pytest-patterns is a plugin for pytest that provides a pattern matching engine optimized for testing.

Benefits:

• provides easy to read reporting for complex patterns in long strings (1000+ lines)
• assists in reasoning which patterns have matched or not matched – and why
• can deal with ambiguity, optional and repetitive matches, intermingled output from non-deterministic concurrent processes
• helps writing patterns that are easy to read, easy to maintain and easy to adjust in the face of unstable outputs
• helps reusing patterns using pytest fixtures

Long term goals:

Support testing of CLI output, as well as HTML and potentially typical text/* types like JSON, YAML, and others.

Examples

Try and play around using the examples in the source repository. The examples fail on purpose, because the failure reporting is the most important and useful part – aside from making it easier to write the assertions.

$ nix develop
$ hatch run test examples -vv

Basic API

optional matches and variability with the ellipsis ...

If you want to test a complex string, start by pulling in the patterns fixture and create a named pattern that accepts any number of lines that contain the word "better":

import this

zen = "".join([this.d.get(c, c) for c in this.s])


def test_zen(patterns):
    p = patterns.better_things
    p.optional("...better...")
    assert p == zen

This will not give us a green bar, as the pattern does match some lines, but some lines were not matched and thus considered unexpected:

  🟢=EXPECTED | ⚪️=OPTIONAL | 🟡=UNEXPECTED | 🔴=REFUSED/UNMATCHED

  Here is the string that was tested:

  🟡                 | The Zen of Python, by Tim Peters
  🟡                 |
  ⚪️ better_things   | Beautiful is better than ugly.
  ⚪️ better_things   | Explicit is better than implicit.
  ⚪️ better_things   | Simple is better than complex.
  ⚪️ better_things   | Complex is better than complicated.
  ⚪️ better_things   | Flat is better than nested.
  ⚪️ better_things   | Sparse is better than dense.
  🟡                 | Readability counts.
  🟡                 | Special cases aren't special enough to break the rules.
  🟡                 | Although practicality beats purity.
  🟡                 | Errors should never pass silently.
  🟡                 | Unless explicitly silenced.
  🟡                 | In the face of ambiguity, refuse the temptation to guess.
  🟡                 | There should be one-- and preferably only one --obvious way to do it.
  🟡                 | Although that way may not be obvious at first unless you're Dutch.
  ⚪️ better_things   | Now is better than never.
  ⚪️ better_things   | Although never is often better than *right* now.
  🟡                 | If the implementation is hard to explain, it's a bad idea.
  🟡                 | If the implementation is easy to explain, it may be a good idea.
  🟡                 | Namespaces are one honking great idea -- let's do more of those!

The report highlights which lines were matched (and which pattern caused the match) and are fine the way they are. Optional matches are displayed with a white circle (⚪️). The report also highlights those lines that weren't matched and marked with a yellow circle (🟡).

We know the Zen is correct the way it is, so lets use more of the API to continue completing the pattern.

continous matches

Lets use the continuous match which requires lines to come both in a specific order and must not be interrupted by other lines. We create a new named pattern and merge it with the previous pattern:

def test_zen(patterns):
    ...

    p = patterns.conclusio
    p.continous("""
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!
""")
    full_pattern = patterns.full
    full_pattern.merge("better_things", "conclusio")

    assert full_pattern == zen

This gets us a bit further:

  🟢=EXPECTED | ⚪️=OPTIONAL | 🟡=UNEXPECTED | 🔴=REFUSED/UNMATCHED

  Here is the string that was tested:

  🟡                 | The Zen of Python, by Tim Peters
  🟡                 |
  ⚪️ better_things   | Beautiful is better than ugly.
  ⚪️ better_things   | Explicit is better than implicit.
  ⚪️ better_things   | Simple is better than complex.
  ⚪️ better_things   | Complex is better than complicated.
  ⚪️ better_things   | Flat is better than nested.
  ⚪️ better_things   | Sparse is better than dense.
  🟡                 | Readability counts.
  🟡                 | Special cases aren't special enough to break the rules.
  🟡                 | Although practicality beats purity.
  🟡                 | Errors should never pass silently.
  🟡                 | Unless explicitly silenced.
  🟡                 | In the face of ambiguity, refuse the temptation to guess.
  🟡                 | There should be one-- and preferably only one --obvious way to do it.
  🟡                 | Although that way may not be obvious at first unless you're Dutch.
  ⚪️ better_things   | Now is better than never.
  ⚪️ better_things   | Although never is often better than *right* now.
  🟢 conclusio       | If the implementation is hard to explain, it's a bad idea.
  🟢 conclusio       | If the implementation is easy to explain, it may be a good idea.
  🟢 conclusio       | Namespaces are one honking great idea -- let's do more of those!

Note, that lines matched by continuous are highlighted in green as they are considered a stronger match than the optional ones.

in_order matches

There is still stuff missing. Lets make the test green by creating a match for all other lines using in_order, which expects the lines to come in the order given, but might be mixed in with other lines.

def test_zen(patterns):
    ...
    p = patterns.top_and_middle
    p.in_order(
        """
The Zen of Python, by Tim Peters

Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
"""
    )
    full_pattern = patterns.full
    full_pattern.merge("top_and_middle", "better_things", "conclusio")

    assert full_pattern == zen

Shouldn't that have given us a green bar? I can still see a yellow circle there!

  🟢=EXPECTED | ⚪️=OPTIONAL | 🟡=UNEXPECTED | 🔴=REFUSED/UNMATCHED

  Here is the string that was tested:

  🟢 top_and_middle  | The Zen of Python, by Tim Peters
  🟡                 |
  ⚪️ better_things   | Beautiful is better than ugly.
  ⚪️ better_things   | Explicit is better than implicit.
  ⚪️ better_things   | Simple is better than complex.
  ⚪️ better_things   | Complex is better than complicated.
  ⚪️ better_things   | Flat is better than nested.
  ⚪️ better_things   | Sparse is better than dense.
  🟢 top_and_middle  | Readability counts.
  🟢 top_and_middle  | Special cases aren't special enough to break the rules.
  🟢 top_and_middle  | Although practicality beats purity.
  🟢 top_and_middle  | Errors should never pass silently.
  🟢 top_and_middle  | Unless explicitly silenced.
  🟢 top_and_middle  | In the face of ambiguity, refuse the temptation to guess.
  🟢 top_and_middle  | There should be one-- and preferably only one --obvious way to do it.
  🟢 top_and_middle  | Although that way may not be obvious at first unless you're Dutch.
  ⚪️ better_things   | Now is better than never.
  ⚪️ better_things   | Although never is often better than *right* now.
  🟢 conclusio       | If the implementation is hard to explain, it's a bad idea.
  🟢 conclusio       | If the implementation is easy to explain, it may be a good idea.
  🟢 conclusio       | Namespaces are one honking great idea -- let's do more of those!

Handling empty lines with the <empty-line> marker

The previous pattern is not quite perfect because pytest-patterns has a special way to handle newlines both in patterns and in content.

1. In content that is tested we never implicitly accept any lines that were not specified, including empty lines.

2. However, in patterns empty lines are not significant to allow you to use them to make your patterns more readable by grouping lines visually.

We get out of this by using the special marker <empty-line> in our patterns which will match both literally for lines containing <empty-line> and which are empty lines:

def test_zen(patterns):
    ...
    p = patterns.top_and_middle
    p.optional("<empty-line>")
    p.in_order(
        """
The Zen of Python, by Tim Peters

Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
"""
    )
    ...
    assert full_pattern == zen

And we finally get a green bar:

examples/test_examples.py::test_zen_5_1 PASSED

refused matches

Up until now we only created matches that allowed us to write down things that we expect. However, we can also explicitly refuse lines - for example any line containing the word "should":

def test_zen(patterns):
    ...
    p = patterns.no_should
    p.refused("...should...")

    full_pattern = patterns.full
    full_pattern.merge(
        "no_should", "top_and_middle", "better_things", "conclusio"
    )

    assert full_pattern == zen

This is were pytest-patterns really shines. We now can quickly see which parts of our output is OK and which isn't and why:

  🟢=EXPECTED | ⚪️=OPTIONAL | 🟡=UNEXPECTED | 🔴=REFUSED/UNMATCHED

  Here is the string that was tested:

  🟢 top_and_middle  | The Zen of Python, by Tim Peters
  🟡                 |
  ⚪️ better_things   | Beautiful is better than ugly.
  ⚪️ better_things   | Explicit is better than implicit.
  ⚪️ better_things   | Simple is better than complex.
  ⚪️ better_things   | Complex is better than complicated.
  ⚪️ better_things   | Flat is better than nested.
  ⚪️ better_things   | Sparse is better than dense.
  🟢 top_and_middle  | Readability counts.
  🟢 top_and_middle  | Special cases aren't special enough to break the rules.
  🟢 top_and_middle  | Although practicality beats purity.
  🔴 no_should       | Errors should never pass silently.
  🟢 top_and_middle  | Unless explicitly silenced.
  🟢 top_and_middle  | In the face of ambiguity, refuse the temptation to guess.
  🔴 no_should       | There should be one-- and preferably only one --obvious way to do it.
  🟢 top_and_middle  | Although that way may not be obvious at first unless you're Dutch.
  ⚪️ better_things   | Now is better than never.
  ⚪️ better_things   | Although never is often better than *right* now.
  🟢 conclusio       | If the implementation is hard to explain, it's a bad idea.
  🟢 conclusio       | If the implementation is easy to explain, it may be a good idea.
  🟢 conclusio       | Namespaces are one honking great idea -- let's do more of those!`

  These are the matched refused lines:

  🔴 no_should       | ...should...
  🔴 no_should       | ...should...

Re-using patterns with fixtures

Lastly, we'd like to re-use patterns in multiple tests. Let's refactor the current patterns into a separate fixture that can be activated as needed:

import pytest
import this

zen = "".join([this.d.get(c, c) for c in this.s])


@pytest.fixture
def zen_patterns(patterns):
    p = patterns.better_things
    p.optional("...better...")

    p = patterns.conclusio
    p.continuous(
        """\
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!
"""
    )

    p = patterns.top_and_middle
    p.in_order(
        """
The Zen of Python, by Tim Peters

Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
"""
    )


def test_zen(patterns, zen_patterns):
    full_pattern = patterns.full
    full_pattern.merge("better_things", "conclusio", "top_and_middle")

    assert full_pattern == zen

Handling tabs and whitespace

When copying and pasting output from commands its easy to turn tabs from an original source into spaces and then accidentally not aligning things right.

As its more typical to not insert tabs in your code pytest-patterns converts tabs to spaces (properly aligned to 8 character stops as terminals render them):

def test_tabs_and_spaces(patterns):
    data = """
pre>\taligned text
prefix>\tmore aligned text
"""
    tabs = patterns.tabs
    tabs.in_order("""
pre>    aligned text
prefix> aligned text
""")
    assert tabs == data

Development

$ pre-commit install
$ nix develop
$ hatch run test

TODO

• normalization feature

  -> json (+whitespace) -> python object causes serialization -> json object causes deserialization + reserialization (mit readable oder so) -> whitespace normalization

  -> html (+whitespace) -> parse html, then serialize in a normalized way -> whitespace normalization for both pattern and tested content

  -> whitespace (pattern and tested content) -> strip whitespace at beginning and end -> fold multiple spaces into single spaces (makes it harder to diagnose things)

    -> make tab replacement and format with control pictures optional.
    -> also, tab replacement only happens on the input line, not the test line because due to `...` we can't know where the tab will land
  

• proper release process with tagging, version updates, etc.

• Get coverage working correctly (https://pytest-cov.readthedocs.io/en/latest/plugins.html doesnt seem to help ...)

• Get the project fully set up to make sense for interested parties and potential contributors.

• optional reporting without colors

• matrix builds for multiple python versions / use tox locally and in github action?

• highlight whitespace (e.g. ) when reporting unmatched expected lines. this can be confusing if you see an "empty" line because you typoed e.g.:

    outmigrate.optional(
        """
simplevm             waiting                        interval=3 remaining=...
simplevm             check-staging-config           result='none'
simplevm             query-migrate                  arguments={} id=None subsystem='qemu/qmp'
simplevm             migration-status               mbps=... remaining='...' status='active'
simplevm             vm-destroy-kill-vm             attempt=... subsystem='qemu'
    """
    )

Do you see it? There are four spaces on the last line which is now an expected line with four spaces ...

This could also be improved by ignoring whitespace only lines (optionally?)

DONE

• coding style template

• Actual documentation

• differentiate between tolerated and expected in status reporting

• add avoided lines that must not appear

• allow patterns expectations/tolerations/... to have names and use those to mark up the report why things matched?

  • [T] DEBUG | ....
  • MIGRATION |

• how to deal with HTML boilerplate -> use optional("...")

• add lines that must appear in order without being interrupted

Later

• html normalization might want to include a feature to suppress reporting of certain lines (and just add ... in the reporting output, e.g. if something fails do not report the owrap lines

• add line numbers

• report line numbers on matched avoidances

• structlog integration

• more comprehensive docs

Wording

Matches on patterns are adjectives:

• These lines must appear and they must be continuous.
• These lines must appear and they must be in order.
• These lines are optional.
• If those lines appear they are refused.

Modifiers to the pattern itself are verbs:

• Merge the rules from those patterns into this one.
• Normalize the input (and the rules) in this way.s