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Approval testing for Python3

Project description

/go:kjen/ Approve (Norwegian)

Approval testing for Python3.


Install the package:

pip install .

and the pytest plugin dependencies:

pip install .[pytest-plugin]

(This is probably not strictly necessary since the only plugin dependency is pytest).

Then you can use it in your pytest tests with the godkjenn fixture:

def test_my_test(godkjenn):
    new_output = my_function()
    godkjenn.verify(new_output)  # new_output must be a bytes object

godkjenn will look for an existing accepted value for that test function. If accepted data is found and it matches the new data, then the test passes. Otherwise the test fails.

If your test fails, this could mean either a) your code is actually broken or b) you want to accept the new output in place of the previous accepted value (if any). godkjenn will instruct you on how to proceed in that case.


The principle of approval testing simple. Given a function or program that you consider correct, you store its output for a given input. This is the accepted or golden version of its output. Then, as you change your code, you reproduce the output (we call this received output) and compare it to the accepted version. If they match, then the test passes. Otherwise, it fails.

A test failure can mean one of two things. First, it could mean that you actually broke you program and need to fix it so that the received output matches the accepted. Second, it could mean the received output is now correct, the accepted output is now out of date, and you need to update the accepted output with the received.

As an approval testing tool, godkjenn aims to streamline and simplify this kind of testing.

Core elements

There are a few core elements to godkjenn. These are the parts of the approval testing system that are independent of any particular testing framework. Generally speaking, you won’t need to work with these directly; the integration with your testing framework with hide most of the low-level details.


Vaults are where the accepted outputs are stored. (The term vault is a bit of a play on words: the accepted output is “golden”, and you keep gold in vaults.)

The vault abstraction defines an API for storing and retrieving accepted (and received) output.

godkjenn provides a simple vault, FSVault, that stores its data on the filesystem. Other vaults can be provided via a plugin system.


The core verification algorithm compares new received data with the accepted data for a given test. If there’s a mismatch or if not accepted output exists, this triggers test failure and instructs the user on what to do next.


When an approval test fails, godkjenn tries to provide a diff showing the user how the received outputs differs from the accepted. The user can provide a diffing algorithm for producing this diff.


Internally godkjenn uses a simple dict for holding its configuration information. Top-level values are stored as keys in the dict. Each plugin type has a top-level entry pointing to a dict mapping plugin names to plugin-specific configurations. So for example, the configuration might look like this:

    'vault': {
        'fs-vault': {
            ... configuration for fs-vault plugin

Since testing frameworks provide different configuration methods, godkjenn leaves it up to each test framework integration to figure out how to load the configuration. Ideally, godkjenn will have sane-enough defaults to work without explicit configuration; this is an aspiration, and may not be practical in all cases.

For examples of hwo to configure godkjenn, see the “examples” directory.

Configuration with pytest

The pytest integration looks for a “godkjenn_config” entry in the pytest INI configuration. Pytest configuration is fairly complex, so we’ll let its documentation tell you how to specify an INI file. In the end, though, the “godkjenn_config” entry should contain the filename of a TOML file containing the godkjenn configuration. This path is interpreted relative to the pytest root directory; if it’s not specified, godkjenn looks for “godkjenn.toml” (again in the pytest root directory). A missing config file is OK and is not treated as an error.

For example, your pytest.ini might look like this:

godkjenn_config = godkjenn.toml

Then your godkjenn.toml file might look like this:

vault_type = 'fs-vault'

This would specify that you want to use the ‘fs-vault’ plugin for storing results. If there were configuration options for the ‘fs-vault’ plugin, the TOML file might also include something like this:

option_a = 42
option_b = "so long and thanks for all the fish"

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