[Python-Dev] Type hints -- a mediocre programmer's reaction (original) (raw)

Guido van Rossum guido at python.org
Thu Apr 23 02:45:13 CEST 2015

On Wed, Apr 22, 2015 at 2:38 PM, Chris Barker <chris.barker at noaa.gov> wrote:

Oh wait, maybe it won't -- a string IS a sequence of strings. That's why this is an insidious bug in the first place. On Tue, Apr 21, 2015 at 11:32 PM, Terry Reedy <tjreedy at udel.edu> wrote:

I was just thinking today that for this, typing needs a subtraction (difference) operation in addition to an addition (union) operation: Difference(Iterable(str), str) Yup -- that might solve, it, but it feels a bit odd -- I can take any Iterable of string, except a string. -- but what if there are others that won't work??? But I guess that's the core of putting type hints on a dynamic language. Still, I tend to think that this particular issue is really a limitation with Python's type system -- nothing to do with type hinting. I can see that a character type seems useless in Python, but there are lessons from other places: a numpy array is a collection of (usually) numbers that can be treated as a single entity -- much like a string is a collection of characters that is treated as a single entity -- in both cases, it's core to convenience and performance to do that. But with numpy, when you index an array, you get something back with one less dimension: index into a 3-d array, you get a 2-d array index into a 2-d array, you get a 1-d array index into a 1-d array, you get a scalar -- NOT a length-one 1-d array Sometimes this is a pain for generic code, but more often than not it's critical to writing dynamic code -- not because you couldn't do the operations you want, but because it's important to distinguish between a scalar and an array that happens to have only one value. Anyway, the point is that being able to say "all these types, except this one" would solve this particular problem -- but would it solve any others? Do we want this to work around a quirk in Pythons string type? NOTE: I know full well that adding a character type to Python is not worth it.

If you switch to bytes the problem goes away. :-P

More seriously, I doubt there are other important use cases for Difference.

Given that even if Difference existed, and even if we had a predefined type alias for Difference[Iterable[str], str], you' still have to remember to mark up all those functions with that annotation. It almost sounds simpler to just predefine this function:

def make_string_list(a: Union[str, Iterable[str]]) -> Iterable[str]: if isinstance(a, str): return [a] else: return a

and call this in those functions that have an Interable[str] argument. Now instead of getting errors for all the places where a caller mistakenly passes a single str, you've fixed all those call sites. Isn't that more Pythonic? :-)

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