[Python-ideas] Runtime types vs static types

Steven D'Aprano steve at pearwood.info
Sun Jul 2 07:16:08 EDT 2017

On Sat, Jun 24, 2017 at 10:42:19PM +0300, Koos Zevenhoven wrote:

> Clearly, there needs to be some sort of distinction between runtime
> classes/types and static types, because static types can be more precise
> than Python's dynamic runtime semantics.

I think that's backwards: runtime types can be more precise than static 
types. Runtime types can make use of information known at compile time 
*and* at runtime, while static types can only make use of information 
known at compile time. Consider:

     List[str if today == 'Tuesday' else int]

The best that the compile-time checker can do is treat it as 

     List[Union[str, int]]

if even that, but at runtime we can tell whether or not [1, 2, 3] is 
legal or not.

But in any case, *static types* and *dynamic types* (runtime types, 
classes) are distinct concepts, but with significant overlap. Static 
types apply to *variables* (or expressions) while dynamic types apply to 
*values*. Values are, in general, only known at runtime.

> For example, Iterable[int] is an
> iterable that contains integers. For a static type checker, it is clear
> what this means. But at runtime, it may be impossible to figure out whether
> an iterable is really of this type without consuming the whole iterable and
> checking whether each yielded element is an integer.

There's a difference between *requesting* an object's runtime type and 
*verifying* that it is what it says it is.

Of course if we try to verify that an iterator yields nothing but ints, 
we can't do so without consuming the iterator, or possibly even entering 
an infinite loop.

But we can ask an object what type they are, they can tell you that 
they're an Iterable[int], and this could be an extremely fast check.

Assuming you trust the object not to lie.

("Consenting adults" may apply here.)

> Even that is not
> possible if the iterable is infinite. Even Sequence[int] is problematic,
> because checking the types of all elements of the sequence could take a
> long time.
> Since things like isinstance(it, Iterable[int]) cannot guarantee a proper
> answer, one easily arrives at the conclusion that static types and runtime
> classes are just two separate things and that one cannot require that all
> types support something like isinstance at runtime.

That's way too strong.

I agree that static types and runtime types (I don't use the term 
"class" because in principle at least this could include types not 
implemented as a class, e.g. a struct or record or primitive unboxed 
value) are distinct, but they do overlap. To describe them as "separate" 
implies that they are unconnected and that one could sensibly have 
things which are statically typed as (let's say) Sequence[bool] but 
runtime typed as float.

Gradual typing is useful because the static types are at least an 
approximation to the runtime types. If they had no connection at all, 
we'd learn nothing from static type checking and there would be no 
reason to do it.

So static types and runtime types must be at least closely related to be 

> These and other incompatibilities between runtime and static typing will
> create two (or more) different kinds of type-annotated Python:
> runtime-oriented Python and Python with static type checking. These may be
> incompatible in both directions: a static type checker may complain about
> code that is perfectly valid for the runtime folks, and code written for
> static type checking may not be able to use new Python techniques that make
> use of type hints at runtime.

Yes? What's your point? Consenting adults certainly applies here. There 
are lots of reasons why people might avoid "new Python techniques" for 
*anything*, not just type hints:

- they have to support older versions of Python;
- they're stuck on an older version and can't upgrade;
- they just don't like those new techniques.

Nobody forces you to run a static type-checker. If you choose to run one, and 
it gives the wrong answers, then you can:

- stop using it;
- use a better one that gives the right answer;
- fix the broken code that the checker says is broken (regardless 
  of whether it is genuinely broken or not);
- add, remove or modify annotations to satisfy the checker;
- disable type-checking for that code unit (module?) alone.

But the critical thing here is that so long as Python is a dynamically 
typed language, you cannot eliminate runtime type checks. You can choose 
*not* to write them in your code, and rely on duck typing and 
exceptions, but the type checks are still there in the implementation. 
E.g. you have x + 1 in your code. Even if *you* don't guard with an 
type check:

    # if isinstance(x, int):
    y = x + 1

there's still a runtime check in the byte-code which prevents low-level 
machine code errors that could lead to a segmentation fault or worse.

> There may not even be a fully functional 
> subset of the two "languages".

What do you mean by "fully functional"?

Of course there will be working code that can pass both the static 
checks and run without error. Here's a trivial example:

    print("Hello World")

On the other hand, it's trivially true that code which works at runtime 
cannot *always* be statically checked:

    s = input("Type some Python code: ")

The static type checker cannot possibly check code that doesn't even 
exist until runtime!

I don't think it is plausible to say that there is, or could be, no 
overlap between (a) legal Python code that runs under a type-checker, 
and (b) legal Python code that runs without it.

That's literally impossible since the type-checker is not part of the 
Python interpreter, so you can always just *not run the type-checker* to 
turn (a) into (b).

>  Different libraries will adhere to different
> standards and will not be compatible with each other. The split will be
> much worse and more difficult to understand than Python 2 vs 3, peoples
> around the world will suffer like never before, and programming in Python
> will become a very complicated mess.

I think this is Chicken Little "The Sky Is Falling" FUD.

> One way of solving the problem would be that type annotations are only a
> static concept, like with stubs or comment-based type annotations.

I don't agree that there's a problem that needs to be solved.

> This
> would also be nice from a memory and performance perspective, as evaluating
> and storing the annotations would not occupy memory (although both issues
> and some more might be nicely solved by making the annotations lazily
> ealuated).

Sounds like premature optimization to me. How many distinct annotations 
do you have? How much memory do you think they will use?

If you're running 64-bit Python, each pointer to the annotation takes a 
full eight bytes. If we assume that every annotation is distinct, and we 
allow 1000 bytes for each annotation, a thousand annotations would only 
use 1MB of memory. On modern machines, that's trivial.

I don't think this will be a problem for the average developer. 
(Although people programming on embedded devices may be different.)

If we want to support that optimization, we could add an optimization 
flag that strips annotations at runtime, just as the -OO flag strips 
docstrings. That becomes a matter of *consenting adults* -- if you don't 
want annotations, you don't need to keep them, but it then becomes your 
responsibility that you don't try to use them. (If you do, you'll get a 
runtime AttributeError.)

> However, leaving out runtime effects of type annotations is not
> the approach taken, and runtime introspection of annotations seems to have
> some promising applications as well. And for many cases, the traditional
> Python class actually acts very nicely as both the runtime and static type.
> So if type annotations will be both for runtime and for static checking,
> how to make everything work for both static and runtime typing?
> Since a writer of a library does not know what the type hints will be used
> for by the library users, 

No, that's backwards. The library creator gets to decide what their 
library uses annotations for: type-hints, or something else.

As the user of a library, I don't get to decide what the library does 
with its own annotations.

> it is very important that there is only one way
> of making type annotations which will work regardless of what the
> annotations are used for in the end. This will also make it much easier to
> learn Python typing.

I don't understand this.

> Regarding runtime types and isinstance, let's look at the Iterable[int]
> example. For this case, there are a few options:
> 1) Don't implement isinstance
> This is problematic for runtime uses of annotations.
> 2) isinstance([1, '2', 'three'], Iterable[int]) returns True
> This is in fact now the case.

That's clearly a bug. If isinstance(... Iterable[int]) is supported at 
all, then clearly the result should be False.

> 3) Check as much as you can at runtime

For what purpose?

> 4) Do a deeper check than in (2) but trust the annotations
> For example, an instance of a class that has a method like
> def __iter__(self) -> Iterator[int]:
>     some code
> could be identified as Iterable[int] at runtime, even if it is not
> guaranteed that all elements are really integers.

I suggested something similar to this earlier in this post. 

> On the other hand, an object returned by
> def get_ints() -> Iterable[int]:
>     some code
> does not know its own annotations, so the check is difficult to do at
> runtime. And of course, there may not be annotations available.

Right -- when annotations are not available, the type checker will 
either infer types, if it can, or default to the Any type.

I don't really understand where you are going with this. The premise, 
that statically-type-checked Python is fundamentally different from 
Python-without-static-checks, and therefore we have to bring in a bunch 
of extra runtime checks to make them the same, seems wrong to me.

Perhaps I have not understood you.


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