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Python expectaton library

Project Description

The whole point of the expect pattern is to allow concise assertions that generate predictable and good error messages.

Best viewed in an example:

>>> expect(3).to.equal(4)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "", line 146, in __call__
AssertionError: Expect 3 to equal 4

Line noise is reduced as much as possible, so the error message is displayed as near to the problematic code as possible. No stack traces to dig through, clear and consistent error messages that tell you what went wrong. Thats how assertions should work.

Why should I use expect() over self.assert*?

This is best explained in contrast to the classic assertion pattern like the python unittest module uses. In addition to all that however, these assertions can be used independently of any unittest package. But lets start with an example:

self.assertEquals('foo', 'bar')

In this assertion it is not possible to see which of the arguments is the expected and which is the actual value. While this ordering is mostly internally consistent between the different assertions within the unittest package (sadly only mostly), it is not consistent between all the different unit testing packages out there for python and especially not between different languages this pattern has been implemented on.

To add insult to injury some frameworks will then output the error message like this: (Yes unittest I’m looking at you!)

'bar' does not equal 'foo'

It’s easy to spend minutes till you remember or figure out that your framework fooled you and inverted the order of arguments just to make it harder for you to understand the code you are reading.

If you are as annoyed by this as I am, allow me to introduce you to the expect pattern. An assertion like this:

# or this
# or even this
expect('foo') == 'bar'

Makes it absolutely plain what is the expected and what is the actual value. No confusion possible. Also the error messages are designed to map cleanly back to the source code:

Expect 'foo' to equal 'bar'.

Thus the mapping from the error message is immediate and complete saving you minutes each time, enhancing your focus, productivity and - most important - your enjoyment when working with unit tests.

As a bonus all these exceptions are not coupled to any TestCase class so you can easily reuse them anywhere in your code to formalize expectations that your code has about some internal state. Sometimes called ‘Design by Contract’ or ‘Fail Fast’ programming. Oh and these expectations are generally shorter, so you even have less to type while getting clearer and more to the point assertions into your tests. Almost like having a cake and eating it too!

Interesting! So what can it do?

Glad you ask! Here you go:

  1. Lots of included matchers: Take a look at the source to see all the assertions you need to get started. From equals over be or is_ and raises till matches - we’ve got you covered. And not only that, but each matcher has some aliasses so you can use the variant that reads the best in your assertion, or that you are more used to when using multiple unit testing frameworks across language boundaries (python/js anyone?).

    Some examples:

    expect(True) == True

    Should an important alias be missing, pull requests are welcome.

  2. Ease of use: expect() can be arbitrary chained with whatever you can think of (provided it’s a valid python identifier) to give you the cleanest description of your assertion possible:

    # or go all out - but just because it works doesn't mean it's sensible
    # Here .segment(23) would be the matcher that is called

    Choose whatever makes sense for your specific test to read well so that reading the test later feels natural and transports the meaning of the code as best as possible.

  3. Simplicity of extension: In all other python expect implementations I’ve looked at, at least some aspects of them are way more complicated. Each matcher is a class or something that needs to be registered via a more or less complicated process, arguments are not just straightforward method arguments, not is not supported as a native framework concept…

    In contrast in pyexpect if you want to register a new matcher, it’s as easy as defining a method and then assigning it to as many instance method names as you want, asserting what you want to assert within and clearly define the error message that is going to be raised:

    def falseish(self):
        # See expect() source for availeable helpers
        self._assert(bool(self._actual) is False, "to be falseish")
    expect.is_falsish = expect.is_falseish = expect.falsish = expect.falseish = falseish


    Also note how the matcher clearly communicates what is important: what it asserts, and what error message it generates. No fluff included™!

  4. Native not support: If you define a matcher, you don’t have to define the inverse of it too or do anything special to get it. That means that for every matchers like equals, you automatically get the inverse of that, i.e. not_equals. This inverse can be invoked in a number of ways:

    You can just prefix the matcher with not_ like this


    You can include not as part of the path before the matcher like this:

    # or go all out - but just because it works doesn't mean it's sensible

    That is you can include the word not at the beginning, middle or end of an identifier - just ensure to separate it from the identifier by snakecase.

    This works for all aliasses of each matcher, so no additional work there.

    For more examples, have a look at the testsuite for the matchers.

    If you want to add your own matchers, sometimes the inverse doesn’t work automatically if you implement your expectations as multiple consecutive checks. In that case the inverse matcher might assert the wrong thing, because the order of the checks doesn’t make sense in the inverted case. Should that happen, take a look at expect._assert_if_positive(), expect._assert_if_negative() and expect._is_negative(). Be advised however, that good matchers should need this only very rarely.

  5. Great error messages: pyexpect takes great care to ensure every aspect of experiencing an error is as concise and usefull as possible. All error messages have the same format that always starts with what is expected and then is customized by the matcher to pack as much information as possible.

    Expect 23 not to equal 23

    If you write your own assertion methods to enhance your unit testing, it’s quite easy to get long stack traces because the actuall assertion happens some stack frames down in one of the called matchers.

    Consider assertions like this (a little fabricated, ok. But I’m sure you’ve seen this happen in your projects):

    from unittest import TestCase, main
    class Test(TestCase):
        def assert_equals(self, actual, expected):
            self.assertEquals(actual, expected)
        def assert_something(self, something):
            self.assert_equals(something, 'something')
        def test_something(self):

    It will give you output like this:

    FAIL: test_something (__main__.Test)
    Traceback (most recent call last):
      method test_something in at line 11
      method assert_something in at line 8
        self.assert_equals(something, 'something')
      method assert_equals in at line 5
        self.assertEquals(actual, expected)
    AssertionError: 'fnord' != 'something'

    Even in this simple case the actual error is 4 lines removed from the actual error.

    Using pyexpect however a test like this:

    from pyexpect import expect
    def something(self):
        self._assert(self._actual == 'something', "to be 'something'")
    expect.something = something
    from unittest import TestCase, main
    class Test(TestCase):
        def test_something(self):

    Gives you this output (standard unittest.main()):

    FAIL: test_something (__main__.Test)
    Traceback (most recent call last):
      File "", line 11, in test_something
      File "/Users/dwt/Code/Projekte/pyexpect/  # pyexpect_internals_hidden_in_backtraces
        raise assertion
    AssertionError: Expect 'fnord' to be something

    In nosetests:

    FAIL: test_example:Test.test_something
      mate +11  # test_something
      mate +150  # pyexpect_internals_hidden_in_backtraces
        raise assertion
    AssertionError: Expect 'fnord' to be something

    py.test this is even prettier:

    self = <test_example.Test testMethod=test_something>
    def test_something(self):
    >       expect('fnord').to_be.something()
    E       AssertionError: Expect 'fnord' to be something

    Notice here, that the actual matcher someting() calls another method _assert to do the actual assertion and compose the error message, but none of this is visible in the stack trace? That is true for any methods you call in the matcher, so call into your api or whatever you need to trigger the assertion and enjoy the readability of the generated error messages.

    Another common cause for really hard to read error messages are too long messages. Ever had something like this?

    FAIL: test_example (__main__.DemoTest)
    Traceback (most recent call last):
      File "untitled", line 15, in test_example
        self.assertEquals(long_actual, long_expected)
    AssertionError: ['foo', 'bar', 'baz', 'quoox', 'quaax', 'quuuxfoo', 'bar', 'baz', 'quoox', 'quaax', 'quuux'] != ['foo', 'bar', 'baz', 'quoox', 'quaax', 'quuux', 'foo', 'bar', 'baz', 'quoox', 'quaax', 'quuux', 'foo', 'bar', 'baz', 'quoox', 'quaax', 'quuux']

    Good luck finding where the output of the expected and actual object even are separated, let alone what is different between them.

    pyexpect formats long error messages on multiple lines, so you always see where the complaint starts and have a much easier time mentally diffing the two objects:

    FAIL: test_example (__main__.DemoTest)
    Traceback (most recent call last):
      File "", line 16, in test_example
        expect(long_actual) == long_expected
      File "pyexpect/", line 17, in pyexpect_internals_hidden_in_backtraces
        raise exception
    AssertionError: Expect ['foo', 'bar', 'baz', 'quoox', 'quaax', 'quuuxfoo', 'bar', 'baz', 'quoox', 'quaax', 'quuux']
    to equal ['foo', 'bar', 'baz', 'quoox', 'quaax', 'quuux', 'foo', 'bar', 'baz', 'quoox', 'quaax', 'quuux', 'foo', 'bar', 'baz', 'quoox', 'quaax', 'quuux']

    tl;dr: error messages are much easier to read and there is less fluff in between the error and the cause to distract you. As it should be.

  6. Usage outside of unit tests: You can use this package as a standalone assertion package that gives you much more expressive assertions than just using using assert and refined error messages to boot.

    Just assert away wherever you need it to get robust code by failing fast:

    from pyexpect import expect
    def some_method(some_argument):

    And should you need it, you can switch the assertions from throwing to returning a (bool, string) tuple so you can reuse it in your api.

    from pyexpect import expect
    def some_api(something):
        was_success, explanation = expect.returning(something) == 23
        if not was_success: register_error(explanation)

    And should you need it, you can override the error messages generated without needing to change the matchers. See expect.with_message() for details.

  7. Test coverage: Of course pyexpect has full test coverage ensuring that it does exactly what you expect it to do.

  8. Think something could be better in this documentation? Send a pull request. :)

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