named_axis: fix typo and avoid dictionary copies where possible

src/awkward/_namedaxis.py

@@ -145,7 +145,7 @@ def _get_named_axis(
     if isinstance(ctx, MaybeSupportsNamedAxis):
         return _get_named_axis(ctx.attrs)
     elif isinstance(ctx, tp.Mapping) and NAMED_AXIS_KEY in ctx:
-        return dict(ctx[NAMED_AXIS_KEY])
+        return ctx[NAMED_AXIS_KEY]
     else:
         return {}
 
@@ -306,7 +306,7 @@ def _prepare_named_axis_for_attrs(
     if isinstance(named_axis, tuple):
         _named_axis = _axis_tuple_to_mapping(named_axis)
     elif isinstance(named_axis, dict):
-        _named_axis = dict(named_axis)
+        _named_axis = named_axis
     else:
         raise TypeError(
             f"named_axis must be a mapping or a tuple, got {named_axis=} [{type(named_axis)=}]"
@@ -380,7 +380,7 @@ def _named_axis_to_positional_axis(
 # The possible strategies are:
 # - "keep all" (_keep_named_axis(..., None)): Keep all named axes in the output array, e.g.: `ak.drop_none`
 # - "keep one" (_keep_named_axis(..., int)): Keep one named axes in the output array, e.g.: `ak.firsts`
-# - "keep up to" (_keep_named_axis_up_to(..., int)): Keep all named axes upto a certain positional axis in the output array, e.g.: `ak.local_index`
+# - "keep up to" (_keep_named_axis_up_to(..., int)): Keep all named axes up to a certain positional axis in the output array, e.g.: `ak.local_index`
 # - "remove all" (_remove_all_named_axis): Removes all named axis, e.g.: `ak.categories`
 # - "remove one" (_remove_named_axis): Remove the named axis from the output array, e.g.: `ak.sum`
 # - "add one" (_add_named_axis): Add a new named axis to the output array, e.g.: `ak.concatenate`
@@ -409,7 +409,7 @@ def _keep_named_axis(
         {"x": 0, "y": 1, "z": 2}
     """
     if axis is None:
-        return dict(named_axis)
+        return named_axis
     return {k: 0 for k, v in named_axis.items() if v == axis}
 
 
@@ -577,10 +577,7 @@ def _adjust(pos: int, axis: int, direction: int) -> int:
             else:
                 return pos
 
-    out = dict(named_axis)
-    for k, v in out.items():
-        out[k] = _adjust(v, axis, direction)
-    return out
+    return {k: _adjust(v, axis, direction) for k, v in named_axis.items()}
 
 
 def _add_named_axis(
@@ -608,8 +605,7 @@ def _add_named_axis(
     if total is None:
         total = len(named_axis)
 
-    out = dict(named_axis)
-    return _adjust_pos_axis(out, axis, total, direction=+1)
+    return _adjust_pos_axis(named_axis, axis, total, direction=+1)
 
 
 def _unify_named_axis(