Configuration

Aside from running tach mod and tach sync, you can configure Tach by creating or modifying the configuration file as described below.

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tach.toml

This is the project-level configuration file which should be in the root of your project.

modules defines the modules in your project, and accepts the keys described below.

exclude accepts a list of directory patterns to exclude from checking. These are treated as regex/glob paths (depending on use_regex_matching) which match from the beginning of a given file path. For example: when using glob matching project/*.tests would match any path beginning with project/ and ending with .tests.

ignore_type_checking_imports is a boolean which, when enabled, silences tach check failures caused by imports under a TYPE_CHECKING conditional block.

exact is a boolean which causes tach check to fail if any declared dependencies are found to be unused.

forbid_circular_dependencies is a boolean which, when enabled, causes tach check to fail if any circular dependencies are detected.

root_module takes a string enum value, and determines how Tach treats code which lives within the project but is not covered by an explicit module. This is described in detail below

rules allows precise configuration of the severity of certain types of issues. See below for more details.

use_regex_matching is a boolean which, when enabled, uses regex (default) matching to exclude patterns else uses globs matching.

exclude = [
    ".*__pycache__",
    "build/",
    "dist/",
    "docs/",
    "python/tests/",
    "tach.egg-info/",
    "venv/",
]
source_roots = ["python"]
exact = true
ignore_type_checking_imports = true
forbid_circular_dependencies = true

root_module = "allow"

[[modules]]
path = "tach"
depends_on = []
strict = true

[[modules]]
path = "tach.__main__"
unchecked = true
depends_on = [{ path = "tach.start" }]

[[modules]]
path = "tach.errors"
depends_on = []
strict = true
utility = true

[[modules]]
path = "tach.parsing"
depends_on = [{ path = "tach" }, { path = "tach.filesystem" }]
visibility = ["tach.check"]
strict = true

[[modules]]
path = "tach.check"
depends_on = [
    { path = "tach.extension" },
    { path = "tach.filesystem" },
    { path = "tach.parsing" },
]
strict = true

...

[cache]
file_dependencies = ["python/tests/**", "src/*.rs"]

[external]
exclude = ["pytest"]

[rules]
unused_ignore_directives = "warn"

​

Modules

Each module listed under the modules key above can accept the following attributes:

• path: the Python import path to the module (e.g. a.b for <root>/a/b.py)
• depends_on: a list of the other modules which this module can import from (default: [])
• visibility: a list of other modules which can import from this module (default: ['*'])
• strict: enables strict mode for the module (boolean)
• utility: marks this module as a Utility, meaning all other modules may import from it without declaring an explicit dependency (boolean)
• unchecked: marks this module as unchecked, meaning Tach will not check its imports (boolean)

​

The Root Module

By default, Tach checks all of the source files beneath all of the configured source roots. This means that some code may not be contained within any configured module.

For example, given the file tree below:

my_repo/
  tach.toml
  script.py
  lib/
    module1.py
    module2/
      __init__.py
      service.py
    module3.py
  docs/
  tests/

If lib.module1, lib.module2, and lib.module3 are the only configured modules, then the code in script.py would be automatically part of the <root> module.

This module can declare its own dependencies with depends_on and use the rest of the available module configuration. Further, other modules need to declare an explicit dependency on <root> to use code which rolls up to the root.

Tach allows configuring how the root module should be treated through the root_module key in tach.toml. It may take one of the following values:

• (default) "allow": Treat <root> as a catch-all rollup module which must be explicitly declared as a dependency and must declare its own dependencies on other modules.
• (permissive) "ignore": Disable all checks related to the <root> module. tach check will never fail due to code in the <root> module, and tach sync will never add <root> to tach.toml
• (stricter) "dependenciesonly": Forbid any module from listing <root> as a dependency, but allow <root> to declare its own dependencies.
• (strictest) "forbid": Forbid any reference to the <root> module in tach.toml. This means that all code in source roots MUST be contained within an explicitly configured module.

​

Source Roots

The source_roots key is required for Tach to understand the imports within your project. If it is not set explicitly, source_roots defaults to your project root path: ['.']. This means Tach will expect that your Python imports are resolved relative to the directory in which tach.toml exists.

Below are typical cases in which modifying source_roots is necessary.

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Example: Python below project root

Suppose your repository contains a subfolder where all of your Python code lives. This could be a web server, a collection of serverless functions, or even utility scripts. In this example we will assume the Python code in our repo lives in the backend/ folder.

my_repo/
  tach.toml
  backend/
    module1.py
    module2/
      __init__.py
      service.py
    module3.py
  docs/
  tests/

In a Python module such as backend/module1.py, we can see imports from other modules.

# In backend/module1.py

import module3
from module2.service import MyService

Notice that these import paths (module3, module2.service.MyService) are rooted in the backend/ folder, NOT the project root.

To indicate this structure to Tach, set:

source_roots = ["backend"]

in your tach.toml, or use tach mod and mark the backend folder as the only source root.

​

Example: Monorepo

Suppose you work on a ‘monorepo’, in which Python packages which import from each other are located in distinct project directories. You may package your utility libraries in a utilityfolder, while your core packages live in core_one and core_two. You may also use a namespace package to share a common top-level namespace. In this example we’ll use myorg as the namespace package.

The file tree in a case like this might look like:

my_repo/
  tach.toml
  utility/
    pyproject.toml
    myorg/
      utils/
        __init__.py
  core_one/
    pyproject.toml
    myorg/
      core_one/
        __init__.py
        module1.py
        module2/
          __init__.py
          service.py
        module3.py
  core_two/
    pyproject.toml
    myorg/
      core_two/
        __init__.py
        module1.py
        module2/
          __init__.py
          service.py
        module3.py
  docs/
  tests/

In a Python module such as core_one/myorg/core_one/module1.py, there may be imports from other packages:

# In core_one/myorg/core_one/module1.py

from myorg.utils import utility_fn

Notice that this import path (myorg.utils.utility_fn) is rooted in the utility folder, NOT the project root.

To indicate the project structure to Tach, you would set:

source_roots = [
  "utility",
  "core_one",
  "core_two"
]

in your tach.toml, or use tach mod and mark the same folders as source root.

In tach.toml, each entry in source_roots is interpreted as a relative path from the project root.

​

Cache

Tach allows configuration of the computation cache it uses to speed up tasks like testing.

The file_dependencies key accepts a list of glob patterns to indicate additional file contents that should be considered when checking for cache hits. This should typically include files outside of your source roots which affect your project’s behavior under test, including the tests themselves. Additionally, if you have non-Python files which affect your project’s behavior (such as Rust or C extensions), these should be included as well.

The env_dependencies key accepts a list of environment variable names whose values affect your project’s behavior under test. This may include a DEBUG flag, or database connection parameters in the case of tests which use a configurable database.

​

External Checks

When running check-external, Tach allows excluding certain modules from validation.

Adding the top level module name to the exclude key (underneath the external key) will allow all usages of the corresponding module.

​

Rules

Tach allows configuring the severity of certain issues.

The unused_ignore_directives rule determines whether unnecessary tach-ignore comments trigger a warning, an error, or nothing at all.

Example:

# tach-ignore valid_import_fn
from other.valid import valid_import_fn

[rules]
# "warn" is the default for this rule,
# other options are "error", "off"
unused_ignore_directives = "warn"