[Python-ideas] Type Hinting Kick-off
steve at pearwood.info
Sat Dec 20 08:59:45 CET 2014
On Fri, Dec 19, 2014 at 04:55:37PM -0800, Guido van Rossum wrote:
> A few months ago we had a long discussion about type hinting. I've thought
> a lot more about this. I've written up what I think is a decent "theory"
> document -- writing it down like this certainly helped *me* get a lot of
> clarity about some of the important issues.
Very interesting indeed.
Some questions, which you may not have answers to yet :-)
(1) Under "General rules", you state "No type can be subclassed unless
stated." That's not completely clear to me. I presume you are talking
about the special types like Union, Generic, Sequence, Tuple, Any etc.
Is that correct?
(2) Under "Types", you give an example Tuple[t1, t2, ...], a tuple
whose items are instances of t1 etc. To be more concrete, a declaration
of Tuple[int, float, str] will mean "a tuple with exactly three items,
the first item must be an int, the second item must be a float, the
third item must be a string." Correct?
(3) But there's no way of declaring "a tuple of any length, which each
item is of type t". We can declare it as Sequence[t], but cannot specify
that it must be a tuple, not a list. Example:
def startswith(self, prefix:Union[str, ???])->bool:
There's nothing I can use instead of ??? to capture the current
behaviour of str.startswith. Correct?
(4) Under "Pragmatics", you say "Don't use dynamic type expressions; use
builtins and imported types only. No 'if'." What's the reason for this
rule? Will it be enforced by the compiler?
(5) I presume (4) above also applies to things like this:
X = Var('X', str)
X = Var('X', bytes)
# Later on
def spam(arg: X)-> X:
How about this?
from condition_is_true import X # str
from condition_is_false import X # bytes
(6) Under "Generic types", you have:
X = Var('X'). Declares a unique type variable.
The name must match the variable name.
To be clear, X is a type used only for declarations, right? By (1)
above, it cannot be instantiated? But doesn't that make it impossible to
satisfy the declaration? I must be misunderstanding something.
I imagine the declaration X = Var("X") to be something equivalent to:
except that X cannot be instantiated.
(7) You have an example:
AnyStr = Var('AnyStr', str, bytes)
def longest(a: AnyStr, b: AnyStr) -> AnyStr:
Would this be an acceptable syntax?
def longest(a:str|bytes, b:str|bytes) -> str|bytes
It seems a shame to have to invent a name for something you might only
use once or twice.
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