[Python] - Dictionary

Dictionary

---

type is immutable key and mutable value mapping not sorted set
No index , so access the key

 

• Syntax

# Default type
{"a" : 1, "b":2}

e = {}
f = dict()
newdict = dict( alice = 5, bob = 20, tony= 15, suzy = 30)

• Key is not allowed mutable key like list , set , dict

# Error case
a = { {1, 3}: 5, {3,5}: 3}     #set
a = {[1,3]: 5, [3,5]: 3}       #list
a = { {"a":1}: 5, "abc": 3}    #dict

# Key immuttable
a = {1: 5, 2: 3}   # int 사용
a = {(1,5): 5, (3,3): 3} # tuple사용
a = { True: 5, "abc": 3} # bool 사용

 

• list of list and list of tuple and tuple of list and tuple of tuple can changed the dictionary

name_and_ages = [['alice', 5], ['Bob', 13]]
name_and_ages = [('alice', 5), ('Bob', 13)]
name_and_ages = (('alice', 5), ('Bob', 13))
name_and_ages = (['alice', 5], ['Bob', 13])

# all case change the dict
dict(name_and_ages)

 

복사

---

shallow copy

얕은 복사는 동일한 주소를 할당 받기 때문에 변경된다.

a = {'alice': [1, 2, 3], 'bob': 20, 'tony': 15, 'suzy': 30}

b =a.copy()
c = dict(a)

a['alice'].append(5)

# change b['alice'] / c['alice']
# why? allocated same address

deep copy

import copy
a = {'alice': [1, 2, 3], 'bob': 20, 'tony': 15, 'suzy': 30}
b = copy.deepcopy(a)

 

Dictionary function

---

a = {'alice': [1, 2, 3], 'bob': 20, 'tony': 15, 'suzy': 30}

# key
a.keys()
# value
a.values()
# key-value
a.items()
# Passing value if key does not exist
a.get( 'alice' , 0 )
> [1, 2, 3]
a.get( 'tt' , 0 )
> 0

# dictionary 'in' use key
'alice' in a
> True

# delete key
del a['alice']

 

 

Defaultdict

---

• Dictionary key에 대한 Value 초기화 값 설정 할 때 사용한다.

from collections import defaultdict

a = defaultdict(int)
a[1]
> 0

a = defaultdict(lambda :3 )
a[1]
> 3