Utilise defaultdicts

This code originates from the Python 2.4 supporting version of Flask,
with defaultdicts being added in 2.5. Using defaultdict makes the
intentional usage clearer, and slightly simplifies the code.

src/flask/app.py

@@ -1292,7 +1292,7 @@ class Flask(Scaffold):
             (request.blueprint, None),
             (None, None),
         ):
-            handler_map = self.error_handler_spec.setdefault(name, {}).get(c)
+            handler_map = self.error_handler_spec[name][c]
 
             if not handler_map:
                 continue
@@ -1753,10 +1753,10 @@ class Flask(Scaffold):
 
         .. versionadded:: 0.7
         """
-        funcs = self.url_default_functions.get(None, ())
+        funcs = self.url_default_functions[None]
         if "." in endpoint:
             bp = endpoint.rsplit(".", 1)[0]
-            funcs = chain(funcs, self.url_default_functions.get(bp, ()))
+            funcs = chain(funcs, self.url_default_functions[bp])
         for func in funcs:
             func(endpoint, values)
 
@@ -1794,13 +1794,13 @@ class Flask(Scaffold):
 
         bp = _request_ctx_stack.top.request.blueprint
 
-        funcs = self.url_value_preprocessors.get(None, ())
+        funcs = self.url_value_preprocessors[None]
         if bp is not None and bp in self.url_value_preprocessors:
             funcs = chain(funcs, self.url_value_preprocessors[bp])
         for func in funcs:
             func(request.endpoint, request.view_args)
 
-        funcs = self.before_request_funcs.get(None, ())
+        funcs = self.before_request_funcs[None]
         if bp is not None and bp in self.before_request_funcs:
             funcs = chain(funcs, self.before_request_funcs[bp])
         for func in funcs:
@@ -1857,7 +1857,7 @@ class Flask(Scaffold):
         """
         if exc is _sentinel:
             exc = sys.exc_info()[1]
-        funcs = reversed(self.teardown_request_funcs.get(None, ()))
+        funcs = reversed(self.teardown_request_funcs[None])
         bp = _request_ctx_stack.top.request.blueprint
         if bp is not None and bp in self.teardown_request_funcs:
             funcs = chain(funcs, reversed(self.teardown_request_funcs[bp]))

src/flask/blueprints.py

@@ -251,7 +251,7 @@ class Blueprint(Scaffold):
             """
             for key, values in self_dict.items():
                 key = self.name if key is None else f"{self.name}.{key}"
-                app_dict.setdefault(key, []).extend(values)
+                app_dict[key].extend(values)
 
         def merge_dict_nested(self_dict, app_dict):
             """Merges self_dict into app_dict. Replaces None keys with self.name.

src/flask/scaffold.py

@@ -1,3 +1,4 @@
+from collections import defaultdict
 from functools import update_wrapper
 
 from werkzeug.exceptions import default_exceptions
@@ -86,21 +87,21 @@ class Scaffold(_PackageBoundObject):
         #:
         #: To register an error handler, use the :meth:`errorhandler`
         #: decorator.
-        self.error_handler_spec = {}
+        self.error_handler_spec = defaultdict(lambda: defaultdict(dict))
 
         #: A dictionary with lists of functions that will be called at the
         #: beginning of each request. The key of the dictionary is the name of
         #: the blueprint this function is active for, or ``None`` for all
         #: requests. To register a function, use the :meth:`before_request`
         #: decorator.
-        self.before_request_funcs = {}
+        self.before_request_funcs = defaultdict(list)
 
         #: A dictionary with lists of functions that should be called after
         #: each request.  The key of the dictionary is the name of the blueprint
         #: this function is active for, ``None`` for all requests.  This can for
         #: example be used to close database connections. To register a function
         #: here, use the :meth:`after_request` decorator.
-        self.after_request_funcs = {}
+        self.after_request_funcs = defaultdict(list)
 
         #: A dictionary with lists of functions that are called after
         #: each request, even if an exception has occurred. The key of the
@@ -112,7 +113,7 @@ class Scaffold(_PackageBoundObject):
         #: :meth:`teardown_request` decorator.
         #:
         #: .. versionadded:: 0.7
-        self.teardown_request_funcs = {}
+        self.teardown_request_funcs = defaultdict(list)
 
         #: A dictionary with list of functions that are called without argument
         #: to populate the template context.  The key of the dictionary is the
@@ -120,7 +121,9 @@ class Scaffold(_PackageBoundObject):
         #: requests.  Each returns a dictionary that the template context is
         #: updated with.  To register a function here, use the
         #: :meth:`context_processor` decorator.
-        self.template_context_processors = {None: [_default_template_ctx_processor]}
+        self.template_context_processors = defaultdict(
+            list, {None: [_default_template_ctx_processor]}
+        )
 
         #: A dictionary with lists of functions that are called before the
         #: :attr:`before_request_funcs` functions. The key of the dictionary is
@@ -129,7 +132,7 @@ class Scaffold(_PackageBoundObject):
         #: :meth:`url_value_preprocessor`.
         #:
         #: .. versionadded:: 0.7
-        self.url_value_preprocessors = {}
+        self.url_value_preprocessors = defaultdict(list)
 
         #: A dictionary with lists of functions that can be used as URL value
         #: preprocessors.  The key ``None`` here is used for application wide
@@ -141,7 +144,7 @@ class Scaffold(_PackageBoundObject):
         #: automatically again that were removed that way.
         #:
         #: .. versionadded:: 0.7
-        self.url_default_functions = {}
+        self.url_default_functions = defaultdict(list)
 
     def _is_setup_finished(self):
         raise NotImplementedError
@@ -258,7 +261,7 @@ class Scaffold(_PackageBoundObject):
         non-None value, the value is handled as if it was the return value from
         the view, and further request handling is stopped.
         """
-        self.before_request_funcs.setdefault(None, []).append(f)
+        self.before_request_funcs[None].append(f)
         return f
 
     @setupmethod
@@ -272,7 +275,7 @@ class Scaffold(_PackageBoundObject):
         As of Flask 0.7 this function might not be executed at the end of the
         request in case an unhandled exception occurred.
         """
-        self.after_request_funcs.setdefault(None, []).append(f)
+        self.after_request_funcs[None].append(f)
         return f
 
     @setupmethod
@@ -311,7 +314,7 @@ class Scaffold(_PackageBoundObject):
            debugger can still access it.  This behavior can be controlled
            by the ``PRESERVE_CONTEXT_ON_EXCEPTION`` configuration variable.
         """
-        self.teardown_request_funcs.setdefault(None, []).append(f)
+        self.teardown_request_funcs[None].append(f)
         return f
 
     @setupmethod
@@ -334,7 +337,7 @@ class Scaffold(_PackageBoundObject):
         The function is passed the endpoint name and values dict. The return
         value is ignored.
         """
-        self.url_value_preprocessors.setdefault(None, []).append(f)
+        self.url_value_preprocessors[None].append(f)
         return f
 
     @setupmethod
@@ -343,7 +346,7 @@ class Scaffold(_PackageBoundObject):
         application.  It's called with the endpoint and values and should
         update the values passed in place.
         """
-        self.url_default_functions.setdefault(None, []).append(f)
+        self.url_default_functions[None].append(f)
         return f
 
     @setupmethod
@@ -416,8 +419,7 @@ class Scaffold(_PackageBoundObject):
                 " instead."
             )
 
-        handlers = self.error_handler_spec.setdefault(key, {}).setdefault(code, {})
+        self.error_handler_spec[key][code][exc_class] = f
-        handlers[exc_class] = f
 
     @staticmethod
     def _get_exc_class_and_code(exc_class_or_code):