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                # 每天學點Python之dict 字典用來存儲鍵值對,在Python中同一個字典中的鍵和值都可以有不同的類型。 ### 字典的創建 創建一個空的字典有兩種方法: ~~~ d = {} d = dict() ~~~ 而創建一個包含元素的字典方法比較多,下面操作結果相同: ~~~ >>> a = dict(one=1, two=2, three=3) >>> b = {'one': 1, 'two': 2, 'three': 3} >>> c = dict(zip(['one', 'two', 'three'], [1, 2, 3])) >>> d = dict([('two', 2), ('one', 1), ('three', 3)]) >>> e = dict({'three': 3, 'one': 1, 'two': 2}) ~~~ ### 字典的查詢 獲取字典中的值只需通過dict[key]來獲取,如果不存在該鍵,會拋出KeyError錯誤;也可用`dict.get(k[,default])`方法來獲取,這個方法即使鍵不存在也不會拋出錯誤,也可以給一個默認的值,在鍵值對不存在時返回: ~~~ >>> a = dict(one=1, two=2, three=3) >>> a['one'] 1 >>> a['four'] Traceback (most recent call last): File "<input>", line 1, in <module> KeyError: 'four' >>> a.get('four') >>> a.get('four',4) 4 ~~~ ### 字典的修改 要修改字典中的鍵對應的值,只需要取出來直接對它賦值就行,需要注意的是,如果修改的鍵原來不存在,就變成了向字典中增加了一個鍵值對: ~~~ >>> a = dict(one=1, two=2, three=3) >>> a['one']=10 >>> a {'two': 2, 'one': 10, 'three': 3} >>> a['four']=4 >>> a {'two': 2, 'four': 4, 'one': 10, 'three': 3} ~~~ 而`dict.setdefault(key[,default])`在key存在時,不做修改返回該值,如果不存在則添加鍵值對(key, default),其中default默認為None: ~~~ >>> a = dict(one=1, two=2, three=3) >>> a.setdefault('one') 1 >>> a {'two': 2, 'one': 1, 'three': 3} >>> a.setdefault('four') >>> a {'two': 2, 'four': None, 'one': 1, 'three': 3} ~~~ 批量增加有`dict.update(p)`方法。 ### 字典的刪除 如果要刪除一個鍵值對,直接用del方法,試圖刪除一個不存在的鍵值對會拋出KeyError錯誤: ~~~ >>> a {'two': 2, 'four': 4, 'one': 10, 'three': 3} >>> del a['four'] >>> a {'two': 2, 'one': 10, 'three': 3} >>> del a['four'] Traceback (most recent call last): File "<input>", line 1, in <module> KeyError: 'four' ~~~ 此外同樣可以調用dict.clear()來清空字典。還有`dict.pop(key[,default])`和`dict.popitem()`,這兩個一個是彈出特定鍵對應的鍵值對,另一個彈出任意的一個鍵值對: ~~~ >>> a {'two': 2, 'one': 10, 'three': 3} >>> a.pop("one") 10 >>> a {'two': 2, 'three': 3} >>> a.popitem() ('two', 2) >>> a {'three': 3} ~~~ ### 字典的集合 字典可以分別返回它所有的鍵值對、鍵和值,它們的類型都是字典內置類型,可以通過list()轉化為列表: ~~~ >>> a = dict(one=1, two=2, three=3) >>> a {'two': 2, 'one': 1, 'three': 3} >>> a.items() dict_items([('two', 2), ('one', 1), ('three', 3)]) >>> a.values() dict_values([2, 1, 3]) >>> a.keys() dict_keys(['two', 'one', 'three']) >>> list(a.keys()) ['two', 'one', 'three'] ~~~
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