On 11/22/20 10:15 AM, Guido van Rossum wrote:
> - We intentionally don't support things like `isinstance(x, List[str])`
> because that would require checking all the items with `isinstance(item,
> str)`, and that seems a speed trap. Reverting this decision would be
> hard work.
Aye. I imagine many folks would expect isinstance() to be "fast" and
altering
it to do recursive checks on its argument would lose its current O(1) time.
I imagine I *could* implement my own kind of isinstance() that would
work on
TypedDict values that would still be recognized by typecheckers. Say I
write
an implementation for the following method:
from typing import Optional, Type, TypeVar, TypedDict
TD = TypeVar(bound=TypedDict)
def try_cast(type: Type[TD], value: object) -> Optional[TD]:
"""Returns `value` if it can be parsed as a `type`, otherwise
None."""
raise NotImplementedError()
Then I could use that method in a similar way as my earlier example to parse
a value very concisely:
if (shape := try_cast(Shape, request.json)) is not None:
draw_shape(shape) # is narrowed to Shape
else:
return HTTPResponse(status=400) # Bad Request
Going further, I could extend try_cast() to accept any (non-None) JSON-like
value as the top-level object (not just TypedDicts):
from typing import Dict, List, Optional, Type, TypeVar, TypedDict,
Union
TD = TypeVar('TD', bound=TypedDict)
JsonValue = Union[
TD,
Dict[str, 'OptionalJV'],
List['OptionalJV'],
Dict, # heterogeneous Dict
List, # heterogeneous List
float,
int, # because json.loads may return an int when parsing a number
str,
bool,
]
JV = TypeVar('JV', bound=JsonValue)
OptionalJV = TypeVar('OptionalJV', bound=Union[JsonValue, None])
def try_cast(type: Type[JV], value: object) -> Optional[JV]:
"""Returns `value` if it can be parsed as a `type`, otherwise
None."""
raise NotImplementedError()
Now, I'm not sure if mypy can handle that kind of recursive TypedDict
definition :), but it *will* work at runtime.
I'll see about implementing a function like try_cast() as a separate
package.
This should be fun. :)
--
David Foster | Seattle, WA, USA
Contributor to TypedDict support for mypy
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