bmesh py api:

Wrap customdata, so far you can access the data layers in a pythonic way but not manipulate the customdata yet.

provides dictionary like access to customdata layers, eg:
  texpoly = bm.faces.tex["UVMap"]
  print(bm.verts.shape.keys())  # un-intended pun, keys() works on all layers.
  print("MyInt" in bm.edges.int)  # __contains__
  layer = bm.faces.get("CheckForLayer")

source/blender/python/intern/bpy_rna.c

@@ -3927,7 +3927,7 @@ PyDoc_STRVAR(pyrna_prop_collection_keys_doc,
 "   (matching pythons dict.keys() functionality).\n"
 "\n"
 "   :return: the identifiers for each member of this collection.\n"
-"   :rtype: list of stings\n"
+"   :rtype: list of strings\n"
 );
 static PyObject *pyrna_prop_collection_keys(BPy_PropertyRNA *self)
 {