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Tach helps Python teams prevent accidental coupling by checking which modules may import one another and which parts of a module are public. Add it with pip install tach, initialize boundaries using tach init, and run tach check to find violations. Its rules live in tach.toml and can be enforced in CI or pre-commit.
What Tach enforces
Tach is an open-source command-line tool for Python projects, implemented in Rust. It checks declared module dependencies, whether imports use public interfaces, and cycles in the dependency graph. The project says Tach has no runtime impact; that is a project claim, not a benchmark result. See the official README.
These checks address related but distinct architecture questions: which modules may depend on other modules, and which imports from an allowed module are part of its supported surface. An allowed module dependency does not necessarily grant permission to import that module’s internals.
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From the project environment, install Tach with
pip install tach. -
Run
tach initfrom the project root. Its interactive setup helps mark module boundaries. You can begin with top-level packages or select only the modules you want to govern. -
Review and edit
tach.tomlto describe allowed dependencies and public interfaces. If your Python import roots are under a subdirectory such asbackend/, configuresource_roots; it defaults to the project root (.) when omitted. -
Run
tach checkand review any reported imports. Decide whether the code should change or the intended architecture rule should be adjusted.DriversOutdated Drivers Are Slowing You DownPerformanceWindows Errors? Fix Them Before They SpreadDriversCrashes, No Sound, or Screen Glitches?Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy. -
Add the check to CI or a pre-commit workflow if you want violations caught during routine development. Tach documents both integration approaches in its README.
Declare dependencies separately from public interfaces
Tach’s configuration distinguishes a module’s dependency rules from an interface’s public import patterns. The configuration options and their syntax are documented in the configuration reference.
Allow or forbid module dependencies
Use depends_on to list modules a module is allowed to import. Use cannot_depend_on to forbid particular imports; prohibitions take precedence when a pattern could match both settings. Patterns can cover groups of modules.
If a module omits depends_on, Tach treats its dependencies permissively: it may depend on any other module, subject to the other module’s public-interface rules. Omission therefore does not grant access to every internal import.
The configuration also supports visibility, utility modules, unchecked modules, and deprecating individual dependencies. These options let teams express exceptions or transitional rules, but they should be chosen deliberately so an exception does not quietly become the architecture.
Expose stable import surfaces
Define interfaces separately, using an interface’s expose patterns to identify imports that consumers may treat as public. Optional from, visibility, and exclusive settings let you scope which consumers adopt or may use an interface.
This separation is useful when one module is allowed to depend on another but should import only its stable surface, not implementation details. It makes the intended boundary explicit rather than relying on conventions that contributors may not know.
Use layers when dependency direction is the main rule
Layers express a high-to-low architecture: higher layers may import from lower layers, but lower layers may not import from higher ones. They provide a concise way to describe direction across tiers, while exact module dependency declarations remain a separate choice.
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By default, a higher layer may use modules in lower layers without listing each one in depends_on. Modules in the same layer still need explicit dependency declarations. Set layers_explicit_depends_on = true if cross-layer imports must also be declared explicitly. A layer can be marked closed to prevent higher layers from bypassing it and importing directly from still-lower layers.
The documented UI, commands, and core example illustrates the distinction: commands can use core without listing every dependency by default, while modules within core still declare their dependencies. Layer direction alone is not the same as a complete inventory of allowed module-to-module imports. See the layers documentation.
Inspect dependencies and investigate violations
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tach showgenerates a local DOT dependency graph that can help you see the relationships represented by the configuration. -
tach reportinspects dependencies and usages for a path, which can help explain how a particular part of the codebase participates in the graph.Quick wins for a faster PC:
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tach show --webgenerates the graph remotely. The README says this sends the contents oftach.tomlto the remote graph-generation service, so consider whether that is appropriate for your project before using it.
These commands complement tach check: graph and report output help explore structure, while the check is the documented enforcement step and returns a non-zero exit status for violations, making it suitable for automated workflows. Command guidance is in the official README.
Monorepos: distinguish module rules from package dependencies
Tach’s configuration documentation describes multiple source roots for monorepos and namespace packages. Within a Python package, module rules govern internal imports. Separately, tach check-external checks whether imports between packages are declared in the relevant package’s pyproject.toml. For example, if a consumer package imports a utility package, the consumer’s package dependencies should declare it.
Those checks answer different questions: an internal module graph is not a substitute for declaring package-level dependencies. The documentation covers source roots and external dependency checks in the configuration reference.
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Where Tach fits—and what it does not establish
Tach is useful when a team wants executable rules for module dependencies, public import surfaces, and dependency cycles, rather than relying only on review conventions. It does not by itself define the right architecture: the team must decide its module boundaries, exceptions, and stable interfaces.
A public PostHog architecture document describes using Tach interfaces to govern imports across product boundaries and running checks in CI. That document also describes a separate tool for checking structure inside a product. It is one implementation example, not evidence that Tach covers every architectural concern or that every team needs the same division of tools.
The official README and documentation are rolling pages rather than version-pinned release notes, so consult them for the configuration supported by the Tach version you install. They do not provide a quantitative benchmark necessary to assess this use case.
Quick Recap
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