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Mypy – static type checking for Python 3

70 points by HerrMonnezza 12y ago

HerrMonnezza 12y ago
Former HN discussion at: https://news.ycombinator.com/item?id=4561973 https://news.ycombinator.com/item?id=4561973 Although it seems that MyPy has done a lot of progress since, most notably adopting Python 3 syntax for type annotations.
ceronman 12y ago
Just a few days ago Guido proposed to adopt the Mypy syntax for type annotations in Python 3.5. There is a big discussion going on on the python-ideas mailing list about that and the general idea of having optional static typing in Python: https://mail.python.org/pipermail/python-ideas/2014-August/028618.html https://mail.python.org/pipermail/python-ideas/2014-August/0... https://mail.python.org/pipermail/python-ideas/2014-August/028742.html https://mail.python.org/pipermail/python-ideas/2014-August/0... Recommended reading for those interested in the topic.
- bsaul 12y ago
  This is probably the best thing i've heard about python in the last 2 years. Guido's point are all 100% valid. Python should take example on Typescript. It's light and smart. And that's enough to cover 99% of the cases. Just let me specify that function foo's first parameter is an array of "stuff", and i'll be the happiest man. Leave the complicated cases for future versions.
- oblique63 12y ago
  Been using optional typing for almost a year now with Dart, and it's pretty awesome. Really hoping this comes to python!
Goranek 12y ago
It would be awesome to combine pypy and mypy. Pypy could probable use static typing to gain some speed.
- thristian 12y ago
  Alex Gaynor, one of the PyPy developers said (on the mailing-list where Guido proposed this): You're right. None of this would provide any value for PyPy. https://mail.python.org/pipermail/python-ideas/2014-August/028623.html https://mail.python.org/pipermail/python-ideas/2014-August/0...
  - Goranek 12y ago
    I certainly didn't expect this, but he is right.
  - andreasvc 12y ago
    Why is that?
    - untothebreach 12y ago
      Because in a dynamically typed language like Python, it is perfectly feasible that the type declared in the code, and type-checked before the program runs, is different when the piece of code actually _runs_. Pypy uses runtime types to make decisions about optimizations, and since those may be different than the types declared at compile time, it will always have to inspect the runtime type anyway.
      - andreasvc 12y ago
        Then the type annotation is wrong, and it shouldn't be there. Type annotations only make sense if you know exactly what the types may be at runtime. I still don't see why correct annotations couldn't help PyPy.
        untothebreach 12y ago
        Type-checking _via_ those annotations, are, like Guido said, basically a souped-up linter. With a dynamic language like Python, there is _never_ a guarantee that the types that type-checked during compilation are the types that will be observed during runtime. Type-checking Python code via function annotations is nothing like type checking a Haskell program, where you can be reasonably certain that a program that type-checks will run correctly. Even if you write a program that you can provably show will never have any runtime types that differ from compile time types, Pypy can't assume that will be the case with every program that it tries to run. I don't have any specific knowledge of the optimizations that Pypy runs, but if one of the core contributors of Pypy says it can't be done, I'm inclined to believe him.
seivan 12y ago
Has anyone used this with Django? If it works well I might replace Ruby with Mypy. Doing iOS (with Swift) for a while has made me dislike not having static typing.
ak217 12y ago
I wrote a decorator-driven library to do optional type checking at runtime with no need for a special interpreter :) https://github.com/kislyuk/ensure https://github.com/kislyuk/ensure (Having optional static type checks in the core interpreter would be nice, though Alex Gaynor's concerns linked in another comment are very valid.)
- untothebreach 12y ago
  I think one of the strengths of Mypy's approach is the fact that it _doesn't_ do type checking at runtime. It may be inconvenient to have a separate step for type checking, but it also doesn't impart any runtime performance penalty.

70 points by HerrMonnezza 12y ago

Former HN discussion at: https://news.ycombinator.com/item?id=4561973 https://news.ycombinator.com/item?id=4561973 Although it seems that MyPy has done a lot of progress since, most notably adopting Python 3 syntax for type annotations.

ceronman 12y ago

Just a few days ago Guido proposed to adopt the Mypy syntax for type annotations in Python 3.5. There is a big discussion going on on the python-ideas mailing list about that and the general idea of having optional static typing in Python: https://mail.python.org/pipermail/python-ideas/2014-August/028618.html https://mail.python.org/pipermail/python-ideas/2014-August/0... https://mail.python.org/pipermail/python-ideas/2014-August/028742.html https://mail.python.org/pipermail/python-ideas/2014-August/0... Recommended reading for those interested in the topic.

bsaul 12y ago
This is probably the best thing i've heard about python in the last 2 years. Guido's point are all 100% valid. Python should take example on Typescript. It's light and smart. And that's enough to cover 99% of the cases. Just let me specify that function foo's first parameter is an array of "stuff", and i'll be the happiest man. Leave the complicated cases for future versions.
oblique63 12y ago
Been using optional typing for almost a year now with Dart, and it's pretty awesome. Really hoping this comes to python!

Goranek 12y ago

It would be awesome to combine pypy and mypy. Pypy could probable use static typing to gain some speed.

thristian 12y ago
Alex Gaynor, one of the PyPy developers said (on the mailing-list where Guido proposed this): You're right. None of this would provide any value for PyPy. https://mail.python.org/pipermail/python-ideas/2014-August/028623.html https://mail.python.org/pipermail/python-ideas/2014-August/0...
- Goranek 12y ago
  I certainly didn't expect this, but he is right.
- andreasvc 12y ago
  Why is that?
  - untothebreach 12y ago
    Because in a dynamically typed language like Python, it is perfectly feasible that the type declared in the code, and type-checked before the program runs, is different when the piece of code actually _runs_. Pypy uses runtime types to make decisions about optimizations, and since those may be different than the types declared at compile time, it will always have to inspect the runtime type anyway.
    - andreasvc 12y ago
      Then the type annotation is wrong, and it shouldn't be there. Type annotations only make sense if you know exactly what the types may be at runtime. I still don't see why correct annotations couldn't help PyPy.
      - untothebreach 12y ago
        Type-checking _via_ those annotations, are, like Guido said, basically a souped-up linter. With a dynamic language like Python, there is _never_ a guarantee that the types that type-checked during compilation are the types that will be observed during runtime. Type-checking Python code via function annotations is nothing like type checking a Haskell program, where you can be reasonably certain that a program that type-checks will run correctly. Even if you write a program that you can provably show will never have any runtime types that differ from compile time types, Pypy can't assume that will be the case with every program that it tries to run. I don't have any specific knowledge of the optimizations that Pypy runs, but if one of the core contributors of Pypy says it can't be done, I'm inclined to believe him.

seivan 12y ago

Has anyone used this with Django? If it works well I might replace Ruby with Mypy. Doing iOS (with Swift) for a while has made me dislike not having static typing.

ak217 12y ago

I wrote a decorator-driven library to do optional type checking at runtime with no need for a special interpreter :) https://github.com/kislyuk/ensure https://github.com/kislyuk/ensure (Having optional static type checks in the core interpreter would be nice, though Alex Gaynor's concerns linked in another comment are very valid.)

untothebreach 12y ago
I think one of the strengths of Mypy's approach is the fact that it _doesn't_ do type checking at runtime. It may be inconvenient to have a separate step for type checking, but it also doesn't impart any runtime performance penalty.