A dict object can have any mix of data types as its values, including other dict objects. In other words, you can have a dictionary-of-dictionaries such as the one below:
# The 100m medal winners at the Tokyo 2020 Olympics
athletics = {
'100m': {
'Men': {
'Gold': 'Marcell Jacobs',
'Silver': 'Fred Kerley',
'Bronze': 'Andre De Grasse'
},
'Women': {
'Gold': 'Elaine Thompson-Herah',
'Silver': 'Shelly-Ann Fraser-Pryce',
'Bronze': 'Shericka Jackson'
}
}
}This is a pretty useful way of storing data because it’s logical: to get the name of the gold medallist in the men’s 100m you can look up '100m' > 'Men' > 'Gold' as shown below:
print(athletics['100m']['Men']['Gold'])## Marcell JacobsAdd another item (key+value) to a nested dictionary by indexing an as-of-yet non-existent key and assigning a value to it:
# Add a key-value pair to the nested dictionary
athletics['100m']['Women']['Fourth'] = 'Marie-Josée Ta Lou'Add another dictionary to a nested dictionary in a similar way:
# The 200m medal winners at the Tokyo 2020 Olympics
results200m = {
'Men': {
'Gold': 'Andre De Grasse',
'Silver': 'Kenneth Bednarek',
'Bronze': 'Noah Lyles',
},
'Women': {
'Gold': 'Elaine Thompson-Herah',
'Silver': 'Christine Mboma',
'Bronze': 'Gabrielle Thomas',
}
}
# Add to the 'athletics' dictionary
athletics['200m'] = results200mAnother way to do it:
results100m = athletics['100m']
# Combine the two dictionaries
athletics = {
'100m': results100m,
'200m': results200m,
}The above examples used dictionaries where all the keys and values were strings. Of course, this doesn’t have to be the case: keys can be strings, numbers or tuples and values can be any data type:
# The 400m medal winners at the Tokyo 2020 Olympics
athletics['400m'] = {
'Men': {
1: 'Steven Gardiner',
2: 'Anthony Zambrano',
3: 'Kirani James',
},
'Women': {
(1, 'Name'): 'Shaunae Miller-Uibo',
(1, 'Time'): 48.36,
(2, 'Name'): 'Marileidy Paulino',
(2, 'Time'): 49.20,
(3, 'Name'): 'Allyson Felix',
(3, 'Time'): 49.46,
}
}Indexing values now looks like this:
print(athletics['400m']['Men'][1])## Steven Gardinerprint(athletics['400m']['Women'][(2, 'Time')])## 49.2As mentioned, the values in a dict can be of any data type. Often it will be useful to have them be lists and this leads onto the next section…
Dictionaries containing a mix of both lists and dictionaries is a very common format for data to be stored in. Usually, these will appear in the form of JavaScript Object Notation (JSON) files which are, once imported into Python, essentially exactly this. Here’s an example of what the contents of a file like this might look like:
# The 800m medal winners at the Tokyo 2020 Olympics
results800m = {
'Women': {
1: ['Athing Mu', 'United States', '1:55.21'],
2: ['Keely Hodgkinson', 'Great Britain', '1:55.88'],
3: ['Raevyn Rogers', 'United States', '1:56.81'],
},
'Men': [
['Gold', 'Emmanuel Korir', 'Kenya', '1:45.06'],
['Silver', 'Ferguson Rotich', 'Kenya', '1:45.23'],
['Bronze', 'Patryk Dobek', 'Poland', '1:45.39'],
]
}print(results800m['Women'][1][2])## 1:55.21Note that, in the indexing above, the [1] is the key of a value in a dictionary and the [2] is the position of an element in a list.
You can, of course, also have a list of dictionaries:
results800m = [
{1: 'Athing Mu', 2: 'Keely Hodgkinson', 3: 'Raevyn Rogers'},
{1: 'Emmanuel Korir', 2: 'Ferguson Rotich', 3: 'Patryk Dobek'},
]print(results800m[0][1])## Athing MuNested dictionaries can be merged in the same way as any dictionaries:
athletics = {**athletics, **{'800m': results800m}}
print(athletics.keys())## dict_keys(['100m', '200m', '400m', '800m'])When you loop through a dictionary, you will be looping over the keys, and only the top-level keys at that:
for key in athletics:
print(key)## 100m
## 200m
## 400m
## 800mPrinting the value(s) of each key will show the entire dictionary’s contents
for key in athletics:
print(athletics[key])## {'Men': {'Gold': 'Marcell Jacobs', 'Silver': 'Fred Kerley', 'Bronze': 'Andre De Grasse'}, 'Women': {'Gold': 'Elaine Thompson-Herah', 'Silver': 'Shelly-Ann Fraser-Pryce', 'Bronze': 'Shericka Jackson', 'Fourth': 'Marie-Josée Ta Lou'}}
## {'Men': {'Gold': 'Andre De Grasse', 'Silver': 'Kenneth Bednarek', 'Bronze': 'Noah Lyles'}, 'Women': {'Gold': 'Elaine Thompson-Herah', 'Silver': 'Christine Mboma', 'Bronze': 'Gabrielle Thomas'}}
## {'Men': {1: 'Steven Gardiner', 2: 'Anthony Zambrano', 3: 'Kirani James'}, 'Women': {(1, 'Name'): 'Shaunae Miller-Uibo', (1, 'Time'): 48.36, (2, 'Name'): 'Marileidy Paulino', (2, 'Time'): 49.2, (3, 'Name'): 'Allyson Felix', (3, 'Time'): 49.46}}
## [{1: 'Athing Mu', 2: 'Keely Hodgkinson', 3: 'Raevyn Rogers'}, {1: 'Emmanuel Korir', 2: 'Ferguson Rotich', 3: 'Patryk Dobek'}]The above is equivalent to:
for key, value in athletics.items():
print(value)In order to iterate over every item in a dictionary, as opposed to just the top-level keys, you will need to use a series of loops (or one recursive loop, as demonstrated below):
def get_all_values(obj, level=0):
"""Walk through a dictionary of dicts and lists."""
if type(obj) is dict:
for key, value in obj.items():
if type(value) in [dict, list]:
print(' ' * level, key, sep='')
level = level + 1
get_all_values(value, level)
level = level - 1
else:
print(' ' * (level), key, ': ', value, sep='')
elif type(obj) is list:
for i, element in enumerate(obj):
if type(element) in [dict, list]:
print(' ' * level, i, sep='')
level = level + 1
get_all_values(element, level)
level = level - 1
else:
print(' ' * (level), element, sep='')
else:
raise ValueError
get_all_values(athletics)## 100m
## Men
## Gold: Marcell Jacobs
## Silver: Fred Kerley
## Bronze: Andre De Grasse
## Women
## Gold: Elaine Thompson-Herah
## Silver: Shelly-Ann Fraser-Pryce
## Bronze: Shericka Jackson
## Fourth: Marie-Josée Ta Lou
## 200m
## Men
## Gold: Andre De Grasse
## Silver: Kenneth Bednarek
## Bronze: Noah Lyles
## Women
## Gold: Elaine Thompson-Herah
## Silver: Christine Mboma
## Bronze: Gabrielle Thomas
## 400m
## Men
## 1: Steven Gardiner
## 2: Anthony Zambrano
## 3: Kirani James
## Women
## (1, 'Name'): Shaunae Miller-Uibo
## (1, 'Time'): 48.36
## (2, 'Name'): Marileidy Paulino
## (2, 'Time'): 49.2
## (3, 'Name'): Allyson Felix
## (3, 'Time'): 49.46
## 800m
## 0
## 1: Athing Mu
## 2: Keely Hodgkinson
## 3: Raevyn Rogers
## 1
## 1: Emmanuel Korir
## 2: Ferguson Rotich
## 3: Patryk DobekQuite simply, a list comprehension creates a new list while a dictionary comprehension creates a new dictionary. So, if we have a list as follows:
list_x = [0, 1, 2, 3, 4, 5]An example of a list comprehension would be:
# Square the numbers in list_x
square_numbers = [x**2 for x in list_x]
print(square_numbers)## [0, 1, 4, 9, 16, 25]While an example of a dictionary comprehension would be:
# Square the numbers in list_x
square_numbers = {x: x**2 for x in list_x}
print(square_numbers)## {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}If we have a second list as follows:
list_y = ['zero', 'one', 'two', 'three', 'four', 'five']Then we can combine the two lists into one dictionary. Any of the following will work for this (ie these will produce the same output):
dct = {x: list_y[x] for x in list_x}
dct = {i: v for i, v in enumerate(list_y)}
dct = {n: list_y[n] for n in range(len(list_y))}
print(dct)## {0: 'zero', 1: 'one', 2: 'two', 3: 'three', 4: 'four', 5: 'five'}But the following will NOT work as expected:
dct = {x: y for y in list_y for x in list_x}
dct = {x: y for x in list_x for y in list_y}
print(dct)## {0: 'five', 1: 'five', 2: 'five', 3: 'five', 4: 'five', 5: 'five'}Here’s an example that counts the letters of the alphabet:
import string
lt = string.ascii_letters
dct = {n: lt[n] for n in range(10)}
print(dct)## {0: 'a', 1: 'b', 2: 'c', 3: 'd', 4: 'e', 5: 'f', 6: 'g', 7: 'h', 8: 'i', 9: 'j'}This isn’t the same as a dictionary comprehension! It’s merely a list comprehension that happens to use a dictionary:
# The men's 100m medal winners at the Tokyo 2020 Olympics
dct = {
'Gold': 'Marcell Jacobs',
'Silver': 'Fred Kerley',
'Bronze': 'Andre De Grasse'
}
ls = [v for k, v in dct.items()]
print(ls)## ['Marcell Jacobs', 'Fred Kerley', 'Andre De Grasse']We can also get selective about which values we include in the output of our list comprehension:
# Use "k" and "v" for "key" and "value"
ls = [v for k, v in dct.items() if k != 'Bronze']
print(ls)## ['Marcell Jacobs', 'Fred Kerley']