A module can enable ‘strict mode’ by setting strict: true in its configuration within tach.toml.

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How does it work?

When a module is in strict mode, other modules may only import names declared in __all__. For Python packages marked as modules, __all__ is checked within __init__.py. This creates an explicit public interface for the module which prevents coupling to implementation details, and makes future changes easier.

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Example

Given modules called ‘core’ and ‘parsing’, we may have tach.toml contents like this:

# tach.toml
[[modules]]
path = "parsing"
depends_on = [
    { path = "core" }
]
strict = true

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

Then, in parsing.py, we may have:

from core.main import get_data  # This import fails

get_data()

This import would fail tach check with the following error:

❌ parsing.py[L1]: Module 'core' is in strict mode. Only imports from the public interface of this module are allowed. The import 'core.main.get_data' (in module 'parsing') is not included in __all__.

If get_data should actually be part of the public interface of ‘core’, it needs to be specified in __all__ of core/__init__.py or core.py:

core/__init__.py

from .main import get_data

__all__ = ["get_data"]

which would allow ‘parsing’ to depend on this interface:

from core import get_data  # This import is OK

get_data()

tach check will now pass!

✅ All module dependencies validated!
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