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Guidance for Project Agents

Project Overview

Pyre is Meta's performant type checker for Python (PEP 484 compliant).

It includes Pysa, a security-focused static analysis tool for taint analysis.

The core engine is written in OCaml (source/), with a Python CLI client (client/). Note: Pyrefly is the next-generation replacement.

Build and Test Commands

Detecting Meta Internal vs Open Source

Assume the user is a Meta internal user by default. If the facebook/ directory is missing from the root of the project, the user is an open source user. All buck and arc commands are only available to Meta internal users.

Configure

If the file source/dune is missing, this must be a fresh repository clone. In that case, run the following command:

# For Meta internal users (facebook/ directory exists)
./facebook/scripts/setup.sh --local
# For open source users
./scripts/setup.sh --local

Building the OCaml binary

cd source && make

The built binary is at source/_build/default/main.exe.

Running OCaml tests

# All OCaml tests
cd source && make test

# Specific OCaml test - e.g. analysis/test/integration/methodTest.ml:
cd source && dune exec analysis/test/integration/methodTest.exe

Running Python tests

# For Meta internal users
buck test fbcode//tools/pyre/client/...

Building the Python CLI (Buck)

# For Meta internal users
buck build fbcode//tools/pyre/client:pyre

Linting and Formatting

IMPORTANT: Always run linting and formatting after editing files.

For Meta internal users:

# Formatting
arc f
# Linting
arc lint

Testing against real code

Set PYRE_BINARY to your built binary to override the default:

export PYRE_BINARY=/path/to/source/_build/default/main.exe
pyre -n check  # Use one-off check mode, not the server, when recompiling frequently

Pyrefly Integration

Pyrefly is the default type provider for Pysa tests. The setup script (./facebook/scripts/setup.sh --local) automatically builds the Pyrefly binary and symlinks it to source/pyrefly.exe. After setup, make test uses Pyrefly automatically — no manual steps needed.

To manually rebuild the Pyrefly binary (Meta internal only):

cd source && make pyrefly

To override the Pyrefly binary path (e.g., for a custom build):

cd source && PYREFLY_BINARY=<path-to-binary> make test

Coding Style

  • Avoid abbreviations
  • Prefer snake_case for variables/functions, CamelCase for modules/classes