Add headers.get and headers.getlist

[Julien00859]

Hello there, first time contributing :)

I wanted to access a header value with little code, like req.headers[b"content-type"] but since the headers are implemented as a Sequence there are no easy way beside iterating all the headers to find the one that interest me.

I am not challenging the internal data-structure, list(tuple(bytes, bytes, bytes)) is kinda the only sane structure because of headers like Set-Cookie.

In this work I add two helpers get(name) -> bytes and getlist(name) -> list[bytes] that searches the headers for me, so I don't have to write this next(generator, None) everywhere 😄 The two method names at coined at the Headers class of werkzeug. We might also add the type= parameter for total symmetry, you tell me.

The implementation is simple stupid, it iterates over the headers looking for one that matches the name.

I did experiment with another implementation, as follow (didn't test it):

     def __init__(self, full_items: List[Tuple[bytes, bytes, bytes]]) -> None:
         self._full_items = full_items
+        self._index = {}
+        for idx, (_, name, value) in enumerate(full_items):
+            self._index.setdefault(name, []).append(idx)
 
     def __bool__(self) -> bool:
         return bool(self._full_items)
@@ -118,6 +133,15 @@ class Headers(Sequence[Tuple[bytes, bytes]]):
         _, name, value = self._full_items[idx]
         return (name, value)
 
+    def get(self, name: bytes, default: T = None) -> Union[bytes, T]:
+        try:
+            self[self._index[name][0]][1]
+        except KeyError:
+            return default
+
+    def getlist(self, name: bytes) -> List[bytes]:
+        return [self[idx][1] for idx in self._index.get(name, ())]

But I don't think it deserves an index. I think the O(n) search is fine.

[Julien00859]

My IDE was telling me AnyStr and cast were not used, I used the opportunity of changing the line to remove them

[Julien00859]

Hello @njsmith, do you consider this work valuable? If not would you like to close this PR?