# Source: https://github.com/DetachHead/basedpyright/blob/main/docs/development/internals.md

# Internals

## Code Structure

* `packages/vscode-pyright/src/extension.ts`: Language Server Protocol (LSP) client entry point for VS Code extension.
* `packages/pyright-internal/typeshed-fallback/`: Recent copy of Typeshed type stub files for Python stdlib
* `packages/pyright-internal/src/pyright.ts`: Main entry point for command-line tool
* `packages/pyright-internal/src/server.ts`: Main entry point for LSP server
* `packages/pyright-internal/src/analyzer`: Modules that perform analysis passes over Python parse tree
* `packages/pyright-internal/src/common`: Modules  that are common to the parser and analyzer
* `packages/pyright-internal/src/parser`: Modules that perform tokenization and parsing of Python source
* `packages/pyright-internal/src/tests`: Tests for the parser and analyzer
* `packages/pyright`: basedpyright npm package (used internally by the pypi package)
* `packages/browser-pyright`: basedpyright build that can run in a browser (used internally by [the playground](https://basedpyright.com))
* `basedpyright`: pypi package wrapper for the npm package, so that users don't need to install nodejs themselves
* `docstubs`: stubs with [docstrings on compiled modules](../benefits-over-pyright/pylance-features.md#docstrings-for-compiled-builtin-modules) (generated from `packages/pyright-internal/typeshed-fallback/` when building the pypi package)

## Core Concepts

Pyright implements a [service](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/service.ts), a persistent in-memory object that controls the order of analysis and provides an interface for the language server. For multi-root workspaces, each workspace gets its own service instance.

The service owns an instance of a [program](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/program.ts), which tracks the configuration file and all of the source files that make up the source base that is to be analyzed. A source file can be added to a program if it is a) referenced by the config file, b) currently open in the editor, or c) imported directly or indirectly by another source file. The program object is responsible for setting up file system watchers and updating the program as files are added, deleted, or edited. The program is also responsible for prioritizing all phases of analysis for all files, favoring files that are open in the editor (and their import dependencies).

The program tracks multiple [sourceFile](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/sourceFile.ts) objects. Each source file represents the contents of one Python source file on disk. It tracks the status of analysis for the file, including any intermediate or final results of the analysis and the diagnostics (errors and warnings) that result.

The program makes use of an [importResolver](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/importResolver.ts) to resolve the imported modules referenced within each source file.


## Analysis Phases

Pyright performs the following analysis phases for each source file.

The [tokenizer](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/parser/tokenizer.ts) is responsible for converting the file’s string contents into a stream of tokens. White space, comments, and some end-of-line characters are ignored, as they are not needed by the parser.

The [parser](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/parser/parser.ts) is responsible for converting the token stream into a parse tree. A generalized [parseTreeWalker](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/parseTreeWalker.ts) provides a convenient way to traverse the parse tree. All subsequent analysis phases utilize the parseTreeWalker.

The [binder](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/binder.ts) is responsible for building scopes and populating the symbol table for each scope. It does not perform any type checking, but it detects and reports some semantic errors that will result in unintended runtime exceptions. It also detects and reports inconsistent name bindings (e.g. a variable that uses both a global and nonlocal binding in the same scope). The binder also builds a “reverse code flow graph” for each scope, allowing the type analyzer to determine a symbol’s type at any point in the code flow based on its antecedents.

The [checker](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/checker.ts) is responsible for checking all of the statements and expressions within a source file. It relies heavily on the [typeEvaluator](https://github.com/microsoft/pyright/blob/main/packages/pyright-internal/src/analyzer/typeEvaluator.ts) module, which performs most of the heavy lifting. The checker doesn’t run on all files, only those that require full diagnostic output. For example, if a source file is not part of the program but is imported by the program, the checker doesn’t need to run on it.