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

​
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 - see details.

interfaces defines the interfaces of modules in your project (optional) - see details.

layers defines the layers of modules in your project (optional) - see details.

exclude accepts a list of directory patterns to exclude from checking. These should be glob paths which match from the beginning of a given file path. For example: project/*.tests would match any path beginning with project/ and ending with .tests.

Tach uses forward slashes to match path separators, even on Windows.

ignore_type_checking_imports (default: true) is a flag which silences tach check failures caused by imports under a TYPE_CHECKING conditional block.

exact (default: false) is a flag which causes tach check to fail if any declared dependencies are found to be unused.

forbid_circular_dependencies (default: false) is a flag which 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.

[DEPRECATED] use_regex_matching (default: false) is a flag which controls how exclude patterns are interpreted. By default, exclude patterns are interpreted as globs. But when this flag is true, exclude patterns are interpreted as regex.

The use_regex_matching configuration option will be removed in a future version of Tach. If you have already customized your exclude list, it is likely that you will need to update your patterns to globs.
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

layers = [
  "ui",
  "commands",
  "core"
]

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

[[modules]]
path = "tach.__main__"
layer = "ui"

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

[[modules]]
path = "tach.parsing"
depends_on = ["tach", "tach.filesystem"]
layer = "core"
visibility = ["tach.check"]

[[modules]]
path = "tach.check"
depends_on = [
    "tach.extension",
    "tach.filesystem",
    "tach.parsing",
]
layer = "commands"

[[interfaces]]
expose = ["types.*"]

[[interfaces]]
expose = [
    "parse_project_config",
    "dump_project_config_to_toml",
]
from = [
    "tach.parsing",
]

...

[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)

A module can also define paths as a shorthand for multiple module definitions. This allows specifying allowed dependencies and other attributes as a group.

Example: paths = ["a.b", "a.c"]

  • depends_on a list of the other modules which this module can import from
Omitting the depends_on field means the module will be allowed to import from any other module. However, it will still be subject to those modules’ public interfaces.
  • layer (default: null) the layer which holds this module
  • visibility (default: ['*']) a list of other modules which can import from this module
  • utility (default: false) marks this module as a Utility, meaning all other modules may import from it without declaring an explicit dependency
  • unchecked (default: false) marks this module as unchecked, meaning Tach will not check its imports

Tach also supports deprecating individual dependencies.

​
Interfaces

Public interfaces are defined separately from modules, and define the imports that are allowed from that module.

For example, if a module should expose everything from a nested ‘services’ folder, the config would look like:

[[interfaces]]
expose = ["services.*"]
from = ["my_module"]

More specifically:

  • expose: a list of regex patterns which define the public interface
  • from (optional): a list of regex patterns which define the modules which adopt this interface

More details here.

If an interface entry does not specify from, all modules will adopt the interface.

A module can match multiple interface entries - if an import matches any of the entries, it will be considered valid.

​
Layers

An ordered list of layers can be configured at the top level of tach.toml, and modules can each be assigned to a specific layer.

layers = [
  "ui",
  "commands",
  "core"
]

[[modules]]
path = "tach.check"
layer = "commands"

[[modules]]
path = "tach.cache"
layer = "core"

The configuration above defines three layers, with ui being the highest layer, and core being the lowest layer. It also tags tach.check as a module in the commands layer, and tach.cache in core.

More details here.

​
The Root Module

By default, Tach checks all of the source files beneath all of the configured source roots, and will ignore dependencies which are not contained by modules.

However, Tach allows configuration of how to treat code which is within a source root, but not contained by a 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 would 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:

  • (permissive default) "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) "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.
  • (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.

​
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.

​
tach.domain.toml

Tach allows splitting your configuration into ‘domains’, or sub-folders of your project. You can define modules and interfaces in a tach.domain.toml file which lives right next to the module code itself.

This enables multiple developers or teams to independently own and maintain their modules and interfaces.

​
Modules

Within a tach.domain.toml file, you can define modules similar to modules in tach.toml. The key difference is that paths are relative to the location of the domain by default, and there is a [root] module syntax for describing the parent folder itself.

For example, a tach.toml file containing the module definitions:

[[modules]]
path = "tach.filesystem"
depends_on = [
  "tach.hooks",
  "tach.filesystem.service",
]

[[modules]]
path = "tach.filesystem.service"
depends_on = []

could be expressed instead in a tach.domain.toml file at <source root>/tach/filesystem/tach.domain.toml:

[root]  # This defines "tach.filesystem"
depends_on = [
  "//tach.hooks",  # This refers to "tach.hooks" (outside of this domain)
  "service",  # This refers to "tach.filesystem.service"
]

[[modules]]
path = "service"  # This defines "tach.filesystem.service"
depends_on = []

Note that the tach.domain.toml file uses its location relative to the nearest source root to implicitly determine its [root] module path ("tach.filesystem").

The domain configuration also uses special syntax to refer to dependencies on module paths outside of the domain, prefixing the absolute path with "//".

Module paths are generally interpreted relative to the location of the tach.domain.toml file, which can be seen in the definition for the "service" module. This path is interpreted relative to the domain root, meaning it refers to the "tach.filesystem.service" module.

​
Interfaces

Interfaces are defined in largely the same way as in tach.toml, with the key difference being the treatment of paths in the from field. These will be interpreted as paths relative to the domain root.

For example, a tach.toml file containing the interface definitions:

[[interfaces]]
expose = ["service.*"]
from = ["tach.filesystem"]

[[interfaces]]
expose = ["client.*"]
from = ["tach.filesystem.git_ops"]

could be expressed instead in a tach.domain.toml file at <source root>/tach/filesystem/tach.domain.toml:

[[interfaces]]
expose = ["service.*"]
from = ["<domain_root>"]  # This matches "tach.filesystem"

[[interfaces]]
expose = ["client.*"]
from = ["git_ops"]  # This matches "tach.filesystem.git_ops"

​
Example: CODEOWNERS

Tach domain configuration files enable smooth integration with CODEOWNERS:

# Domain ownership for different teams

/tach.toml    @platform-team

/payments/tach.domain.toml    @payments-team
/auth/tach.domain.toml    @auth-team
/data/analytics/tach.domain.toml    @analytics-team @data-team
/mobile/tach.domain.toml    @mobile-team
/libs/shared/tach.domain.toml    @platform-team

This allows a team to own their public interface, without imposing a bottleneck on other teams’ configuration changes.

​
External

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.

Example:

[external]
exclude = ["PIL"]

Tach also allows supplying a rename field to handle cases where the top level module name does not match the name of the package.

For example, the pillow package supplies the PIL module, so Tach needs to map imports from PIL to the pillow package specifier in your requirements.

[external]
rename = [
  # Format "[module name]:[package name]"
  "PIL:pillow",
  ...
]

In most cases you should not need to specify rename manually (see the Note below).

It is recommended to run Tach within a virtual environment containing all of your dependencies across all packages. This is because Tach uses the distribution metadata to map module names like ‘git’ to their distributions (‘GitPython’).

​
Rules

Tach allows configuring the severity of certain issues. Each entry in the rules table can be set to error, warn, or off.

The available rules and their defaults are listed below.

  • unused_ignore_directives (default: warn): catch tach-ignore comments which are unused (e.g. do not suppress any errors or warnings)
  • require_ignore_directive_reasons (default: off): require every tach-ignore comment to have a reason
  • unused_external_dependencies (default: error): catch declared 3rd party dependencies which are not imported in your code

​
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.

CommandsInterfaces