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A plugin for snapshot testing with pytest.
This library was inspired by jest's snapshot testing
_.
Snapshot testing can be used to test that the value of an expression does not change unexpectedly.
The added benefits of snapshot testing are that
Instead of manually updating tests when the expected value of an expression changes, the developer simply needs to
pytest --snapshot-update
to update the snapshot testsPyPy
_pytest
_ 3.0+You can install "pytest-snapshot" via pip
from PyPI
::
$ pip install pytest-snapshot
A classic equality test looks like:
.. code-block:: python
def test_function_output():
assert foo('function input') == 'expected result'
It could be re-written using snapshot testing as:
.. code-block:: python
def test_function_output_with_snapshot(snapshot):
snapshot.snapshot_dir = 'snapshots' # This line is optional.
snapshot.assert_match(foo('function input'), 'foo_output.txt')
The author of the test should then
pytest --snapshot-update
to generate the snapshot file snapshots/foo_output.txt
containing the output of foo()
.Now, whenever the test is run, it will assert that the output of foo()
is equal to the snapshot.
What if the behaviour of foo()
changes and the test starts to fail?
In the first example, the developer would need to manually update the expected result in test_function_output
.
This could be tedious if the expected result is large or there are many tests.
In the second example, the developer would simply
pytest --snapshot-update
Snapshot testing can be used for expressions whose values are strings or bytes.
For other types, you should first create a human readable representation of the value.
For example, to snapshot test a json-serializable value, you could either convert it into json
or preferably convert it into the more readable yaml format using PyYAML
_:
.. code-block:: python
snapshot.assert_match(yaml.dump(foo()), 'foo_output.yml')
When snapshot testing a collection of values, assert_match_dir
comes in handy.
It will save a snapshot of a collection of values as a directory of snapshot files.
assert_match_dir
takes a dictionary from file name to value.
Dictionaries can also be nested to create nested directories containing snapshot files.
For example, the following code creates the directory snapshots/people
containing files john.json
and jane.json
.
.. code-block:: python
def test_something(snapshot):
snapshot.snapshot_dir = 'snapshots'
snapshot.assert_match_dir({
'john.json': '{"first name": "John", "last name": "Doe"}',
'jane.json': '{"first name": "Jane", "last name": "Doe"}',
}, 'people')
When running pytest --snapshot-update
, snapshot files will be added, updated, or deleted as necessary.
As a safety measure, snapshots will only be deleted when using the --allow-snapshot-deletion
flag.
A quick way to create snapshot tests is to create a directory containing many test case directories. In each test case, add files containing the inputs to the function you wish to test. For example:
.. code-block::
test_cases
case1
input.json
case2
input.json
...
Next, add a test that is parametrized on all test case directories. The test should
.. code-block:: python
import json
import os
import pytest
import yaml
from pathlib import Path
def json_to_yaml(json_string):
obj = json.loads(json_string)
return yaml.dump(obj, indent=2)
@pytest.mark.parametrize('case_dir', list(Path('test_cases').iterdir()))
def test_json(case_dir, snapshot):
# Read input files from the case directory.
input_json = case_dir.joinpath('input.json').read_text()
# Call the tested function.
output_yaml = json_to_yaml(input_json)
# Snapshot the return value.
snapshot.snapshot_dir = case_dir
snapshot.assert_match(output_yaml, 'output.yml')
Now, we can run pytest --snapshot-update
to create an output.yml
snapshot for each test case.
If we later decide to modify the tested function's behaviour,
we can fix the test cases with another pytest --snapshot-update
.
Another python package that can be used for snapshot testing is snapshottest
_.
While this package and snapshottest fulfill the same role, there are some differences.
With pytest-snapshot:
This allows the user to organize snapshots in the most human-readable and logical place in their code repository. This is highly beneficial since snapshots will be viewed by users many times during development and code reviews.
Contributions are very welcome. Before contributing, please discuss the change with me. I wish to keep this plugin flexible and not enforce any project layout on the user.
Tests can be run with tox
_ or python -m pytest
.
Distributed under the terms of the MIT
_ license, "pytest-snapshot" is free and open source software.
If you encounter any problems, please file an issue
_ along with a detailed description.
This pytest
plugin was generated with Cookiecutter
along with @hackebrot
's cookiecutter-pytest-plugin
template.
.. Cookiecutter
: https://github.com/audreyr/cookiecutter
.. @hackebrot
: https://github.com/hackebrot
.. MIT
: http://opensource.org/licenses/MIT
.. BSD-3
: http://opensource.org/licenses/BSD-3-Clause
.. GNU GPL v3.0
: http://www.gnu.org/licenses/gpl-3.0.txt
.. Apache Software License 2.0
: http://www.apache.org/licenses/LICENSE-2.0
.. cookiecutter-pytest-plugin
: https://github.com/pytest-dev/cookiecutter-pytest-plugin
.. file an issue
: https://github.com/joseph-roitman/pytest-snapshot/issues
.. pytest
: https://github.com/pytest-dev/pytest
.. tox
: https://tox.readthedocs.io/en/latest/
.. pip
: https://pypi.org/project/pip/
.. PyPI
: https://pypi.org
.. PyPy
: https://www.pypy.org/
.. jest's snapshot testing
: https://jestjs.io/docs/en/snapshot-testing
.. PyYAML
: https://pypi.org/project/PyYAML/
.. snapshottest
: https://github.com/syrusakbary/snapshottest