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Why mypy?
Compile-time type checking
Static typing makes it easier to find bugs and with less debugging (and with less staring at long stack traces).
Stress-free code maintenance
Type declarations act as machine-checked documentation. Static types can make your code easier to understand and modify, for yourself and other programmers.
Grow your programs from dynamic to static typing
You can develop programs with dynamic types and only add static typing after your code has matured. This way you do not have maintain type declarations in initial development when the code is still changing rapidly.
Static typing has the potential to enable high, scalable and predictable efficiency, without the slow warm-up seen in many JIT compilers.
IDE goodness
The type system makes it practical to add support for Eclipse/Visual Studio style IDE convenience for mypy programs, including precise and reliable code completion.


The mypy programming language is an experimental Python variant that aims to combine the benefits of dynamic (or "duck") typing and static typing. Our goal is to have the expressive power and convenience of Python combined with compile-time type checking. The long-term goal is to also support efficient compilation to native code, without the need of a heavy-weight VM.

Mypy is still in development. The current prototype supports type checking and running programs using a standard Python VM, i.e. there is no performance boost yet (development roadmap). A significant subset of Python features is supported.

What's new

Mypy switches to Python-compatible syntax!

2 Jul 2013: Mypy now has a Python-compatible syntax! The implementation is already self-hosting. Have a look at the rewritten Mypy Tutorial (formerly Mypy Overview), README and the code. Also updated the roadmap. Related blog post -Jukka

PyCon Update: Python-compatible syntax for mypy?

15 Apr 2013: I wrote a blog post about my PyCon visit and a new, Python-compatible syntax idea. The short summary is that mypy may get a 100% Python-compatible syntax in the future! Also Python 2.x support, structural subtyping and real multiple inheritance are being planned. -Jukka

Mypy at PyCon US 2013 (Santa Clara, CA)!

13 Mar 2013: I will present a poster about mypy at PyCon US this weekend. If you are coming to PyCon, you are welcome to come for a chat. -Jukka

Older news

Seamless dynamic and static typing

Dynamic typing
def fib(n):
    a, b = 0, 1
    while a < n:
        a, b = b, a+b
Static typing
def fib(n: int) -> None:
    a, b = 0, 1
    while a < n:
        a, b = b, a+b
<Add signature
for static

You can freely mix static and dynamic typing within a program, within a source file or even within a single expression. No need to give up the flexibility of dynamic typing — use static typing when it gives you a benefit. Often the only changes in statically typed code are the addition of function type signatures. Mypy can infer the types of other variables. (The above example was adapted from the Python tutorial.)

Python syntax

Every mypy program is syntactically valid Python. Dynamically typed mypy is almost identical to normal Python; there's practically no learning curve if you know Python. The aim is to support most Python language constructs in mypy.

Access to Python libs

Mypy will support accessing Python modules from mypy programs. Most commonly used libraries also have statically typed interface definitions that allow the type checker to check code that uses the libraries.

And there's more...

An important long-term goal is improved concurrency without the GIL. We aim to eventually get rid of the Global Interpreter Lock (GIL) that is used in CPython in our new VM and allow parallel programs to take advantage of multicore processors within a single process (more about the approach). We plan to support a low-level programming model based on threads and other, more programmer-friendly models built on top of that. One is likely to be based on lightweight processes with disjoint heaps and message passing, inspired by Erlang. The concurrency model will also allow Java threads to be used on future JVM targets efficiently (but the main implementation will compile directly to native code).

Learn more