest(): with pytest.raises(SystemExit): f()

Group multiple tests in a class¶

Once you develop multiple tests, you may want to group them into a class. pytest makes it easy to create a class containing more than one test:

Grouping tests in classes can be beneficial for the following reasons:

• Test organization

• Sharing fixtures for tests only in that particular class

• Applying marks at the class level and having them implicitly apply to all tests

In [5]:

# content of test_class.py
class TestClass:
    def test_one(self):
        x = "this"
        assert "h" in x

    def test_two(self):
        x = "hello"
        assert hasattr(x, "check")

# content of test_class.py class TestClass: def test_one(self): x = "this" assert "h" in x def test_two(self): x = "hello" assert hasattr(x, "check")

pytest discovers all tests following its Conventions for Python test discovery, so it finds both test_ prefixed functions.

• There is no need to subclass anything, but make sure to prefix your class with Test otherwise the class will be skipped. We can simply run the module by passing its filename:

  • pytest -q test_class.py

  

The first test passed and the second failed. You can easily see the intermediate values in the assertion to help you understand the reason for the failure.

Something to be aware of when grouping tests inside classes is that each test has a unique instance of the class.

• Having each test share the same class instance would be very detrimental to test isolation and would promote poor test practices.
  • Do not use bind instance variable!
• This is outlined below:

In [6]:

# content of test_class_demo.py
class TestClassDemoInstance:
    value = 0

    def test_one(self):
        self.value = 1
        assert self.value == 1

    def test_two(self):
        assert self.value == 1

# content of test_class_demo.py class TestClassDemoInstance: value = 0 def test_one(self): self.value = 1 assert self.value == 1 def test_two(self): assert self.value == 1

pytest -k TestClassDemoInstance -q

• In the context of pytest, the -k option is used to filter test functions and classes based on their names using a substring matching.
  • use pytest -k TestClassDemoInstance, it means you are filtering for test functions or classes whose names contain TestClassDemoInstance.
• Note that attributes added at class level are class attributes, so they will be shared between tests.

setup() and teardown()¶

• use pytest - s to show the print info
• Create shapes.py for test

In [ ]:

import math

class Shape:
    def area(self):
        pass
    def perimeter(self):
        pass
    
class Circle(Shape):
    def __init__(self, radius) -> None:
        self.radius = radius
    
    def area(self):
        return math.pi * (self.radius**2)
    
    def perimeter(self):
        return 2 * math.pi * self.radius

import math class Shape: def area(self): pass def perimeter(self): pass class Circle(Shape): def __init__(self, radius) -> None: self.radius = radius def area(self): return math.pi * (self.radius**2) def perimeter(self): return 2 * math.pi * self.radius

• test_circle.py

import pytest
import math
import shapes

class TestCircle:
    def setup_method(self,method):
        print(f"Setting up {method}")
        self.circle = shapes.Circle(10)

    def teardown_method(self,method):
        print(f"Tear down method")
        del self.circle

    def test_area(self):
        assert self.circle.area() == math.pi * self.circle.radius ** 2

    def test_perimeter(self):
        assert self.circle.perimeter() == 2 * math.pi * self.circle.radius

• pytest test_circle.py

Request a unique temporary directory for functional tests¶

• pytest provides Builtin fixtures/function arguments to request arbitrary resources, like a unique temporary directory:
• List the name tmp_path in the test function signature and pytest will lookup and call a fixture factory to create the resource before performing the test function call.
  • Before the test runs, pytest creates a unique-per-test-invocation temporary directory:
• The use of a unique temporary directory for functional tests, facilitated by pytest's tmp_path fixture, helps ensure that each test operates in an isolated environment.

In [ ]:

# content of test_tmp_path.py
def test_needsfiles(tmp_path):
    print(tmp_path)
    assert 0

# content of test_tmp_path.py def test_needsfiles(tmp_path): print(tmp_path) assert 0

pytest -q test_tmp_path.py