Try to set a dict to true at a key only if the key already exists in the dict. Assume dict is a vanilla dict.

gistfile1.py

def setExistingKeyToTrue(addict, key):
  """Try to set a dict to True at a key only if the key already exists in the dict. Assume addict is a vanilla dict. Try to do this with only a single hash table lookup."""
  try:
    addict[key] = bool(addict[key]) # Fails criteria, two hash lookups
    addict[key] = True # Fails criteria, always sets value
  except KeyError:
    pass