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#5 in Testing

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Apache-2.0

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insta

insta: a snapshot testing library for Rust

What are snapshot tests

Snapshots tests (also sometimes called approval tests) are tests that assert values against a reference value (the snapshot). This is similar to how assert_eq! lets you compare a value against a reference value but unlike simple string assertions snapshot tests let you test against complex values and come with comprehensive tools to review changes.

Snapshot tests are particularly useful if your reference values are very large or change often.

How it operates

This crate exports multiple macros for snapshot testing:

  • assert_snapshot! for comparing basic string snapshots.
  • assert_debug_snapshot! for comparing Debug outputs of values.
  • assert_display_snapshot! for comparing Display outputs of values.
  • assert_yaml_snapshot! for comparing YAML serialized output of types implementing serde::Serialize.
  • assert_ron_snapshot! for comparing RON serialized output of types implementing serde::Serialize. (requires the ron feature)
  • assert_json_snapshot! for comparing JSON serialized output of types implementing serde::Serialize.

Snapshots are stored in the snapshots folder right next to the test file where this is used. The name of the file is <module>__<name>.snap where the name of the snapshot. Snapshots can either be explicitly named or the name is derived from the test name.

Additionally snapshots can also be stored inline. In that case the cargo-insta tool is necessary. See inline snapshots for more information.

For macros that work with serde::Serialize this crate also permits redacting of partial values. See redactions for more information.

What it looks like:

This is what the cargo insta review UI looks like:

There is also a screencast that shows the entire workflow: watch the insta introduction screencast

Example

Install insta:

Recommended way if you have cargo-edit installed:

$ cargo add --dev insta

Alternatively edit your Cargo.toml manually and add insta as manual dependency.

And for an improved review experience also install cargo-insta:

$ cargo install cargo-insta
use insta::assert_debug_snapshot;

#[test]
fn test_snapshots() {
    let value = vec![1, 2, 3];
    assert_debug_snapshot!(value);
}

The recommended flow is to run the tests once, have them fail and check if the result is okay. By default the new snapshots are stored next to the old ones with the extra .new extension. Once you are satisifed move the new files over. To simplify this workflow you can use cargo insta review which will let you interactively review them:

$ cargo test
$ cargo insta review

For more information on updating see Snapshot Updating.

Snapshot files

The committed snapshot files will have a header with some meta information that can make debugging easier and the snapshot:

---
expression: "&User{id: Uuid::new_v4(), username: \"john_doe\".to_string(),}"
source: tests/test_user.rs
---
[
    1,
    2,
    3
]

Snapshot updating

During test runs snapshots will be updated according to the INSTA_UPDATE environment variable. The default is auto which will write all new snapshots into .snap.new files if no CI is detected.

INSTA_UPDATE modes:

  • auto: the default. no for CI environments or new otherwise
  • always: overwrites old snapshot files with new ones unasked
  • new: write new snapshots into .snap.new files.
  • no: does not update snapshot files at all (just runs tests)

When new is used as mode the cargo-insta command can be used to review the snapshots conveniently:

$ cargo install cargo-insta
$ cargo test
$ cargo insta review

"enter" or "a" accepts a new snapshot, "escape" or "r" rejects, "space" or "s" skips the snapshot for now.

For more information invoke cargo insta --help.

Test assertions

By default the tests will fail when the snapshot assertion fails. However if a test produces more than one snapshot it can be useful to force a test to pass so that all new snapshots are created in one go.

This can be enabled by setting INSTA_FORCE_PASS to 1:

$ INSTA_FORCE_PASS=1 cargo test --no-fail-fast

A better way to do this is to run cargo insta test --review which will run all tests with force pass and then bring up the review tool:

$ cargo insta test --review

Named snapshots

All snapshot assertion functions let you leave out the snapshot name in which case the snapshot name is derived from the test name (with an optional leading test_ prefix removed.

This works because the rust test runner names the thread by the test name and the name is taken from the thread name. In case your test spawns additional threads this will not work and you will need to provide a name explicitly.

Explicit snapshot naming can also otherwise be useful to be more explicit when multiple snapshots are tested within one function as the default behavior would be to just count up the snapshot names.

To provide an explicit name provide the name of the snapshot as first argument to the macro:

#[test]
fn test_something() {
    assert_snapshot!("first_snapshot", "first value");
    assert_snapshot!("second_snapshot", "second value");
}

This will create two snapshots: first_snapshot for the first value and second_snapshot for the second value. Without explicit naming the snapshots would be called something and something-2.

Redactions

Feature: redactions

For all snapshots created based on serde::Serialize output insta supports redactions. This permits replacing values with hardcoded other values to make snapshots stable when otherwise random or otherwise changing values are involved. Redactions became an optional feature in insta 0.11 and can be enabled with the redactions feature./

Redactions can be defined as the third argument to those macros with the syntax { selector => replacement_value }.

The following selectors exist:

  • .key: selects the given key
  • ["key"]: alternative syntax for keys
  • [index]: selects the given index in an array
  • []: selects all items on an array
  • [:end]: selects all items up to end (excluding, supports negative indexing)
  • [start:]: selects all items starting with start
  • [start:end]: selects all items from start to end (end excluding, supports negative indexing).
  • .*: selects all keys on that depth

Example usage:

#[derive(Serialize)]
pub struct User {
    id: Uuid,
    username: String,
    extra: HashMap<String, String>,
}

assert_yaml_snapshot!(&User {
    id: Uuid::new_v4(),
    username: "john_doe".to_string(),
    extra: {
        let mut map = HashMap::new();
        map.insert("ssn".to_string(), "123-123-123".to_string());
        map
    },
}, {
    ".id" => "[uuid]",
    ".extra.ssn" => "[ssn]"
});

Inline Snapshots

Additionally snapshots can also be stored inline. In that case the format for the snapshot macros is assert_snapshot!(reference_value, @"snapshot"). The leading at sign (@) indicates that the following string is the reference value. cargo-insta will then update that string with the new value on review.

Example:

#[derive(Serialize)]
pub struct User {
    username: String,
}

assert_yaml_snapshot!(User {
    username: "john_doe".to_string(),
}, @"");

After the initial test failure you can run cargo insta review to accept the change. The file will then be updated automatically.

Features

The following features exist:

  • ron: enables RON support (assert_ron_snapshot!)
  • redactions: enables support for redactions

Settings

There are some settings that can be changed on a per-thread (and thus per-test) basis. For more information see settings.

Legacy Snapshot Formats

With insta 0.11 the snapshot format was improved for inline snapshots. The old snapshot format will continue to be available but if you want to upgrade them make sure the tests pass first and then run the following command to force a rewrite of them all:

$ cargo insta test --accept --force-update-snapshots

License: Apache-2.0

Dependencies

~3.5–4.5MB
~103K SLoC