Pytype

Statically check and infer types for unannotated Python code. (This is not an official Google product.)

Abstract

Pytype is a static analyzer that helps you find type errors in Python code. It can type-check code with or without type annotations, as well as generate annotations. Pytype runs under Python 2.7 or 3.5+ and analyzes both Python 2 and Python 3 code.

Example

Below, print_greeting calls make_greeting incorrectly:

$ cat foo.py

def make_greeting(user_id):
    return 'hello, user' + user_id

def print_greeting():
    print(make_greeting(0))

Run pytype to catch the bug:

$ pytype foo.py

File "foo.py", line 2, in make_greeting: Function str.__add__ was called with the wrong arguments [wrong-arg-types]
  Expected: (self, y: str)
  Actually passed: (self, y: int)
Traceback:
  line 5, in print_greeting

Merge pytype's generated type information back into foo.py:

$ cat pytype_output/foo.pyi

def make_greeting(user_id) -> str: ...
def print_greeting() -> None: ...

$ merge-pyi -i foo.py pytype_output/foo.pyi
$ cat foo.py

def make_greeting(user_id) -> str:
    return 'hello, user' + user_id

def print_greeting() -> None:
    print(make_greeting(0))

Requirements

You need a Python 2.7 or 3.5+ interpreter to run pytype, as well as an interpreter in $PATH for the Python version of the code you're analyzing.

Platform support:

• Pytype is currently developed and tested on Linux, which is the main supported platform.
• Installation on MacOSX requires OSX 10.7 or higher and Xcode v8 or higher.
• Windows is currently not supported.

Quickstart resources

The rest of this document provides complete instructions for installing and using pytype. To quickly get started with some common workflows, check out the following docs:

• Quickstart
• Error classes

Installing

Pytype can be installed via pip. Note that the installation requires wheel and setuptools. (If you're working in a virtualenv, these two packages should already be present.)

pip install pytype

Or from the source code on GitHub.

git clone --recurse-submodules https://github.com/google/pytype.git
cd pytype
pip install -U .

Instead of using --recurse-submodules, you could also have run

git submodule init
git submodule update

in the pytype directory.

Usage

usage: pytype [options] input [input ...]

positional arguments:
  input                 file or directory to process

Common options:

• -V, --python-version: Python version (major.minor) of the target code. Defaults to 3.6.
• -o, --output: The directory into which all pytype output goes, including generated .pyi files. Defaults to pytype_output.
• -P, --pythonpath. Paths to source code directories, separated by ':'. Defaults to an educated guess based on input.
• -d, --disable. Comma separated list of error names to ignore. Detailed explanations of pytype's error names are in this doc. Defaults to empty.

For a full list of options, run pytype --help.

In addition to the above, you can direct pytype to use a custom typeshed installation instead of its own bundled copy by setting $TYPESHED_HOME.

Config File

For convenience, you can save your pytype configuration in a file. The config file is an INI-style file with a [pytype] section; if an explicit config file is not supplied, pytype will look for a [pytype] section in the first setup.cfg file found by walking upwards from the current working directory.

Start off by generating a sample config file:

$ pytype --generate-config pytype.cfg

Now customize the file based on your local setup, keeping only the sections you need. Directories may be relative to the location of the config file, which is useful if you want to check in the config file as part of your project.

For example, suppose you have the following directory structure and want to analyze package ~/repo1/foo, which depends on package ~/repo2/bar:

~/
├── repo1
│   └── foo
│       ├── __init__.py
│       └── file_to_check.py
└── repo2
    └── bar
        ├── __init__.py
        └── dependency.py

Here is the filled-in config file, which instructs pytype to treat its input as Python 3.6 code and ignore attribute errors. Notice that the path to a package does not include the package itself.

$ cat ~/repo1/pytype.cfg

# NOTE: All relative paths are relative to the location of this file.

[pytype]
# Python version (major.minor) of the target code.
python_version = 3.6

# Paths to source code directories, separated by ':'.
pythonpath =
    .:
    ~/repo2

disable=attribute-error

We could've discovered that ~/repo2 needed to be added to the pythonpath by running pytype's broken dependency checker:

$ pytype --config=~/repo1/pytype.cfg ~/repo1/foo/*.py --unresolved

Unresolved dependencies:
  bar.dependency

Subtools

Pytype ships with three scripts in addition to pytype itself:

• merge-pyi, for merging type information from a .pyi file into a Python file.
• pytd, a parser for .pyi files.
• pytype-single, a debugging tool for pytype developers, which analyzes a single Python file assuming that .pyi files have already been generated for all of its dependencies.

Roadmap

• Windows support
• A rerun mode to only reanalyze files that have changed since the last run

License

Apache 2.0