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DICTionary

Literal syntax

d = {}
d = {'key': 'value'}

# makes a shallow copy of otherdict
d = {**otherdict}
# also updates the shallow copy with the contents of the yetanotherdict.
d = {**otherdict, **yetanotherdict}
# dict comprehension
d = {k:v for k,v in [('key', 'value',)]}

Dict.get method

One common pitfall when using dictionaries is to access a non-existent key. This typically results in a KeyError exception. One way to avoid key errors is to use the dict.get method, which allows you to specify a default value to return in the case of an absent key.

value = mydict.get(key, default_value)

Which returns mydict[key] if it exists, but otherwise returns default_value . Note that this doesn’t add key to mydict . So if you want to retain that key value pair, you should use mydict.setdefault(key, default_value) , which does store the key value pair.

mydict = {}
print(mydict)
# {}
print(mydict.get("foo", "bar"))
# bar
print(mydict)
# {}
print(mydict.setdefault("foo", "bar"))
# bar
print(mydict)
# {'foo': 'bar'}

Merging dictionaries

# Consider the following dictionaries:
fish = {'name': "Nemo", 'hands': "fins", 'special': "gills"}
dog = {'name': "Clifford", 'hands': "paws", 'color': "red"}

fishdog = {**fish, **dog}
print(fishdog)
# {'hands': 'paws', 'color': 'red', 'name': 'Clifford', 'special': 'gills'}

As this example demonstrates, duplicate keys map to their lattermost value (for example “Clifford” overrides “Nemo”).

Unpacking dictionaries using the ** operator

You can use the ** keyword argument unpacking operator to deliver the key -value pairs in a dictionary into a function’s arguments.

def parrot(voltage, state, action):
  print("This parrot wouldn't", action, end=' ')
  print("if you put", voltage, "volts through it.", end=' ')
  print("E's", state, "!")

d = {"voltage": "four million", "state": "bleedin' demised", "action": "VOOM"}
parrot(**d)

# This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !

Dictionary Comprehensions

A dictionary comprehension is similar to a list comprehension except that it produces a dictionary object instead of a list.

F=dict((x, x * x) for x in (1, 2, 3, 4))
print(F)
# Out: {1: 1, 2: 4, 3: 9, 4: 16}

As with a list comprehension, we can use a conditional statement inside the dict comprehension to produce only the dict elements meeting some criterion.

F={name: len(name) for name in ('Fcuk', 'The', 'Code') if len(name) > 3}
print(F)
# Out: {'Fcuk': 4, 'Code': 4}

Starting with a dictionary and using dictionary comprehension as a key-value pair filter

initial_dict = {'F': 1, 'C': 2}
F={key: value for key, value in initial_dict.items() if key == 'F'}
print(F)
# Out: {'F': 1}

Switching key and value of dictionary (invert dictionary)

If you have a dict containing simple hashable values (duplicate values may have unexpected results):

my_dict = {1: 'a', 2: 'b', 3: 'c'}

you wanted to swap the keys and values you can take several approaches depending on your coding style:

'''Approaches'''
''' #1 '''swapped = {v: k for k, v in my_dict.items()}
''' #2 '''swapped = dict((v, k) for k, v in my_dict.iteritems())
''' #3 '''swapped = dict(zip(my_dict.values(), my_dict))
''' #4 '''swapped = dict(zip(my_dict.values(), my_dict.keys()))
''' #5 '''swapped = dict(map(reversed, my_dict.items()))
print(swapped)
'''All leads to same output '''
# Output: {a: 1, b: 2, c: 3}

If your dictionary is large, consider importing itertools and utilize izip or imap .

Merging Dictionaries

Combine dictionaries and optionally override old values with a nested dictionary comprehension.

dict1 = {'w': 1, 'x': 1}
dict2 = {'x': 2, 'y': 2, 'z': 2}
F={k: v for d in [dict1, dict2] for k, v in d.items()}
print(F)
# Out: {'w': 1, 'x': 2, 'y': 2, 'z': 2}

#Python 3.x Version
F={**dict1, **dict2}
print(F)
# Out: {'w': 1, 'x': 2, 'y': 2, 'z': 2}

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