pytest-snapshot 0.1.0

A plugin to enable snapshot testing with pytest.

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

• They are easy to create.

• They are easy to update due to changes in the expected value.

Instead of manually updating tests when the expected value of an expression changes, the developer simply needs to

1. run pytest --snapshot-update to update the snapshot tests

2. verify that the snapshot files contain the new expected results

3. commit the snapshot changes to version control

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This pytest plugin was generated with Cookiecutter along with @hackebrot’s cookiecutter-pytest-plugin template.

Features

• snapshot testing of strings

• paths to snapshot files are controlled by the user

Requirements

• Python 2.7 or 3.5+ or PyPy

• pytest 3.0+

Installation

You can install “pytest-snapshot” via pip from PyPI:

$ pip install pytest-snapshot

Usage

A classic test could look like:

>>> def test_function_output():
...     assert foo('function input') == 'expected result'

It could be re-written using snapshot testing as:

>>> def test_function_output_with_snapshot(snapshot):
...     snapshot.snapshot_dir = 'snapshots/'
...     snapshot.assert_match(foo('function input'), 'foo_output.txt')

The author of the test should then

1. run pytest --snapshot-update to create the snapshot file snapshots/foo_output.txt containing the output of foo().

2. verify that the content of the snapshot file is valid.

3. commit it to version control.

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 very large, or there are many tests.

In the second example, the developer would need to simply

1. run pytest --snapshot-update

2. verify that the snapshot file contains the new expected result

3. commit it to version control.

Snapshot testing can be used for expressions whose values are strings. For other types, you should first create a human readable textual 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:

>>> snapshot.assert_match(yaml.dumps(foo()), 'foo_output.yml')

Contributing

Contributions are very welcome. Tests can be run with tox, please ensure the coverage at least stays the same before you submit a pull request.

License

Distributed under the terms of the MIT license, “pytest-snapshot” is free and open source software

Issues

If you encounter any problems, please file an issue along with a detailed description.