ruff/crates/ty_test

Writing type-checking / type-inference tests

Any Markdown file can be a test suite.

In order for it to be run as one, ty_test::run must be called with its path; see crates/ty_python_semantic/tests/mdtest.rs for an example that treats all Markdown files under a certain directory as test suites.

A Markdown test suite can contain any number of tests. A test consists of one or more embedded "files", each defined by a triple-backticks fenced code block. The code block must have a tag string specifying its language. We currently support py (Python files) and pyi (type stub files), as well as typeshed VERSIONS files and toml for configuration.

The simplest possible test suite consists of just a single test, with a single embedded file:

```py
reveal_type(1)  # revealed: Literal[1]
```

When running this test, the mdtest framework will write a file with these contents to the default file path (/src/mdtest_snippet.py) in its in-memory file system, run a type check on that file, and then match the resulting diagnostics with the assertions in the test. Assertions are in the form of Python comments. If all diagnostics and all assertions are matched, the test passes; otherwise, it fails.

See actual example mdtest suites in crates/ty_python_semantic/resources/mdtest.

Note

If you use dir-test, rstest or similar to generate a separate test for all Markdown files in a certain directory, as with the example in crates/ty_python_semantic/tests/mdtest.rs, you will likely want to also make sure that the crate the tests are in is rebuilt every time a Markdown file is added or removed from the directory. See crates/ty_python_semantic/build.rs for an example of how to do this.

This is because these macros generate their tests at build time rather than at runtime. Without the build.rs file to force a rebuild when a Markdown file is added or removed, a new Markdown test suite might not be run unless some other change in the crate caused a rebuild following the addition of the new test file.

Assertions

Two kinds of assertions are supported: # revealed: (shown above) and # error:.

Assertion kinds

revealed

A # revealed: assertion should always be paired with a call to the reveal_type utility, which reveals (via a diagnostic) the inferred type of its argument (which can be any expression). The text after # revealed: must match exactly with the displayed form of the revealed type of that expression.

The reveal_type function can be imported from the typing standard library module (or, for older Python versions, from the typing_extensions pseudo-standard-library module¹):

from typing import reveal_type

reveal_type("foo")  # revealed: Literal["foo"]

For convenience, type checkers also pretend that reveal_type is a built-in, so that this import is not required. Using reveal_type without importing it issues a diagnostic warning that it was used without importing it, in addition to the diagnostic revealing the type of the expression.

The # revealed: assertion must always match a revealed-type diagnostic, and will also match the undefined-reveal diagnostic, if present, so it's safe to use reveal_type in tests either with or without importing it. (Style preference is to not import it in tests, unless specifically testing something about the behavior of importing it.)

error

A comment beginning with # error: is an assertion that a type checker diagnostic will be emitted, with text span starting on that line. The matching can be narrowed in three ways:

• # error: [invalid-assignment] requires that the matched diagnostic have the rule code invalid-assignment. (The square brackets are required.)
• # error: "Some text" requires that the diagnostic's full message contain the text Some text. (The double quotes are required in the assertion comment; they are not part of the matched text.)
• # error: 8 [rule-code] or # error: 8 "Some text" additionally requires that the matched diagnostic's text span begins on column 8 (one-indexed) of this line.

Assertions must contain either a rule code or a contains-text, or both, and may optionally also include a column number. They must come in order: first column, if present; then rule code, if present; then contains-text, if present. For example, an assertion using all three would look like # error: 8 [invalid-assignment] "Some text".

Error assertions in tests intended to test type checker semantics should primarily use rule-code assertions, with occasional contains-text assertions where needed to disambiguate or validate some details of the diagnostic message.

Assertion locations

An assertion comment may be a line-trailing comment, in which case it applies to the line it is on:

x: str = 1  # error: [invalid-assignment]

Or it may be a comment on its own line, in which case it applies to the next line that does not contain an assertion comment:

# error: [invalid-assignment]
x: str = 1

Multiple assertions applying to the same line may be stacked:

# error: [invalid-assignment]
# revealed: Literal[1]
x: str = reveal_type(1)

Intervening empty lines or non-assertion comments are not allowed; an assertion stack must be one assertion per line, immediately following each other, with the line immediately following the last assertion as the line of source code on which the matched diagnostics are emitted.

Literate style

If multiple code blocks (without an explicit path, see below) are present in a single test, they will be merged into a single file in the order they appear in the Markdown file. This allows for tests that interleave code and explanations:

# My literate test

This first snippet here:

```py
from typing import Literal

def f(x: Literal[1]):
    pass
```

will be merged with this second snippet here, i.e. `f` is defined here:

```py
f(2)  # error: [invalid-argument-type]
```

Diagnostic Snapshotting

In addition to inline assertions, one can also snapshot the full diagnostic output of a test. This is done by adding a <!-- snapshot-diagnostics --> directive in the corresponding section. For example:

## Unresolvable module import

<!-- snapshot-diagnostics -->

```py
import zqzqzqzqzqzqzq  # error: [unresolved-import] "Cannot resolve import `zqzqzqzqzqzqzq`"
```

The snapshot-diagnostics directive must appear before anything else in the section.

This will use insta to manage an external file snapshot of all diagnostic output generated.

Inline assertions, as described above, may be used in conjunction with diagnostic snapshotting.

At present, there is no way to do inline snapshotting or to request more granular snapshotting of specific diagnostics.

Multi-file tests

Some tests require multiple files, with imports from one file into another. For this purpose, tests can specify explicit file paths in a separate line before the code block (b.py below):

```py
from b import C
reveal_type(C)  # revealed: Literal[C]
```

`b.py`:

```py
class C: pass
```

Relative file names are always relative to the "workspace root", which is also an import root (that is, the equivalent of a runtime entry on sys.path).

The default workspace root is /src/. Currently it is not possible to customize this in a test, but this is a feature we will want to add in the future.

So the above test creates two files, /src/mdtest_snippet.py and /src/b.py, and sets the workspace root to /src/, allowing imports from b.py using the module name b.

Multi-test suites

A single test suite (Markdown file) can contain multiple tests, by demarcating them using Markdown header lines:

# Same-file invalid assignment

```py
x: int = "foo"  # error: [invalid-assignment]
```

# Cross-file invalid assignment

```py
from b import y
x: int = y  # error: [invalid-assignment]
```

`b.py`:

```py
y = "foo"
```

This test suite contains two tests, one named "Same-file invalid assignment" and the other named "Cross-file invalid assignment". The first test involves only a single embedded file, and the second test involves two embedded files.

The tests are run independently, in independent in-memory file systems and with new ty Salsa databases. This means that each is a from-scratch run of the type checker, with no data persisting from any previous test.

It is possible to filter to individual tests within a single markdown file using the MDTEST_TEST_FILTER environment variable. This variable will match any tests which contain the value as a case-sensitive substring in its name. An example test name is unpacking.md - Unpacking - Tuple - Multiple assignment, which contains the name of the markdown file and its parent headers joined together with hyphens.

Structured test suites

Markdown headers can also be used to group related tests within a suite:

# Literals

## Numbers

### Integer

```py
reveal_type(1)  # revealed: Literal[1]
```

### Float

```py
reveal_type(1.0)  # revealed: float
```

## Strings

```py
reveal_type("foo")  # revealed: Literal["foo"]
```

This test suite contains three tests, named "Literals - Numbers - Integer", "Literals - Numbers - Float", and "Literals - Strings".

A header-demarcated section must either be a test or a grouping header; it cannot be both. That is, a header section can either contain embedded files (making it a test), or it can contain more deeply-nested headers (headers with more #), but it cannot contain both.

Configuration

The test framework supports a TOML-based configuration format, which is a subset of the full ty configuration format. This configuration can be specified in fenced code blocks with toml as the language tag:

```toml
[environment]
python-version = "3.10"
```

This configuration will apply to all tests in the same section, and all nested sections within that section. Nested sections can override configurations from their parent sections.

To enable logging in an mdtest, set log = true at the top level of the TOML block. See MarkdownTestConfig for the full list of supported configuration options.

Specifying a custom typeshed

Some tests will need to override the default typeshed with custom files. The [environment] configuration option typeshed can be used to do this:

```toml
[environment]
typeshed = "/typeshed"
```

For more details, take a look at the custom-typeshed Markdown test.

Mocking a Python environment

Mdtest supports mocking a Python environment for a specific test at an arbitrary location, again using the [environment] configuration option:

```toml
[environment]
python = ".venv"
```

Mdtest also makes it easy to write Python packages to the mock environment's site-packages directory using the <path-to-site-packages> magic path segment. This would otherwise be hard, due to the fact that the site-packages subdirectory in an environment is located at a different relative path depending on the platform. In the following test, mdtest will write the Python file to .venv/Lib/site-packages/foo.py in its in-memory filesystem used for the test if the test is being executed on Windows, and .venv/lib/python3.13/site-packages/foo.py otherwise:

```toml
[environment]
python = ".venv"
python-version = "3.13"
```

`.venv/<path-to-site-packages>/foo.py`:

```py
X = 1
```

Documentation of tests

Arbitrary Markdown syntax (including of course normal prose paragraphs) is permitted (and ignored by the test framework) between fenced code blocks. This permits natural documentation of why a test exists, and what it intends to assert:

Assigning a string to a variable annotated as `int` is not permitted:

```py
x: int = "foo"  # error: [invalid-assignment]
```

Running the tests

All Markdown-based tests are executed in a normal cargo test / cargo run nextest run. If you want to run the Markdown tests only, you can filter the tests using mdtest__:

cargo test -p ty_python_semantic -- mdtest__

Alternatively, you can use the mdtest.py runner which has a watch mode that will re-run corresponding tests when Markdown files change, and recompile automatically when Rust code changes:

uv run crates/ty_python_semantic/mdtest.py

Planned features

There are some designed features that we intend for the test framework to have, but have not yet implemented:

Multi-line diagnostic assertions

We may want to be able to assert that a diagnostic spans multiple lines, and to assert the columns it begins and/or ends on. The planned syntax for this will use <<< and >>> to mark the start and end lines for an assertion:

(3  # error: 2 [unsupported-operands] <<<
  +
 "foo")  # error: 6 >>>

The column assertion 6 on the ending line should be optional.

In cases of overlapping such assertions, resolve ambiguity using more angle brackets: <<<< begins an assertion ended by >>>>, etc.

Configuring search paths and kinds

The ty TOML configuration format hasn't been finalized, and we may want to implement support in the test framework for configuring search paths before it is designed. If so, we can define some configuration options for now under the [tests] namespace. In the future, perhaps some of these can be replaced by real ty configuration options; some or all may also be kept long-term as test-specific options.

Some configuration options we will want to provide:

• We should be able to configure the default workspace root to something other than /src/ using a workspace-root configuration option.

• We should be able to add a third-party root using the third-party-root configuration option.

• We may want to add additional configuration options for setting additional search path kinds.

Paths for workspace-root and third-party-root must be absolute.

Relative embedded-file paths are relative to the workspace root, even if it is explicitly set to a non-default value using the workspace-root config.

I/O errors

We could use an error= configuration option in the tag string to make an embedded file cause an I/O error on read.

Asserting on full diagnostic output

Note

At present, one can opt into diagnostic snapshotting that is managed via external files. See the section above for more details. The feature outlined below, inline diagnostic snapshotting, is still desirable.

The inline comment diagnostic assertions are useful for making quick, readable assertions about diagnostics in a particular location. But sometimes we will want to assert on the full diagnostic output of checking an embedded Python file. Or sometimes (see “incremental tests” below) we will want to assert on diagnostics in a file, without impacting the contents of that file by changing a comment in it. In these cases, a Python code block in a test could be followed by a fenced code block with language output; this would contain the full diagnostic output for the preceding test file:

# full output

```py
x = 1
reveal_type(x)
```

This is just an example, not a proposal that ty would ever actually output diagnostics in
precisely this format:

```output
mdtest_snippet.py, line 1, col 1: revealed type is 'Literal[1]'
```

We will want to build tooling to automatically capture and update these “full diagnostic output” blocks, when tests are run in an update-output mode (probably specified by an environment variable.)

By default, an output block will specify diagnostic output for the file <workspace-root>/mdtest_snippet.py. An output block can be prefixed by a <path>: label as usual, to explicitly specify the Python file for which it asserts diagnostic output.

It is an error for an output block to exist, if there is no py or python block in the same test for the same file path.

Incremental tests

Some tests should validate incremental checking, by initially creating some files, checking them, and then modifying/adding/deleting files and checking again.

We should add the capability to create an incremental test by using the stage= option on some fenced code blocks in the test:

# Incremental

## modify a file

Initial file contents:

```py
from b import x
reveal_type(x)
```

`b.py`:

```py
x = 1
```

Initial expected output for the unnamed file:

```output
/src/mdtest_snippet.py, line 1, col 1: revealed type is 'Literal[1]'
```

Now in our first incremental stage, modify the contents of `b.py`:

`b.py`:

```py stage=1
# b.py
x = 2
```

And this is our updated expected output for the unnamed file at stage 1:

```output stage=1
/src/mdtest_snippet.py, line 1, col 1: revealed type is 'Literal[2]'
```

(One reason to use full-diagnostic-output blocks in this test is that updating inline-comment
diagnostic assertions for `mdtest_snippet.py` would require specifying new contents for
`mdtest_snippet.py` in stage 1, which we don't want to do in this test.)

It will be possible to provide any number of stages in an incremental test. If a stage re-specifies a filename that was specified in a previous stage (or the initial stage), that file is modified. A new filename appearing for the first time in a new stage will create a new file. To delete a previously created file, specify that file with the tag delete in its tag string (in this case, it is an error to provide non-empty contents). Any previously-created files that are not re-specified in a later stage continue to exist with their previously-specified contents, and are not "touched".

All stages should be run in order, incrementally, and then the final state should also be re-checked cold, to validate equivalence of cold and incremental check results.

---

1. typing-extensions is a third-party module, but typeshed, and thus type checkers also, treat it as part of the standard library. ↩︎