Trezor Core coding style

Python coding style

Run make pystyle from repository root to perform all style checks and auto-format where possible.

General style notes

See rules for exceptions in the Exceptions documentation.

Type annotations

We prefer Python 3.10 style annotations:

instead of List[int], use list[int], dtto for Tuple, Dict and Set
instead of Optional[int], use int | None
instead of Union[int, str], use int | str

This also applies inside if TYPE_CHECKING branches.

Type-checking imports

At run-time, the typing module is not available. There is compile-time magic that removes all from typing imports and contents of if TYPE_CHECKING branches.

It is important to put typing-only imports into if TYPE_CHECKING, to make sure that these modules are not needlessly pulled in at run-time.

Due to the compile-time magic, it is always possible to put a from typing import at top level. The style for doing that are as follows:

If the module needs to import other modules, create type aliases, TypeVars or Protocols, the only top-level import should be TYPE_CHECKING. Everything else (including other items from typing module) should be imported in the TYPE_CHECKING branch:
```
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing import Any, TypeVar, Union
    from trezor.messages import SomeMessage

    TypeAlias = Union[int, str]
    T = TypeVar("T")
```
If the module only needs items from typing, you should not create a TYPE_CHECKING branch, and instead import all required items on top level:
```
from typing import Any, Iterator, Sequence
```

Tools

Configurations of specific Python style tools (isort, flake8, pylint) can be found in root setup.cfg.

Formatting

We are auto-formatting code with black and use the black code style.

We use isort to organize imports.

Linting

We use flake8 lints, disabling only those that conflict with black code style.

We use a select subset of pylint checks that are hard-enforced.

Type checking

We use pyright for type-checking. The codebase is fully type-checked, except for the Monero app (as of 2022-01).

C coding style

Formatting is done by clang-format. We are using the Google code style.

Run make cstyle from repository root to auto-format.

Rust coding style

Formatting is done by rustfmt. We are using the Rust style.

Run make ruststyle from repository root to auto-format.

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