dig.py

def dig(d, keys, *args):
  """Locate a value in a nested dictionary.
  
  Args:
    d (dict): the dictionary
    keys (list): list of keys to index the dictionary with
    default (value|callable): value or callable to return in case of missing key
    
  Returns: 
    Value at `d[k0][k1][...]` if all keys are present
    Value of `default` if missing key encountered
    
  Raises: 
    KeyError: if missing key (unless `default` provided)
  
  Accepts a nested dictionary, a sequence of keys, and an optional
  default return value in case of an exception. 
  Given the dictionary `dict` and the keys `[k1, k2, k3]`, attempts 
  to return the value located at dict[k1][k2][k3]. 
  
  Unless the `default` argument is provided, a missing key will cause 
  a KeyError exception to be raised.
  
  If the `default` argument is provided, it will be the return value
  in case of a KeyError (rather than the exception). The value for
  `default` can be either a plain value or a function. If it is a value,
  it is returned as-is. If it is a function, the function is called 
  with the last found value and the missing key as arguments and the 
  function's return value is returned to the caller.
  
  d = { 'a': { 'b': { 'c': { 'd': True } } } }

  dig(d, ['a', 'b', 'c', 'd'])         # True
  dig(d, ['a', 'c'])                   # raises KeyError
  dig(d, ['a', 'c', 'd'], None)        # None
  dig(d, ['a', 'b', 'd'], lambda v: v) # d['a']['b'] == {'c': {'d': True}}
  """
  def error(d, k): raise
  default = args[0] if args else error
  try:
    value = d[keys[0]]
  except KeyError:
    return default(d, keys[0]) if callable(default) else default
  return dig(value, keys[1:], default) if len(keys) > 1 else value


d = { 'a': { 'b': { 'c': { 'd': True } } } }

dig(d, ['a', 'b', 'c', 'd'])            # True
dig(d, ['a', 'e'])                      # raises KeyError
dig(d, ['a', 'b', 'e'], None)           # None
dig(d, ['a', 'b', 'e'], lambda d, k: d) # d['a']['b'] == {'c': {'d': True}}
dig(d, ['a', 'b', 'e'], lambda d, k: d).setdefault('e', "Hello, world.") 
dig(d, ['a', 'b', 'e'])                 # "Hello, world."