The mypy parser uses the CPython "ast" module to convert source code into anabstract syntax tree (AST). The mypy parser then translates the CPython AST into a mypy AST defined inmypy/nodes.py.(The termparse tree is sometimes used informally as a synonym of AST.)

We don't use the CPython AST directly since it doesn't contain all the node types and attributes mypy needs. Having a custom AST implementation also gives us more flexibility and is more efficient.

You can manually call themypy.parse.parse function to experiment with it (first install test dependencies in the current virtualenv):

$ cd mypy  # mypy repo$ python>>> from mypy.options import Options>>> from mypy.parse import parse>>> print(parse("print('hello, world')", "hello.py", None, None, Options()))MypyFile:1(  hello.py  ExpressionStmt:1(    CallExpr:1(      NameExpr(print)      Args(        StrExpr(hello, world)))))

The namesMypyFile,ExpressionStmt,CallExpr,NameExpr andStrExpr refer to AST node classes defined inmypy/nodes.py.The numbers after colons are line numbers.

The parser does only a minimal amount of consistency checks. As such it also accepts many invalid programs.The next compiler pass,Semantic Analyzer, performs additional checks.