A static type analyzer for Python code

Developer guide
Development process
Python version upgrades
Supporting new features
Program analysis
Main loop
Stack frames
Data representation
Abstract values
Special builtins
Type annotations
Type stubs

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Developer guide

This documentation is for developers of and contributors to pytype. It covers pytype’s core concepts and code layout, as well as tips on suggested workflow.


To get started with contributing to pytype, we recommend familiarizing yourself with the codebase and workflow by fixing a small issue. See the Issue Tracker section for tips on finding issues.


Pytype is built around a “shadow bytecode interpreter”, which traces through a program’s bytecode, mimicking the effects of the cpython interpreter but tracking types rather than values.

A good starting point is to trace through the details of pytype’s main loop and get a feel for how the bytecode interpreter works.

Basic concepts

pytype’s bytecode interpreter is referred to as the Virtual Machine, or VM.

As the VM traces a program, it builds up a Typegraph: a graph that maps the flow of types through a program. Each Node in the typegraph roughly correlates with a single statement in the program. A Variable tracks the type information for a variable in the program being analyzed. A variable has one or more Bindings, each associating it with a value at a particular node. These associated values are known as Abstract Values, or sometimes Data in the context of the typegraph, and represent types in the program.

A bytecode operation is modeled by popping variables from the VM’s data stack, manipulating them, and pushing the result back onto the stack. If an operation cannot be completed legally, then pytype reports a type error. Throughout this process, the VM queries the typegraph for what values a variable may be holding at the current node.

Once the VM has finished execution, it converts the program’s top-level definitions into PyTD format, an AST representation that is easy to serialize and deserialize, and writes the AST to a file. If a later invocation of pytype analyzes a program that depends on this one, it will read the file and convert the PyTD nodes back to abstract values for internal use.

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