[Python] Dictionary Data Structure

Dictionary is a data structure with key and value.

Dictionary is internally implemented as hash table, so dictionary is a fast data structure.

Dictionary is unordered before Python 3.6, and ordered after Python 3.7.

The key of dictionary must be unique.

Creating Dictionary

There are many ways to create dictionary.

### {} method
emptyDict = {}
testDict = {
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
}
print(testDict)
# {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}


### dict() function
testDict = dict([
    ['key1', 'value1'], ['key2', 'value2'], ['key3', 'value3']
])
print(testDict)
# {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}


### Dictionary Comprehension
word = 'letters'
letter_counts = {letter: word.count(letter) for letter in set(word)}
print(letter_counts)
# {'s': 1, 't': 2, 'e': 2, 'r': 1, 'l': 1}


### from Tuple
testTuple = ('ab', 'cd', 'ef')      # possible only 2 characters
testDict = dict(testTuple)
print(testDict)
# {'a': 'b', 'c': 'd', 'e': 'f'}


### from List
testList = ['ab', 'cd', 'ef']       # possible only 2 characters
testDict = dict(testList)
print(testDict)
# {'a': 'b', 'c': 'd', 'e': 'f'}

testList = [['r', 'red'], ['g', 'green'], ['b', 'blue']]
testDict = dict(testList)
print(testDict)
# {'r': 'red', 'g': 'green', 'b': 'blue'}


### from Mixed type
test = (['r', 'red'], ['g', 'green'], ['b', 'blue'])
testDict = dict(test)
print(testDict)
# {'r': 'red', 'g': 'green', 'b': 'blue'}

test = [('r', 'red'), ('g', 'green'), ('b', 'blue')]
testDict = dict(test)
print(testDict)
# {'r': 'red', 'g': 'green', 'b': 'blue'}

Getting Item from Dictionary

testDict = {"key1": "value1", "key2": "value2", "key3": "value3"}


### [key]
print(testDict['key2'])
# value2


### get()
print(testDict.get('key3'))
# value3


### keys()
print(testDict.keys())
# dict_keys(['key1', 'key2', 'key3'])


### values()
print(testDict.values())
# dict_values(['value1', 'value2', 'value3'])


### items()
print(testDict.items())
# dict_items([('key1', 'value1'), ('key2', 'value2'), ('key3', 'value3')])

Setting or Adding Item to Dictionary

testDict = {"key1": "value1", "key2": "value2"}
testDict['key1'] = 'value11'
testDict['key3'] = 'value3'
print(testDict)
# {'key1': 'value11', 'key2': 'value2', 'key3': 'value3'}

Updating Item of Dictionary

testDict1 = {"key1": "value1", "key2": "value2", "key3": "value3"}
testDict2 = {"key4": "value4", "key5": "value5"}
testDict1.update(testDict2)
print(testDict1)
# {'key1': 'value1', 'key2': 'value2', 'key3': 'value3', 'key4': 'value4', 'key5': 'value5'}

testDict3 = {"key2": "value22", "key6": "value6"}
testDict1.update(testDict3)
print(testDict1)
# {'key1': 'value1', 'key2': 'value22', 'key3': 'value3', 'key4': 'value4', 'key5': 'value5', 'key6': 'value6'}

Removing Item from Dictionary

testDict = {"key1": "value1", "key2": "value2", "key3": "value3"}


### del
del testDict['key1']
print(testDict)
# {'key2': 'value2', 'key3': 'value3'}


### clear()
testDict.clear()
print(testDict)
# {}

Checking Item of Dictionary

testDict = {"key1": "value1", "key2": "value2", "key3": "value3"}

print('key1' in testDict)
# True

print('key4' in testDict)
# False

Searching Items from Dictionary

testDict = {"key1": "value1", "key2": "value2", "key3": "value3"}

for k, v in testDict.items():
    print(f"{k}: {v}")
# key1: value1
# key2: value2
# key3: value3

Getting Length of Dictionary

testDict = {"key1": "value1", "key2": "value2", "key3": "value3"}

print(len(testDict))
# 3

Copying Dictionary

testDict = {"key1": "value1", "key2": "value2", "key3": "value3"}

copiedDict1 = testDict          # same as testDict
copiedDict2 = testDict.copy()   # different

testDict['key2'] = 'value22'

print(copiedDict1)
# {'key1': 'value1', 'key2': 'value22', 'key3': 'value3'}

print(copiedDict2)
# {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}

Dictionary Module

defaultdict Object

When a defaultdict object queries a key that doesn't exist, it generates an item for that key based on the default value instead of an error message.

import collections

testDict = collections.defaultdict(int)
testDict['key1'] = 1
print(testDict)
# defaultdict(<class 'int'>, {'key1': 1})

testDict['key2']
print(testDict)
# defaultdict(<class 'int'>, {'key1': 1, 'key2': 0})

Counter Object

Counter object counts the number of items and returns them to the dictionary.

most_common() method returns the most frequent items.

The argument represents how many items return.

import collections

testList = [1, 2, 3, 1, 2, 4, 1]
testDict = collections.Counter(testList)
print(testDict)
# Counter({1: 3, 2: 2, 3: 1, 4: 1})

print(testDict.most_common(2))
# [(1, 3), (2, 2)]

Ordered Object

In most languages, data types using hash tables don't maintain the input order.

Python 3.6 and before were the same, so it provided an object that maintained the input order.

import collections

testList = collections.OrderedDict({})
testList['key2'] = 'value2'
testList['key1'] = 'value1'
testList['key3'] = 'value3'
print(testList)
# OrderedDict([('key2', 'value2'), ('key1', 'value1'), ('key3', 'value3')])

However, dictionary in Python 3.7 and later keep its input order.