expecttest

expecttest

This library implements expect tests (also known as "golden" tests). Expect tests are a method of writing tests where instead of hard-coding the expected output of a test, you run the test to get the output, and the test framework automatically populates the expected output. If the output of the test changes, you can rerun the test with the environment variable EXPECTTEST_ACCEPT=1 to automatically update the expected output.

Somewhat unusually, this library implements inline expect tests: that is to say, the expected output isn't saved to an external file, it is saved directly in the Python file (and we modify your Python file when updating the expect test.)

The general recipe for how to use this is as follows:

• Write your test and use assertExpectedInline() instead of a normal assertEqual. Leave the expected argument blank with an empty string:

  self.assertExpectedInline(some_func(), """""")
  

• Run your test. It should fail, and you get an error message about accepting the output with EXPECTTEST_ACCEPT=1

• Rerun the test with EXPECTTEST_ACCEPT=1. Now the previously blank string literal will contain the expected value of the test.

  self.assertExpectedInline(some_func(), """my_value""")
  

A minimal working example

# test.py
import unittest
from expecttest import TestCase

class TestStringMethods(TestCase):
    def test_split(self):
        s = 'hello world'
        self.assertExpectedInline(str(s.split()), """""")

if __name__ == '__main__':
    unittest.main()

Run EXPECTTEST_ACCEPT=1 python test.py , and the content in triple-quoted string will be automatically updated.

For people who use pytest:

from expecttest import assert_expected_inline

def test_split():
    s = 'hello world'
    assert_expected_inline(str(s.split()), """""")

Run EXPECTTEST_ACCEPT=1 pytest test.py , and the content in triple-quoted string will be automatically updated.

For parameterized tests, advanced fixturing and other cases where the expectation is in a different place than the assertion, use Expect:

from expecttest import Expect

def test_removing_spaces(s: str, expected: Expect) -> None:
    expected.assert_expected(s.replace(" ", ""))

test_removing_spaces("foo bar", Expect("""foobar"""))
test_removing_spaces("foo bar !!", Expect(""""""))

Run EXPECTTEST_ACCEPT=1 pytest test.py , and the content in triple-quoted string will be automatically updated.

Some tips and tricks

• Often, you will want to expect test on a multiline string. This framework understands triple-quoted strings, so you can just write """my_value""" and it will turn into triple-quoted strings.

• Take some time thinking about how exactly you want to design the output format of the expect test. It is often profitable to design an output representation specifically for expect tests.

Similar libraries

• expect-test for Rust