Added PhotosDB() behavior to open last library if no args passed but also added cautionary note to README

README.md

@@ -268,6 +268,7 @@ if __name__ == "__main__":
 #### Read a Photos library database
 
 ```python
+osxphotos.PhotosDB() # not recommended, see Note below
 osxphotos.PhotosDB(path)
 osxphotos.PhotosDB(dbfile=path)
 ```
@@ -278,6 +279,8 @@ Pass the path to a Photos library or to a specific database file (e.g. "/Users/s
 
 If an invalid path is passed, PhotosDB will raise `ValueError` exception.
 
+**Note**: If neither path or dbfile is passed, PhotosDB will use get_last_library_path to open the last opened Photos library.  This usually works but is not 100% reliable.  It can also lead to loading a different library than expected if the user has held down *option* key when opening Photos to switch libraries.  It is therefore recommended you explicitely pass the path to `PhotosDB()`.  
+
 Open the default (last opened) Photos library. (E.g. this is the library that would open if the user opened Photos.app)
 
 ```python

osxphotos/photosdb.py

@@ -165,11 +165,10 @@ class PhotosDB:
                     dbfile_,
                 )
         elif dbfile is None:
-            raise TypeError(
-                f"photos database path must be specified as argument or named parameter dbfile",
-                dbfile_,
-                dbfile,
-            )
+            dbfile = get_last_library_path()
+            if dbfile is None:
+                # get_last_library_path must have failed to find library
+                raise FileNotFoundError("Could not get path to photo library database")
 
         if os.path.isdir(dbfile):
             # passed a directory, assume it's a photoslibrary