[Python-ideas] type checking using PyContracts; was: Proposal: Use mypy syntax for function annotations

[Tennessee Leeuwenburg]

I like your comments about making the type system easily extensible. I can
well imagine this being used, particularly for data types with an inherent
structure such as tuples, lists, dicts and namedtuples.


On Wed, Aug 20, 2014 at 3:49 AM, Andrea Censi <censi at mit.edu> wrote:

> Dear all,
>
> (Apologies for not replying to the original thread,  I only signed up
> to the mailing list today)
>
> I wrote PyContracts[1], which implements type-checking functionality
> using decorators for Python 2.7+ and annotations in Python 3.
>
>  [1] http://andreacensi.github.com/contracts/
>
> > def word_count(input: List[str]) -> Dict[str, int]:
>
> Here's that example done using PyContracts and Python 3,
> and note the extra constraint '>=1' that PyContract can express:
>
>     from contracts import contract
>
>     @contract
>     def word_count(input:'list(str)') -> 'dict(str:int,>=1):
>         ...
>
> Here's that example done using PyContracts and Python 2.7, using
> a decorator instead of annotations:
>
>     from contracts import contract
>
>     @contract(input='list(str)', returns='dict(str:int,>=1)')
>     def word_count(input):
>         ...
>
> I would like to make a few points from my experience of writing
> PyContracts regarding the general issues that one encounters in adding
> type checking to Python.
>
> 1) Once one starts to think about it, "types" are very broad and if you
>    design a system from scratch, make sure it is extensible. For example,
>    you might want to have "positive integer" in addition to "integer"
>    ('int', and 'int,>=0' in PyContracts). Also, let the users add
> their own types.
>
> 2) Dependent types are a must and they must have a simple syntax.
>    For example, this is a PyContracts annotation that says the function
> takes
>    a list of strings and returns a list of integers of the same size N:
>
>        @contract(x='list[N](str)', returns='list[N](int)')
>        def f(x):
>            return list(map(len, x))
>
>    PyContract's convention is that and a,b,c,...,y,z bind to any variables
>    and A,B,C,...,Z bind to integer variables. This makes numpy contracts
>    very nice looking:
>
>        @contract
>        def gray2rgb(gray:'array[HxW](uint8)') -> 'array[HxWx3](uint8)':
>          ...
>
> 3) There is an argument for using regular strings to describe annotations
>    rather than Python constructs such as MyPy. One advantage
>    is that the system is backwards compatible. It also makes the code
>    much cleaner because you don't need to import the various symbols.
>
>    Here's a simple example in MyPy:
>
>         from typing typevar, Sequence
>
>         T = typevar('T')      # Declare type variable
>
>         def first(seq: Sequence[T]) -> T:
>             return seq[0]
>
>    Here's the same example in PyContracts (using Python 3's annotations):
>
>         from contracts import contract
>
>         @contract
>         def first(s: 'seq(type(t))') -> 'type(t)':
>             return s[0]
>
>    Same example in Python 2.7+ (using decorators):
>
>      from contracts import contract
>
>         @contract(s='seq(type(t))', returns='type(t)')
>         def first(s):
>             return s[0]
>
> (By the way, after a while, I actually like the decorator version
>  better than Python 3's annotations, because you can see very
> easily the function's name and parameters, and only if you want
> you can then read the function annotations above.)
>
> cheers,
> A.
>
> --
> Andrea Censi | http://censi.mit.edu | "Not all those who wander are lost."
> research scientist @ LIDS / Massachusetts Institute of Technology
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-- 
--------------------------------------------------
Tennessee Leeuwenburg
http://myownhat.blogspot.com/
"Don't believe everything you think"
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