Welcome to inline-snapshot

inline-snapshot can be used for different things:

• golden master/approval/snapshot testing. The idea is that you have a function with a currently unknown result and you want to write a tests, which ensures that the result does not change during refactoring.
• Compare things which are complex like lists with lot of numbers or complex data structures.
• Things which might change during the development like error messages.

inline-snapshot automates the process of recording, storing and updating the value you want to compare with. The value is converted with repr() and stored in the source file as argument of the snapshot() function.

Usage

You can use snapshot() instead of the value which you want to compare with and run the tests to record the correct values.

original code--inline-snapshot=create

from inline_snapshot import snapshot


def something():
    return 1548 * 18489


def test_something():
    assert something() == snapshot()

from inline_snapshot import snapshot


def something():
    return 1548 * 18489


def test_something():
    assert something() == snapshot(28620972)

Your tests will break, if you change your code by adding // 18. Maybe that is correct and you should fix your code, or your code is correct and you want to update your test results.

changed code--inline-snapshot=fix

from inline_snapshot import snapshot


def something():
    return (1548 * 18489) // 18


def test_something():
    assert something() == snapshot(28620972)

from inline_snapshot import snapshot


def something():
    return (1548 * 18489) // 18


def test_something():
    assert something() == snapshot(1590054)

Please verify the new results. git diff will give you a good overview over all changed results. Use pytest -k test_something --inline-snapshot=fix if you only want to change one test.

Supported operations

You can use snapshot(x) like you can use x in your assertion with a limited set of operations:

• value == snapshot() to compare with something,
• value <= snapshot() to ensure that something gets smaller/larger over time (number of iterations of an algorithm you want to optimize for example),
• value in snapshot() to check if your value is in a known set of values,
• snapshot()[key] to generate new sub-snapshots on demand.

Warning

One snapshot can only be used with one operation. The following code will not work:

from inline_snapshot import snapshot


def test_something():
    s = snapshot(5)
    assert 5 <= s
    assert 5 == s


# Error:
# >       assert 5 == s
# E       TypeError: This snapshot cannot be use with `==`, because it was previously used with `x <= snapshot`

Supported usage

It is possible to place snapshot() anywhere in the tests and reuse it multiple times.

original code--inline-snapshot=create

from inline_snapshot import snapshot


def something():
    return 21 * 2


result = snapshot()


def test_something():
    ...
    assert something() == result


def test_something_again():
    ...
    assert something() == result

from inline_snapshot import snapshot


def something():
    return 21 * 2


result = snapshot(42)


def test_something():
    ...
    assert something() == result


def test_something_again():
    ...
    assert something() == result

snapshot() can also be used in loops:

original code--inline-snapshot=create

from inline_snapshot import snapshot


def test_loop():
    for name in ["Mia", "Eva", "Leo"]:
        assert len(name) == snapshot()

from inline_snapshot import snapshot


def test_loop():
    for name in ["Mia", "Eva", "Leo"]:
        assert len(name) == snapshot(3)

or passed as an argument to a function:

original code--inline-snapshot=create

from inline_snapshot import snapshot


def check_string_len(string, snapshot_value):
    assert len(string) == snapshot_value


def test_string_len():
    check_string_len("abc", snapshot())
    check_string_len("1234", snapshot())
    check_string_len(".......", snapshot())

from inline_snapshot import snapshot


def check_string_len(string, snapshot_value):
    assert len(string) == snapshot_value


def test_string_len():
    check_string_len("abc", snapshot(3))
    check_string_len("1234", snapshot(4))
    check_string_len(".......", snapshot(7))

Feedback

inline-snapshot provides some advanced ways to work with snapshots.

I would like to know how these features are used to further improve this small library. Let me know if you've found interesting use cases for this library via twitter, fosstodon or in the github discussions.

Sponsors

I would like to thank my sponsors. Without them, I would not be able to invest so much time in my projects.

Bronze sponsor 🥉

Issues

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