Python dictionary
last modified March 24, 2024
In this article we work with a Python dictionary.
Python dictionary is a container of key-value pairs. It is mutable and can contain mixed types. A dictionary is an unordered collection. Python dictionaries are called associative arrays or hash tables in other languages. The keys in a dictionary must be immutable objects like strings or numbers. They must also be unique within a dictionary.
Python dictionary creation
First, we show how to create Python dictionaries.
#!/usr/bin/python
# create_dict.py
weekend = { "Sun": "Sunday", "Mon": "Monday" }
vals = dict(one=1, two=2)
capitals = {}
capitals["svk"] = "Bratislava"
capitals["deu"] = "Berlin"
capitals["dnk"] = "Copenhagen"
d = { i: object() for i in range(4) }
print(weekend)
print(vals)
print(capitals)
print(d)
In the example, we create four dictionaries in four different ways. Later we print the contents of these dictionaries to the console.
weekend = { "Sun": "Sunday", "Mon": "Monday" }
We create a weekend dictionary using dictionary literal notation. The key-value
pairs are enclosed by curly brackets. The pairs are separated by commas. The
first value of a pair is a key, which is followed by a colon character and a
value. The "Sun" string is a key and the "Sunday"
string is a value.
vals = dict(one=1, two=2)
Dictionaries can be created using the dict function.
capitals = {}
capitals["svk"] = "Bratislava"
capitals["deu"] = "Berlin"
capitals["dnk"] = "Copenhagen"
In the third way an empty capitals dictionary is created. Three pairs are added to the dictionary. The keys are inside the square brackets, the values are located on the right side of the assignment.
d = { i: object() for i in range(4) }
A dictionary is created using a dictionary comprehension. The comprehension
has two parts. The first part is the i: object expression, which
is executed for each cycle of a loop. The second part is the for i in range(4)
loop. The dictionary comprehension creates a dictionary having four pairs, where
the keys are numbers 0, 1, 2, and 3 and the values are simple objects.
$ ./create_dict.py
{'Sun': 'Sunday', 'Mon': 'Monday'}
{'two': 2, 'one': 1}
{'svk': 'Bratislava', 'dnk': 'Copenhagen', 'deu': 'Berlin'}
{0: <object object at 0xb76cb4a8>, 1: <object object at 0xb76cb4b0>,
2: <object object at 0xb76cb4b8>, 3: <object object at 0xb76cb4c0>}
Python dictionary comprehension
A dictionary comprehension is a syntactic construct which creates a dictionary based on existing dictionary.
D = { expression for variable in sequence [if condition] }
A dictionary comprehension is placed between two curly brackets; it has three parts: for loop, condition, and expression.
In the for loop, we go through the dictionary. The optional if condition specifies a condition which must be met. In the end, the expression is evaluated. The expression produces elements of the output dictionary from members of the input sequence that satisfy the condition.
#!/usr/bin/python
# comprehension.py
capitals = { "Bratislava": 424207, "Vilnius": 556723, "Lisbon": 564657,
"Riga": 713016, "Jerusalem": 780200, "Warsaw": 1711324,
"Budapest": 1729040, "Prague": 1241664, "Helsinki": 596661,
"Yokyo": 13189000, "Madrid": 3233527 }
capitals2 = { key:val for key, val in capitals.items() if val < 1000000 }
print(capitals2)
In the example, we create a new dictionary from an existing dictionary.
capitals = { "Bratislava": 424207, "Vilnius": 556723, "Lisbon": 564657,
"Riga": 713016, "Jerusalem": 780200, "Warsaw": 1711324,
"Budapest": 1729040, "Prague": 1241664, "Helsinki": 596661,
"Yokyo": 13189000, "Madrid": 3233527 }
We have a dictionary of capitals. The capital in a key and the population is the value.
capitals = { key:val for key, val in capitals.items() if val < 1000000 }
A new dictionary is created using a dictionary comprehension. It contains capitals that have a population smaller than one million.
$ ./comprehension.py
{'Bratislava': 424207, 'Vilnius': 556723, 'Jerusalem': 780200, 'Riga': 713016,
'Lisbon': 564657, 'Helsinki': 596661}
These capitals have a population smaller than one million.
Basic operations with Python dictionaries
The following examples shows some basic operations with Python dictionaries.
#!/usr/bin/python
# basics.py
basket = { 'oranges': 12, 'pears': 5, 'apples': 4 }
basket['bananas'] = 5
print(basket)
print("There are {0} various items in the basket".format(len(basket)))
print(basket['apples'])
basket['apples'] = 8
print(basket['apples'])
print(basket.get('oranges', 'undefined'))
print(basket.get('cherries', 'undefined'))
We have a basket with different fruits. We perform some operations on the basket dictionary.
basket = { 'oranges': 12, 'pears': 5, 'apples': 4 }
The basket dictionary is created. It has initially three key-value pairs.
basket['bananas'] = 5
A new pair is created. The 'bananas' string is a key,
the 5 integer is the value.
print("There are {0} various items in the basket".format(len(basket)))
The len function gives the number of pairs in the dictionary.
print(basket['apples'])
The value of the 'apples' key is printed to the terminal.
basket['apples'] = 8
The value of the 'apples' key is modified. It is set to number 8.
print(basket.get('oranges', 'undefined'))
The get method retrieves the value of a specified key. If there is
no such a key, the second parameter of the method is returned.
print(basket.get('cherries', 'undefined'))
This line returns 'undefined'. There are no cherries in the basket.
$ ./basics.py
{'bananas': 5, 'pears': 5, 'oranges': 12, 'apples': 4}
There are 4 various items in the basket
4
8
12
undefined
Python dictionary - fromkeys and setdefault
The next example presents two dictionary methods: fromkeys
and setdefault.
#!/usr/bin/python
# fruits.py
basket = ('oranges', 'pears', 'apples', 'bananas')
fruits = {}.fromkeys(basket, 0)
print(fruits)
fruits['oranges'] = 12
fruits['pears'] = 8
fruits['apples'] = 4
print(fruits.setdefault('oranges', 11))
print(fruits.setdefault('kiwis', 11))
print(fruits)
The fromkeys method creates a new dictionary from a list. The
setdefault method returns a value if a key is present. Otherwise,
it inserts a key with a specified default value and returns the value.
basket = ('oranges', 'pears', 'apples', 'bananas')
We have a list of strings. From this list a new dictionary will be constructed.
fruits = {}.fromkeys(basket, 0)
The fromkeys method creates a new dictionary, where the list items
will be the keys. Each key will be initiated to 0. Note that the
fromkeys method is a class method and needs the class name, which
is {} in our case, to be called.
fruits['oranges'] = 12 fruits['pears'] = 8 fruits['apples'] = 4
Here we add some values to the fruits dictionary.
print(fruits.setdefault('oranges', 11))
print(fruits.setdefault('kiwis', 11))
The first line prints 12 to the terminal. The
'oranges' key exists in the dictionary. In such a case, the method
returns the its value. In the second case, the key does not exist yet. A new
pair 'kiwis': 11 is inserted to the dictionary. And value
11 is printed to the console.
$ ./fruits.py
{'bananas': 0, 'pears': 0, 'oranges': 0, 'apples': 0}
12
11
{'kiwis': 11, 'bananas': 0, 'pears': 8, 'oranges': 12, 'apples': 4}
We receive this output when we launch the fruits.py script.
Python dictionary update method
The next code example shows how to add two Python dictionaries using the
update method.
#!/usr/bin/python
# domains.py
domains = { "de": "Germany", "sk": "Slovakia", "hu": "Hungary"}
domains2 = { "us": "United States", "no": "Norway" }
domains.update(domains2)
print(domains)
Two dictionaries are joined with the update method.
domains.update(domains2)
The domains2 dictionary is added to the domains dictionary with the
update method.
$ ./domains.py
{'sk': 'Slovakia', 'de': 'Germany', 'no': 'Norway',
'us': 'United States', 'hu': 'Hungary'}
The result shows all values.
Python removing items from dictionary
Now we show how to remove a pair from a dictionary.
#!/usr/bin/python
# removing.py
items = { "coins": 7, "pens": 3, "cups": 2,
"bags": 1, "bottles": 4, "books": 5 }
print(items)
item = items.pop("coins")
print("Item having value {0} was removed".format(item))
print(items)
del items["bottles"]
print(items)
items.clear()
print(items)
The items dictionary has six key-value pairs. We will delete pairs from this dictionary.
item = items.pop("coins")
print("Item having value {0} was removed".format(item))
The pop method removes a pair with a specified key; it returns the
value of the removed key.
del items["bottles"]
The del keyword deletes the "bottles": 4 pair
from the items dictionary.
items.clear()
The clear method clears all items from the dictionary.
$ ./removing.py
{'bags': 1, 'pens': 3, 'coins': 7, 'books': 5, 'bottles': 4, 'cups': 2}
Item having value 7 was removed
{'bags': 1, 'pens': 3, 'books': 5, 'bottles': 4, 'cups': 2}
{'bags': 1, 'pens': 3, 'books': 5, 'cups': 2}
{}
Python dictionary keys and values
A Python dictionary consists of key-value pairs. The keys method
returns a list of keys from a dictionary. The values method creates
a list of values. And the items method returns a list of key-value
tuples.
#!/usr/bin/python
# keys_values.py
domains = { "de": "Germany", "sk": "Slovakia", "hu": "Hungary",
"us": "United States", "no": "Norway" }
print(domains.keys())
print(domains.values())
print(domains.items())
print("de" in domains)
print("cz" in domains)
We demonstrate the above mentioned methods. We also check if a key is
present with the in keyword.
print(domains.keys())
We print the list of keys of a domains dictionary with the
keys method.
print(domains.values())
We print the list of values of a domains dictionary with the
values method.
print(domains.items())
And finally, we print the list of key-value tuples of a domains
dictionary using the items method.
print("de" in domains)
print("cz" in domains)
With the in keyword, we check if the "de",
"cz" keys are present in the domains dictionary. The
return value is either True or
False.
$ ./keys_values.py
['sk', 'de', 'no', 'us', 'hu']
['Slovakia', 'Germany', 'Norway', 'United States', 'Hungary']
[('sk', 'Slovakia'), ('de', 'Germany'), ('no', 'Norway'),
('us', 'United States'), ('hu', 'Hungary')]
True
False
Python dictionary looping
Looping through the dictionary is a common programming job. This can be done with the for keyword.
#!/usr/bin/python
# looping.py
domains = { "de": "Germany", "sk": "Slovakia", "hu": "Hungary",
"us": "United States", "no": "Norway" }
for key in domains:
print(key)
for val in domains.values():
print(val)
for k, v in domains.items():
print(": ".join((k, v)))
In the example, we traverse the domains dictionary to print the
keys, values and both keys and values of the dictionary.
for key in domains:
print(key)
This loop prints all the keys of the dictionary.
for val in domains.values():
print(val)
The second loop prints all values of the dictionary.
for k, v in domains.items():
print(": ".join((k, v)))
In the third loop, all keys and values are printed.
$ ./looping.py sk de no us hu Slovakia Germany Norway United States Hungary sk: Slovakia de: Germany no: Norway us: United States hu: Hungary
Python dictionary membership testing
With the in and not in operators we can check if an
key is present in a dictionary.
#!/usr/bin/python
# membership.py
domains = { "de": "Germany", "sk": "Slovakia", "hu": "Hungary",
"us": "United States", "no": "Norway" }
key = "sk"
if key in domains:
print("{0} is in the dictionary".format(domains[key]))
In the example we check if a country is in the dictionary
with the in operator.
Python dictionary sorting
Starting from Python 3.7, dictionaries in CPython (the most common implementation of Python) maintain insertion order. This means the order in which you add key-value pairs to the dictionary is preserved when iterating over it or accessing elements.
We might want to sort the data in a normal or reverse order. We can sort the data by keys or by values.
#!/usr/bin/python
# simple_sort.py
items = { "coins": 7, "pens": 3, "cups": 2,
"bags": 1, "bottles": 4, "books": 5 }
kitems = list(items.keys())
kitems.sort()
for k in kitems:
print(": ".join((k, str(items[k]))))
The first example provides the simplest solution to have the data sorted by the keys.
kitems = items.keys() kitems.sort()
A list of keys is obtained from the dictionary. The list is sorted with the
sort method.
for k in kitems:
print(": ".join((k, str(items[k]))))
In the loop we print the sorted keys together with their values from the dictionary.
$ ./simple_sort.py bags: 1 books: 5 bottles: 4 coins: 7 cups: 2 pens: 3
The items dictionary is sorted by its keys.
More efficient sorting can be done with the
built-in sorted function.
#!/usr/bin/python
# sorting.py
items = { "coins": 7, "pens": 3, "cups": 2,
"bags": 1, "bottles": 4, "books": 5 }
for key in sorted(items.keys()):
print("%{0}: {1}".format(key, items[key]))
print("###############")
for key in sorted(items.keys(), reverse=True):
print("{0}: {1}".format(key, items[key]))
In the example we print sorted data by their keys in ascending and descending
order using the sorted function.
for key in sorted(items.keys()):
print("%{0}: {1}".format(key, items[key]))
In this for loop, we print the pairs sorted in ascending order. The
iteritems function returns an iterator over the dictionary's (key,
value) pairs.
for key in sorted(items.keys(), reverse=True):
print("{0}: {1}".format(key, items[key]))
In the second for loop, the data is sorted in descending order.
The order type is controlled by the reverse parameter.
$ ./sorting.py bags: 1 books: 5 bottles: 4 coins: 7 cups: 2 pens: 3 ############### pens: 3 cups: 2 coins: 7 bottles: 4 books: 5 bags: 1
In the next example, we are going to sort the items by their values.
#!/usr/bin/python
# sorting2.py
items = { "coins": 7, "pens": 3, "cups": 2,
"bags": 1, "bottles": 4, "books": 5 }
for key, value in sorted(items.items(), key=lambda pair: pair[1]):
print("{0}: {1}".format(key, value))
print("###############")
for key, value in sorted(items.items(), key=lambda pair: pair[1], reverse=True):
print("{0}: {1}".format(key, value))
The example prints the data in ascending and descending order by their values.
for key, value in sorted(items.iteritems(),
key=lambda (k,v): (v,k)):
Dictionary pairs are sorted by their values and printed to the console. The key parameter takes a function, which indicates, how the data is going to be sorted.
$ ./sorting2.py bags: 1 cups: 2 pens: 3 bottles: 4 books: 5 coins: 7 ############### coins: 7 books: 5 bottles: 4 pens: 3 cups: 2 bags: 1
From the output we can see that this time the pairs were sorted by their values.
Source
Python data structures - language reference
In this article we have written about Python dictionaries.
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