Dictionary是一種無序性(unordered)的key-value pair。

以下分別就key和value來自問自答

Key

key是否能重複呢?

在python官方的說明文件中提到key有唯一性

an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary)

key可以是什麼呢?

因為需要當作取得value的索引，若key會被外部因素變來變去，造成上一秒指定的key下一秒就被改掉，似乎也不太合理，所以必須為immutable type。舉例來說: strings, numbers, tuple or Enumeration members都可以。

嚴格上來看，若要使用tuple作為key，則tuple內的元素也必須要immutable。

# dict key : Uniqueness, immutable type
demo = { 'height': 170, 'weight': 58, 'height': 20 }
print( "demo data:{}".format(demo) )

# demo data:{'height': 20, 'weight': 58}


demo2 = { 'height': 170, 'weight': 58, 'Height': 20 }
print( "demo2 data:{}".format(demo2) )

# demo2 data:{'height': 170, 'weight': 58, 'Height': 20}

demo3 = { 5: '170', 6: '58' }
print( "demo3 data:{}".format(demo3) )

# demo3 data:{5: '170', 6: '58'}


demo4 = {}
list_as_key = [0, 1, 2 ,3 ,4]
tuple_as_key = ('0', 1, '2' ,3 ,4)
tuple_as_key_with_list = (0, 1, 2 ,3 ,4, list_as_key)
try:
    
    demo4[list_as_key] = 5
    print( "Type Error: {0}".format(e) )
except TypeError as e:
    print( "Type Error: {0}".format(e) )

try:
    demo4[tuple_as_key] = 5
    print( "demo4 data:{}".format(demo4) )
except TypeError as e:
    print( "Type Error: {0}".format(e) )

try:
    demo4[tuple_as_key_with_list] = 5
    print( "demo4 data:{}".format(demo4) )
except TypeError as e:
    print( "Type Error: {0}".format(e) )

# Type Error: unhashable type: 'list'
# demo4 data:{('0', 1, '2', 3, 4): 5}
# Type Error: unhashable type: 'list'

Value

value可以是什麼呢?

理論上甚麼都可以

Mutable

和list相同，dictionary也是mutable

# dict is mutable
demo = { 'height': 170, 'weight': 58, 'age': 20 }
print( "demo data:{}".format(demo) )
print( "id of demo data:{}".format(id(demo)) )

# demo data:{'height': 170, 'weight': 58, 'age': 20}
# id of demo data:139675823957480


demo['height'] = 180
print( "demo data:{}".format(demo) )
print( "id of demo data:{}".format(id(demo)) )

# demo data:{'height': 180, 'weight': 58, 'age': 20}
# id of demo data:139675823957480


demo['name'] = 'Tom'
print( "demo data:{}".format(demo) )
print( "id of demo data:{}".format(id(demo)) )

# demo data:{'height': 180, 'weight': 58, 'age': 20, 'name': 'Tom'}
# id of demo data:139675823957480


del demo['age']
print( "demo data:{}".format(demo) )
print( "id of demo data:{}".format(id(demo)) )

# demo data:{'height': 180, 'weight': 58, 'name': 'Tom'}
# id of demo data:139675823957480

Recap

• key
  • 唯一性
  • immutable type
    • strings, numbers or tuple
• value
• mutable

參考

• python官方
  • tutorial - 5.5. Dictionaries
  • library - 4.10. Mapping Types — dict
• Google for Education - Python Dict and File