Friday, 10 April 2015

Mypy at PyCon 2015 in Montreal

I'm currently at PyCon 2015 in Montreal (and enjoying myself!). I'm happy to chat about anything related to mypy, PEP 484 (type hinting / typing), static typing or static analysis. Say hi if you see me, or meet me at the Dropbox booth or send me an email -- it can be quite difficult to find me among the crowds. There's probably also going to be an open space session that I will attend, maybe on Saturday. I'm going to be around until Wednesday morning.

Sunday, 5 April 2015

Mypy 0.2 Released

I just uploaded mypy 0.2 to PyPI! This release focuses on PEP 484 compatibility, but it also contains many general improvements. The changes are too numerous to enumerate them all. Instead, I'll focus here on the most visible and useful ones.

Install mypy using pip:

$ pip install mypy-lang

Mypy 0.2 breaks backward compatibility. You will probably have to modify your code to be PEP 484 draft compatible. As before, mypy is experimental and still has bugs. Backward compatibility won't be preserved until mypy hits 1.0. However, mypy is committed to becoming compatible with PEP 484.

PEP 484 Syntax Changes

Several changes improve PEP 484 compatibility. Mypy still doesn't implement all of PEP 484, but we've made a lot of progress.

'typevar' Replaced With 'TypeVar'

To define a type variable, use typing.TypeVar instead of typing.typevar. For example:

  from typing import TypeVar, Sequence

  T = TypeVar('T')

  def first(a: Sequence[T]) -> T:
      return a[0]

Function[...] Replaced With Callable[...]

Replace types like Function[...] with Callable[...]. Other than the name change, the types are identical.

No More Type Application Syntax

You can't write code like this any more:

  from typing import List

  names = List[str]()  # ERROR

Use a type comment instead:

  from typing import List

  names = []  # type: List[str]

Types such as List[str] are no longer ordinary classes. Previously, List[int] evaluated to just list at runtime, making it possible to support the idiom in the first example. List[str] now actually creates a new type object that preserves information about the str type argument. This is useful and allows using type information at runtime in a variety of ways. The tradeoff is that constructing instances using generic type objects is no longer possible. As a side benefit, there is usually a single obvious way to declare the type of a container object. Previously there were just too many ways of defining the types of variables and objects.

Any(x) Not Valid As a Cast

Use cast(Any, e) instead of Any(e); the prior is equivalent to the latter, and the latter syntax is no longer supported.

Overloading In Stubs Only

The @overload decorator is only supported in stubs. You can replace many uses of overloading using union types.

Silencing Mypy Errors Locally

You can use a "# type: ignore" comment to silence mypy errors on a particular line. For example, to make mypy not complain about a missing function in a stub:

  import acme

  # Don't complain about missing do_stuff
  acme.do_stuff()  # type: ignore

This is very useful if mypy gets confused about some particular behavior in your code. Instead of trying to refactor your code to pass type checking, you can just locally ignore individual errors.

Better Python Support

Mypy supports additional Python syntax. The mypy parser now can parse most of Python 3.2 standard library, and there's also support for a bunch of features introduced in later 3.x releases.

More Library Modules Supported

Additional stubs for standard library modules are included, and we've started adding support for 3rd party modules such as requests.

This mypy release also makes it easier to work with arbitrary library modules that aren't directly supported my mypy. You can silence errors due to missing library stubs using # type: ignore. For example, if you want to use frobnicate that has no stubs:

  from frobnicate import do_stuff  # type: ignore

  do_stuff(...)  # Okay!

Type Aliases

Mypy supports type aliases that make it easier to use more powerful types such as union types and callable types, and can be used to have more descriptive names for types. Example:

  from typing import Dict, List, Tuple

  DependencyMap = Dict[str, List[str]]
  Location = Tuple[str, str]

  def get_dependencies(base: Location) -> DependencyMap: ...


  • Mypy supports named tuples (both collections.namedtuple and typing.NamedTuple).
  • Mypy supports __call__ and __getattr__ (thanks Sander Kersten!).
  • There's an alternative stub file name extension .pyi. This means that you can have stubs and normal .py files in the same directory. This can also be used to override the default stubs for standard library modules that ship with mypy with local modified variants.
  • Optional[t] allows documenting whether None is a valid value for a type.
  • Better runtime introspection of types (for example, the type parameter int of List[int] is preserved; previously the type parameters were stripped at runtime).
  • The ABCs in typing are compatible with For example, now there's both Sequence and MutableSequence.
  • As always, many bugs were squashed, though there are still remaining open bugs.
  • The @ducktype class decorator was removed. Some built-in types have special handling in the type checker so that int is compatible with float, as it was before.
  • There is no longer typing.AbstractGeneric -- always use typing.Generic when defining generic classes.

There are also some interesting work-in-progress features such as a new automatic stub generator.


Many people contributed to this release. Thanks to everybody who helped! Here is a partial list:

  • Guido van Rossum, Łukasz Langa and others at typehinting (PEP 484)
  • Sander Kersten (continued with strong contributions, including several new Python features and a bunch of code cleanup)
  • Igor Vuk
  • Brodie Rao
  • Anders Schuller
  • Ryan Gonzalez
  • Marek Sapota

An up-to-date list of code contributions is available at the GitHub contributors page.

Saturday, 17 January 2015

Mypy and the PEP 484 (Type Hinting) Draft

Guido van Rossum just yesterday announced that the first draft of PEP 484 is available for review. This Python Enhancement Proposal defines a standard type annotation syntax, which is heavily inspired by mypy, for type hinting. The plan is to get it included in Python 3.5.

I've been contributing to the design and editing process and am very excited about the prospect of mypy supporting a standard annotation syntax. That's why (in addition to holidays) there hasn't been much activity in the mypy repo during the last month or so. Fear not -- there's a lot of things happening around mypy.

The PEP does not define a type checker or many of the details needed to implement a type system -- it gives a syntax and basic guidelines, around which various tools such as type checkers (including mypy), IDEs and maybe even optimizing (JIT) compilers can be implemented. There is still a lot of room for mypy to experiment and innovate around type checking Python code.

Why This Is Great

A standardized syntax could be boon for mypy. Here are just some benefits:

  • More people, projects and companies are likely to hear about and use a syntax that is standard. More users means more polish, more bugs fixed, and overall a better experience for everybody.
  • 3rd party libraries are likely to adopt type annotations when there is a standard. Mypy will catch more bugs when you use these static-typing-aware libraries in your programs.
  • Tools such as IDEs and documentation generators are likely to add support for standard type annotations.
  • There will be less fragmentation caused by multiple incompatible type syntax languages. This makes things less confusing and reduces duplicate work.

How Does This Affect Mypy Features?

The PEP will have some direct implicitations for mypy. As it's still a draft, we don't know what the final proposal will look like, but some aspects of mypy, mostly syntax, will likely change. Here are some of the most prominent differences (some of these are not yet reflected in the PEP draft):

  • The PEP uses Callable[...] instead of Function[...] for callable/function types.
  • Generic types are not valid in expressions. So code like
        x = List[int]()
    will be written like this (this has been supported by mypy since the early days):
        x = []  # type: List[int]
    Syntax for type annotations is still being discussed, and this is clearly one of the more controversial aspects of the proposal.
  • typevar is spelled TypeVar and has other syntactic changes.
  • Function overloading (@overload) is only available in library stubs. A future PEP may add runtime support for function overloading or multiple dispatch, but for now there's not going to be overloading for user code. This isn't really a major issue, since other mypy features such as union types (which are a recent addition and part of the PEP) make overloading rarely useful any more.
  • There's provisions for explicit type checking of None values. Currently mypy implicitly makes None compatible with every type, but this is likely going to change. For example, Optional[int] would be used for int or None (same as Union[int, None]).
  • There's a new type for variable length tuples: Tuple[int, ...] ('...' is part of the syntax).
  • Type arguments of generic types in function annotations will be available for introspection. Currently List[int] evaluates to just list which loses information at runtime.

I've added a bunch of issues that cover differences between the PEP and mypy.

Join the Discussion

Join the discussion of open PEP issues and suggest changes at!

Friday, 23 August 2013

Mypy Development Update #3

This is a short update about what's been happening in the mypy project.

I have (mostly) been on vacation recently, and now it's time to get back to mypy development. My PhD dissertation submission deadline also looms near, so I will have to work on that as well. Actually a lot of things are happening in my life — I will be relocating after finishing with my PhD, and this will involve (the typical) things such as finding a nice neighborhood, new schools/nurseries and shipping our pets and our belongings. I have already been glacial in responding to emails recently, and things may not get better until later this year.

I've been working on several new mypy features during the summer, though some of them are still incomplete. I want to get a "good-enough" prototype mypy implementation for my dissertation, and I really don't have time to polish things at this stage, which is a bit sad but can't really be helped. After submitting I can get back to doing things in a more orderly manner.

I'm really enthusisistic about some of the new features. Here is a sketch of some of them:

  • Multiple (implementation) inheritance now mostly works. This is a side effect of the switch to Python-compatible syntax, as mypy lost the distinction between classes and interfaces.
  • Abstract methods and abstract base classes (ABCs) are supported, using the abstractmethod decorator and the ABCMeta metaclass, as in Python.
  • Static methods are supported, using the staticmethod decorator.
  • There is support for read-only properties. Support for more complex properties is not there yet.
  • Type inference of code that uses isinstance checks is getting better. The nice result is that casts are often no longer needed in cases where they used to be required.
  • Conditional expressions of form x if y else z are supported.
  • For loops over tuple literals are supported.
  • The del statement is improved. You can now delete names and attributes. These don't affect type checking yet, but it makes it easier to adapt existing code that uses del statements to static typing.
  • I'm working on support for a feature which allows a kind of string polymorphism. It lets a single function implementation to support both str and bytes arguments with effective type checking. Previously either some code duplication or using dynamically typed code was needed in cases like this. This seems like a very useful feature especially for library code. I'll get back to this later when the implementation is more complete.
  • There is now some very limited Python 2.x support. It should be straightforward but moderately laborious to finish this, but other tasks are more urgent.
  • As always, several bugs have been fixed and standard library stubs have been improved.

As I had previously planned, I'm not currently working on the compiler. I focus on getting the type system and type checker reasonably feature-complete and stable first.

Tuesday, 2 July 2013

Mypy Switches to Python-Compatible Syntax

Mypy now has a Python-compatible syntax! The implementation is already self-hosting, but it should still be considered very experimental. This is a huge change which touched almost every source code file in the repository, and there are still many unfixed bugs.

Here is small example of the new syntax:

  import typing

  def fib(n: int) -> None:
      a, b = 0, 1
      while a < n:
          a, b = b, a+b

I have mostly updated the web site and documentation to reflect the new syntax. The Mypy Tutorial (formerly Mypy Overview) is largely rewritten:

The update also adds more content to the tutorial that isn't directly related to the syntax switch.

Since the mypy implementation is now a valid Python 3 program, there is no need for a translation step before running it. Thus also the old mypy-py repository is no longer needed to run mypy. Instead, mypy now uses the standard Python mechanism for installation. Have a look at the updated README for details:

I have updated the example programs on the web site:

I also updated the mypy roadmap:

If you are thirsty for more code to look at, point your browser at the git repository:

From now on, the old syntax is considered legacy and will not be supported by the implementation. There will be a tool for migrating mypy code written using the old syntax to the new syntax. The tool is already functional, but it still needs a bit if polish before it's generally useful. I will notify when it's available on this blog. I'll also write a longer blog post discussing the new syntax in more detail at a later date.

Any feedback is appreciated! The syntax is still work in progress, and improvements are possible.

Many thanks to Guido van Rossum, Sebastian Riikonen and Ron Murawski for ideas and comments.

Friday, 28 June 2013

Python-Compatible Syntax for Mypy Is Almost Ready

This is just a quick update about what has been happening in the mypy project recently.

I have been working on the new Python-compatible syntax along with writing my dissertation during the last months. I've also done a lot of design and evaluation work related to potential new exciting mypy features. I won't go into details now, but these include much more powerful type inference, union types and better parametric polymorphism for string types.

The new syntax is currently mostly functional. I'm very happy with the initial results, and this now seems like the obvious way forward for mypy. The most important test was when I translated the entire mypy implementation to use the new syntax this Tuesday. I was able to do it in just a few hours using an automatic translator tool that I had written earlier, though there was quite a bit of manual clean-up work afterwards.

The code is available in the pythonsyntax branch for anybody who wants to play with it:

Note that installing mypy now uses the standard approach instead of the old funky approach of using a git repository to distribute a runnable implementation. Have a look at the updated README for more information.

There is not much documentation, though. The wiki contains a short introruction to the new syntax:

The details of the syntax may still change, and I have many ideas of making the new syntax even better. I'll write about these later.

Monday, 15 April 2013

PyCon Update: Python-Compatible Syntax for Mypy?

It's already weeks since PyCon! Phew, I've been busy recently. Anyway, I had an eventful trip to PyCon in Santa Clara, California. PyCon is the biggest Python conference with about 2500 delegates from all around the world (though most seemed to come from the US).

I had chats with Guido, Armin Rigo (PyPy) and many others. After the conference, I stayed around for a few days in the San Francisco Bay Area and gave a talk at Google Mountain View, and also visited Dropbox in San Francisco.

One of my main goals for the trip was trying to gauge whether mypy is going in the right direction in the eyes of the Python community. There was a lot of interest in the project, but some important issues were raised that I need to discuss in more detail.

Ability to do compile-time checking of programs even without a new VM was interesting to many. This would benefit projects and organizations with large existing Python code bases. However, these organizations also manage risks carefully. Currently mypy can be used on top of CPython, but the sources must always be translated to Python before execution. Adding the mypy tool chain to the core build process is something most seem to be reluctant to do. Obviously this is the case now as mypy is still experimental, but I got the impression that even if mypy would be considered stable and mature, relying on a third-party tool to be able to run their code would be a pretty daring and unlikely move. Also, mypy has the problem of being not-quite compatible with many Python tools such as IDEs. This is a chicken-an-egg problem: tool support probably would fix itself if mypy would be widely used, but it's difficult to get wide use without tool support. Library support is similar. However, there may be a way around this dilemma -- just stay with me for a few more paragraphs.

Many organizations using Python are still stuck with 2.x, and find the transition to Python 3 difficult. Even upgrades from 2.x to 2.x+1 have caused a lot of trouble, and the switch to Python 3 is much trickier, in large part due to changes in string representations (str/unicode in Python 2.x versus bytes/str in Python 3.x). Mypy currently only supports Python 3.x syntax, which limits its usefulness to many.

Some also saw the challenge of developing a production-quality mypy VM to be too large for our team. I think this is to a large part down to how previous projects have succeeded (or not), including PyPy: even after many years, and with several talented developers, still their adoption has been pretty slow in the Python community. Unladen Swallow is another example that showed that speeding up Python is not easy. Of course, mypy has goals different from PyPy and other previous projects, and our approach of targeting ahead-of-time compilation slashes development efforts by a large factor. But I agree that I won't be able to it alone, and getting funding for continued development is hard.

Based on suggestions from Guido and the above observations, I've worked now for some time on a pretty big proposal that would help address all of the above issues in some form or another. This is still in a planning stage, and no concrete plans are yet finalized. However, here are the main points:

  1. For mypy to really take off, we need users. In order to realistically get users, there needs to be a low-risk way of adopting mypy incrementally in current projects implemented in Python.
  2. There is a good amount of interest in optional typing in the Python community, but the approach should be non-invasive to current development processes, tool chains, etc.
  3. The pragmatic way to resolve the two above issues is to make mypy syntax 100% compatible with Python, both Python 2.x and 3.x. There would be no need for a Python translation phase, and a normal Python interpreter could be used to run mypy programs directly. Also all Python tools would pretty much Just Work. Note that as this would be a syntactic change, it would have no significant impact on planned efficiency of the new VM compared to the current syntax and plans, though this would likely result in semantic changes as well (see below for more about these). Also, mypy already supports translation to Python. This would just remove the need for the translation step.
  4. We should first focus most resources on the optional typing part instead of the the new VM and compiler in order to make mypy usable as a static type checker for CPython (and PyPy/Jython).
  5. Now mypy would be much easier to adopt in organizations that would like to use optional typing to get better maintainability and productivity. I think that the above changes could speed up the adoption of mypy a lot. Also, the type checker part of mypy is a fairly straightforward project form an engineering point of view and there is no need for a large team of developers.
  6. If mypy gets significant adoption, there would also be demand for the new VM and the compiler, and it would be easier (but still not exactly easy!) to get contributors, maybe even development funding, etc.

The above plan would imply redesigning the type annotation syntax of mypy. I've given it a lot of thought, and perhaps surprisingly, it seems that there would not be need for many compromises. Generally readability would be similar to the current syntax, and sometimes it would be even better. I'm not going to cover this in detail now, but the main difference would be the introduction of Python 3 style annotation syntax (obviously for Python 3.x only; Python 2.x needs a different approach):

    str greeting(str name):
        return 'hello, ' + name
    def greeting(name:str) -> str:
        return 'hello, ' + name

Mypy uses nominal subtyping, even though structural subtyping would help model 'duck typing' in Python. Many people have expressed their interest in structural subtyping, and I discussed this at PyCon as well. Earlier, I thought that this couldn't be implemented efficiently on platforms that I would eventually like to be able to support, including Dalvik (Android). However, now I think I've figured out how to have efficient structural subtyping on basically any VM than could realistically run mypy, so the main objection is thrown out. Also, with the proposed Python-compatible syntax, structural subtyping could be a win for various reasons. In summary, it now seems likely that mypy will get support for structural subtyping in addition to nominal subtyping. I've started to prepare an enhancement proposal.

There are other, less major changes that Python compatibility would require. Mypy should support multiple inheritance without the current limitations, similar to Python. Again, I previously ruled this out due to efficiency concerns, but I think I was wrong and there is really no technical reason why multiple inheritance needs to be restricted to interfaces like it is now. Also, mypy needs to support metaclasses; this one trickier but I'm optimistic about it as well.

Let me know if you have any opinions on the proposed changes. Write comments below or send me en email.