[Python-Dev] Type hints -- a mediocre programmer's reaction

[Terry Reedy]

On 4/21/2015 6:50 PM, Chris Barker wrote:
> On Tue, Apr 21, 2015 at 11:58 AM, Paul Sokolovsky <pmiscml at gmail.com
> <mailto:pmiscml at gmail.com>> wrote:
>
>     It does, and hope people won't be caught in "static typechecking"
>     loop and consider other usages too.

I an interested is using type hints for automatic or at least 
semi-automatic generation of tests, in particular for parameters hinted 
as generic and abstract.  A parameter "ints: Iterable(int)" should be 
work with empty, length 1, and longer tuples, lists, sets, dicts (with 
int keys) and a custom class.  Unless there is supplementary doc 
otherwise, it should handle negative, 0, and positive ints, small and 
large, in regular and random patterns.  (Thinking about this more, some 
of this could be done without the PEP being approved, but there has to 
be *some* syntax for test generator input information, and using the 
standard vocabulary of typing instead of inventing something would be 
easier.)

> I"m confused -- from the bit I've been skimming the discussion, over on
> python-ideas, and now here, is that this is all about "static typechecking".

The PEP is about a) defining type hints as the primary use of 
annotations* and b) adding a new module to standardize type hints 
instead of having multiple groups define multiple variations on the 
theme.  The urgency of developing a standard now is that there *are* at 
least two or three independent efforts.

* http://www.artima.com/weblogs/viewpost.jsp?thread=85551
(2004) makes it clear that Guido thought of static typing first and 
annotations second, in the form added to 3.0 years later, as an 
implementation thereof.

The primary motivation of the people working on the new module is static 
typechecking by 3rd party programs. This seems to be Guido's primary 
interest and what he believes will be the primary use.  It is also the 
primary promotional argument since type checking already exists.

However, this does not stop others from having other motivations and 
making other uses of the information once available.

> It's not about run-time type checking.

The PEP does not prevent that, and will make it more feasible by 
standardizing on one system.

> It's not about type-base performance optimization.

Nothing is planned for CPython, but type hints are already being used by 
MicroPython for this purpose.  (To me, this is one of the pluses of it 
having targeted 3.3.)

> It's not about any use of annotations other than types.

Right, but there are multiple possible uses for type information.

> What is it about other than static typechecking?

There is also documentation.  Type hints are partially an alternative to 
stylized entries in doc strings.  A program might combine the two or 
check consistency.

-- 
Terry Jan Reedy