Untyped decorator makes function untyped

[aphedges]

When I run mypy --strict on the following file (named flask_types.py),

from typing import Tuple

from flask import Flask

app = Flask(__name__)

@app.errorhandler(404)
def page_not_found(error: Exception) -> Tuple[str, int]:
    return str(error), 404

I get the following error:

flask_types.py:7: error: Untyped decorator makes function "page_not_found" untyped

Instead, mypy should return no errors.

Using --strict (or the equivalent option in setup.cfg) in your tests might have caught an error like this, but given how new your type annotations are, I understand why you aren't using the setting yet.

The root cause appears to be that the implemented decorators often use typing.Callable in type annotations without any type parameters, which causes them to lose type information.

The fix for this specific issue is only a couple of lines (I have a prototype implemented locally), but I will try to add appropriate type parameters for Callable in as many places as possible. I expect to have a PR out for this in a couple of days.
Once I got a fix working for errorhandler(), mypy was able to find an error in my type annotations of page_not_found() as well as some internal issues in Flask itself, which I feel demonstrates how better typing would be useful here.

Environment:

• Python version: 3.7.9
• Flask version: 2.0.0
• mypy version: 0.812

---

PS: I really appreciate the new typing annotations in Flask! They seem like they took a lot of effort, and I have already found them useful. Thank you very much!

[pgjones]

Does it work with this patch applied?

diff --git a/src/flask/scaffold.py b/src/flask/scaffold.py
index f50c9b1b..46d216f6 100644
--- a/src/flask/scaffold.py
+++ b/src/flask/scaffold.py
@@ -642,7 +642,7 @@ class Scaffold:
     @setupmethod
     def errorhandler(
         self, code_or_exception: t.Union[t.Type[Exception], int]
-    ) -> t.Callable:
+    ) -> t.Callable[[ErrorHandlerCallable], ErrorHandlerCallable]:
         """Register a function to handle errors by code or exception class.
 
         A decorator that is used to register a function given an

[aphedges]

That was my initial thought, too, but unfortunately, setupmethod() erases any type signatures it is called on. The minimal patch to stop the error is the following:

diff --git a/src/flask/scaffold.py b/src/flask/scaffold.py
index 20654b6b..f8f94af1 100644
--- a/src/flask/scaffold.py
+++ b/src/flask/scaffold.py
@@ -33,8 +33,10 @@ if t.TYPE_CHECKING:
 # a singleton sentinel value for parameter defaults
 _sentinel = object()
 
+F = t.TypeVar("F", bound=t.Callable[..., t.Any])
 
-def setupmethod(f: t.Callable) -> t.Callable:
+
+def setupmethod(f: F) -> F:
     """Wraps a method so that it performs a check in debug mode if the
     first request was already handled.
     """
@@ -53,7 +55,7 @@ def setupmethod(f: t.Callable) -> t.Callable:
             )
         return f(self, *args, **kwargs)
 
-    return update_wrapper(wrapper_func, f)
+    return t.cast(F, update_wrapper(wrapper_func, f))
 
 
 class Scaffold:

It seems the mypy error was caused by the decorator not having any type, so changing t.Callable to t.Callable[[ErrorHandlerCallable], ErrorHandlerCallable]: is not needed. However, it's still useful for catching errors in user code.

I have a PR with my complete fix: #4071.