thread safe to lock on key,val pairs on a dict instead of entire dict?

[birdsong]

Dictionaries just store references to objects, right?  So is it thread
safe to lock a specific key/val pair on a dictionary and modify its
val and release the lock?

example snippet:
# assuming d_lock  was initialized long ago in a thread-safe manner
d_lock.acquire()
d = {}
d[1] = (threading.Lock(), [])
d_lock.release()

# test key level locking
for key, data in d.items():
  row_lock, rows = data
  row_lock.acquire()
  rows.append(1)
  row_lock.release()

Of course, I'll have to lock the entire dict when adding keys that
dont exist, but further calls and acquire the key specific lock before
doing any write operations.