3.11. Type Annotation Mypyc¶
Mypyc compiles Python modules to C extensions.
It uses standard Python type hints to generate fast code.
Mypyc compiles Python modules to C extensions. It uses standard Python type hints to generate fast code.
The compiled language is a strict, gradually typed Python variant. It restricts the use of some dynamic Python features to gain performance, but it’s mostly compatible with standard Python.
Mypyc uses mypy to perform type checking and type inference. Most type system features in the stdlib typing module are supported.
Compiled modules can import arbitrary Python modules and third-party libraries. You can compile anything from a single performance-critical module to your entire codebase. You can run the modules you compile also as normal, interpreted Python modules.
Existing code with type annotations is often 1.5x to 5x faster when compiled. Code tuned for mypyc can be 5x to 10x faster.
Mypyc currently aims to speed up non-numeric code, such as server applications. Mypyc is also used to compile itself (and mypy).
3.11.2. Differences from Cython¶
Mypyc targets many similar use cases as Cython. Mypyc does many things differently, however:
No need to use non-standard syntax, such as cpdef, or extra decorators to get good performance. Clean, normal-looking type-annotated Python code can be fast without language extensions. This makes it practical to compile entire codebases without a developer productivity hit.
Mypyc has first-class support for features in the typing module, such as tuple types, union types and generics.
Mypyc has powerful type inference, provided by mypy. Variable type annotations are not needed for optimal performance.
Mypyc fully integrates with mypy for robust and seamless static type checking.
Mypyc performs strict enforcement of type annotations at runtime, resulting in better runtime type safety and easier debugging.
Unlike Cython, mypyc doesn’t directly support interfacing with C libraries or speeding up numeric code.
3.11.3. How does it work¶
Mypyc uses several techniques to produce fast code:
Mypyc uses ahead-of-time compilation to native code. This removes CPython interpreter overhead.
Mypyc enforces type annotations (and type comments) at runtime, raising TypeError if runtime values don’t match annotations. Value types only need to be checked in the boundaries between dynamic and static typing.
Compiled code uses optimized, type-specific primitives.
Mypyc uses early binding to resolve called functions and name references at compile time. Mypyc avoids many dynamic namespace lookups.
Classes are compiled to C extension classes. They use vtables for fast method calls and attribute access.
Mypyc treats compiled functions, classes, and attributes declared Final as immutable.
Mypyc has memory-efficient, unboxed representations for integers and booleans.
3.11.4. Development Status¶
Mypyc is currently alpha software. It’s only recommended for production use cases with careful testing, and if you are willing to contribute fixes or to work around issues you will encounter.
>>> import time >>> >>> >>> def fib(n: int) -> int: ... if n <= 1: ... return n ... else: ... return fib(n-2) + fib(n-1) >>> >>> start = time.time() >>> result = fib(32) >>> stop = time.time() >>> >>> duration = stop - start >>> print(duration) 0.4125328063964844
$ python3 fib.py 0.4125328063964844
$ mypyc fib.py $ python3 -c "import fib" 0.04097270965576172
After compilation, the program is about 10x faster.
Mypy will generate a C extension for fib in the current working directory.
For example, on a Linux system the generated file may be called:
Since C extensions can't be run as programs, use
python3 -c to run
the compiled module as a program
would now be
>>> ... from setuptools import setup ... from mypyc.build import mypycify ... ... ... setup( ... name='mylib', ... packages=['mylib'], ... ext_modules=mypycify([ ... 'mylib/__init__.py', ... 'mylib/mod.py', ... ]), ... )
$ python3 setup.py bdist_wheel
The wheel is created under
You can include most mypy command line options in the list of arguments
mypycify(). For example, here we use the
--disallow-untyped-defs flag to require that all functions
have type annotations
>>> ... from setuptools import setup ... from mypyc.build import mypycify ... ... ... setup( ... name='frobnicate', ... packages=['frobnicate'], ... ext_modules=mypycify([ ... '--disallow-untyped-defs', # Pass a mypy flag ... 'frobnicate.py', ... ]), ... )
[tool.mypy] # Import discovery files = ["src"] namespace_packages = false explicit_package_bases = false ignore_missing_imports = false follow_imports = "normal" follow_imports_for_stubs = false no_site_packages = false no_silence_site_packages = false # Platform configuration python_version = "3.10" platform = "linux-64" # Disallow dynamic typing disallow_any_unimported = false # TODO disallow_any_expr = false # TODO disallow_any_decorated = false # TODO disallow_any_explicit = false # TODO disallow_any_generics = true disallow_subclassing_any = true # Untyped definitions and calls disallow_untyped_calls = true disallow_untyped_defs = true disallow_incomplete_defs = true check_untyped_defs = true disallow_untyped_decorators = true # None and Optional handling no_implicit_optional = true strict_optional = true # Configuring warnings warn_redundant_casts = true warn_unused_ignores = true warn_no_return = true warn_return_any = true warn_unreachable = false # GH#27396 # Suppressing errors show_none_errors = true ignore_errors = false enable_error_code = "ignore-without-code" # Miscellaneous strictness flags allow_untyped_globals = false allow_redefinition = false local_partial_types = false implicit_reexport = true strict_equality = true # Configuring error messages show_error_context = false show_column_numbers = false show_error_codes = true
3.11.8. Runtime type checking¶
Non-erased types in annotations will be type checked at runtime. For example, consider this function:
>>> def twice(x: int) -> int: ... return x * 2
If you try to call this function with a float or str argument, you'll get a type error on the call site, even if the call site is not being type checked:
>>> result = twice(2) # OK >>> result = twice(2.0) # TypeError >>> result = twice('two') # TypeError
3.11.9. Final values¶
Compiled code replaces a reference to an attribute declared
with the value of the attribute computed at compile time. This is
an example of early binding. Example:
>>> from typing import Final
>>> MAX: Final = 100 >>> >>> def limit_to_max(x: int) -> int: ... if x > MAX: ... return MAX ... return x
>>> def limit_to_max(x: int) -> int: ... if x > 100: ... return 100 ... return x
The two references to
MAX don't involve any module namespace lookups,
and are equivalent to the second code listing.
3.11.10. Recommended Workflow¶
A simple way to use mypyc is to always compile your code after any code changes, but this can get tedious, especially if you have a lot of code. Instead, you can do most development in interpreted mode. This development workflow has worked smoothly for developing mypy and mypyc (often we forget that we aren’t working on a vanilla Python project):
During development, use interpreted mode. This gives you a fast edit-run cycle.
Use type annotations liberally and use mypy to type check your code during development. Mypy and tests can find most errors that would break your compiled code, if you have good type annotation coverage. (Running mypy is pretty quick.)
After you’ve implemented a feature or a fix, compile your project and run tests again, now in compiled mode. Usually nothing will break here, assuming your type annotation coverage is good. This can happen locally or in a Continuous Integration (CI) job. If you have CI, compiling locally may be rarely needed.
Release or deploy a compiled version. Optionally, include a fallback interpreted version for platforms that mypyc doesn’t support.
This mypyc workflow only involves minor tweaks to a typical Python workflow. Most of development, testing and debugging happens in interpreted mode. Incremental mypy runs, especially when using the mypy daemon, are very quick (often a few hundred milliseconds).