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Mypy provides a simple, stable API for using it as part of a Python application. The API allows you to run Mypy’s type checker programmatically and access the results.

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Core principles

The Mypy API follows these design principles:
  • Simple and non-intrusive: The API mimics command line activation without starting a new interpreter
  • Immediate compatibility: Changes in the command line version of Mypy are immediately usable through the API
  • No incremental support: The API doesn’t support incremental generation of error messages

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Available functions

The mypy.api module provides two main functions:

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mypy.api.run()

Runs Mypy’s type checker on the provided arguments. This is the primary way to use Mypy programmatically. Signature: run(args: list[str]) -> tuple[str, str, int] See the run() documentation for detailed information.

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mypy.api.run_dmypy()

Runs the dmypy daemon client with the provided arguments. Signature: run_dmypy(args: list[str]) -> tuple[str, str, int]
run_dmypy() is not thread-safe and modifies sys.stdout and sys.stderr during its invocation.
See the run() documentation for detailed information.

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Return value format

Both functions return a tuple of three values:
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stdout
str
What Mypy normally writes to standard output (normal report)
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stderr
str
What Mypy normally writes to standard error (error report)
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exit_status
int
The exit status Mypy normally returns to the operating system

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Basic example

Here’s a simple example of using the Mypy API: 
import sys
from mypy import api

result = api.run(sys.argv[1:])

if result[0]:
    print('\nType checking report:\n')
    print(result[0])  # stdout

if result[1]:
    print('\nError report:\n')
    print(result[1])  # stderr

print('\nExit status:', result[2])

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Thread safety

The run() function is thread-safe, but run_dmypy() is not. If you need to run type checking from multiple threads, use run() only.

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Next steps

run() function

Learn about the main API functions

Type system

Explore Mypy’s type system classes

Error codes

Understand error classification