Implementation Overview

This is a general introduction to the mypy implementation. The linked subpages have more detailed information. The file links point to the mypy git repository.

Tests

The file runtests.py runs all the tests. Test suites for individual components are in the files mypy/test/test*.py. You can run many of these individually by doing runtests.py testfoobar.

Many test suites store test case descriptions in text files (mypy/test/data/*.test). The module testdata parses these descriptions. The package mypy.myunit contains the test framework. The unit test use minimal stubs for builtins, so a lot of operations are not possible. You should define any needed functionality within the test case. This way tests run much faster and don't break if the stubs change.

Notes about test cases:

• Python evaluation test cases are a little different from unit tests (mypy/test/testpythoneval.py, mypy/test/data/pythoneval.test). These type check and translate programs to Python and run them. Unlike the unit tests, these use the full builtins and library stubs instead of minimal ones. Run them using runtests.py testpythoneval.

More things (this should be expanded):

• You can run just a subset of tests by giving a wildcard argument, e.g. runtests.py unit-test -a "*Overload*" to run all unit tests with Overload as a substring.
• The builtins used by default in unit tests live in mypy/test/data/lib-stub. Individual tests cases can override the stubs by using [builtins fixtures/foo.py]; this targets files in mypy/test/data/fixtures. You are free add additional stubs to this directory, but generally don't update files in lib-stub without first discussing the addition with other mypy developers.

Structure of the type checker

Common passes:

1. Lexical Analyzer (mypy/lex.py)

• Transform a string that represents mypy source code into a list of tokens

2. Python Parser (mypy/parse.py)

• Transform a list of tokens into an abstract syntax tree (AST)
• Parse tree is defined in the files mypy/nodes.py, mypy/types.py (also noderepr.py and typerepr.py)

3. Semantic Analyzer (mypy/semanal.py, mypy/typeanal.py)

• Bind names to definitions
• Perform various consistency checks

4. Type Checker (mypy/checker.py, mypy/checkexpr.py, mypy/checkmember.py)

• Type check the program
• Perform type inference

Other important files

• Build Manager (mypy/build.py)
  • Coordinate a build, manage dependencies between modules and perform compilation passes in the correct order.
• The driver (mypy/main.py)
  • Perform all the build steps and run the program. Most of the work is done by the build manager.