Dictionary 字典

不像序列是由一個範圍內的數字當作索引，dictionary 是由鍵 (key) 來當索引，鍵可以是任何不可變的類型；字串和數字都可以當作鍵。Tuple 也可以當作鍵，如果他們只含有字串、數字或 tuple；若一個 tuple 直接或間接地含有任何可變的物件，它就不能當作鍵。你無法使用 list 當作鍵，因為 list 可以經由索引指派 (index assignment)、切片指派 (slice assignment) 或是像 append() 和 extend() 等 method 被修改。

思考 dictionary 最好的方式是把它想成是一組鍵值對 (key: value pair) 的 set，其中鍵在同一個 dictionary 裡必須是獨一無二的。使用一對大括號可建立一個空的 dictionary：{}。將一串由逗號分隔的鍵值對置於大括號則可初始化字典的鍵值對。這同樣也是字典輸出時的格式。

基本範例

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
file_counts["txt"]
# Output: 14

檢查索引

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
"jpg" in file_counts
# Output: True

新增元素

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
file_counts["cfg"] = 8
print(file_counts)
# Output {'jpg': 10, 'txt': 14, 'csv': 2, 'py': 23, 'cfg': 8}

變更指定索引的元素

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
file_counts["csv"] = 17
print(file_counts)
# Output {'jpg': 10, 'txt': 14, 'csv': 17, 'py': 23}

刪除指定索引的元素

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23, 'cfg':8}
del file_counts["cfg"]
print(file_counts)
# Output {'jpg': 10, 'txt': 14, 'csv': 2, 'py': 23}

Dictionary methods

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
for extension in file_counts:
  print(extension)

# Output
jpg
txt
csv
py

.items() 

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
for ext, amount in file_counts.items():
  print("There are {} files with the .{} extension".format(amount, ext))

# Output
There are 10 files with the .jpg extension
There are 14 files with the .txt extension
There are 2 files with the .csv extension
There are 23 files with the .py extension

.keys() , .values() 

file_counts = {"jpg":10, "txt":14, "csv":2, "py":23}
file_counts.keys()   # Return dict_keys(['jpg', 'txt', 'csv', 'py'])
file_counts.values() # Return dict_values([10, 14, 2, 23])

file_counts 
= {"jpg":10, "txt":14, "csv":2, "py":23}
for value in file_counts.values():
  print(value)

# Output
10
14
2
23