Update dictionary form list

update_dict_from_list.py

>>> my_list = ['a', 'b', 'c', 'd']

# existing dict object which already has some values in it
>>> my_dict = { 'a' : 5 , 'z': 10 }
>>> default_value = -1
>>> my_dict.update({k: default_value for k in my_list if k not in my_dict.keys()})
>>> my_dict
{'a': 5, 'c': -1, 'z': 10, 'b': -1, 'd': -1} # original values are left as-is

# also works with empty dict
>>> my_dict = {} 
>>> my_dict.update({k: default_value for k in my_list if k not in my_dict.keys()})
>>> my_dict
{'a': -1, 'c': -1, 'b': -1, 'd': -1}

# even if the dict object did not exist
>>> x = dict((k, default_value) for k in my_list)
>>> x
{'a': -1, 'c': -1, 'b': -1, 'd': -1}