March 2018
Beginner to intermediate
624 pages
16h 10m
Japanese
Content preview from Head First Python 第2版 ―頭とからだで覚えるPythonの基本
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setdefault
万歳
for letter in word:
if letter in vowels:
found.setdefault(letter, 0)
found[letter] += 1
120
3
章
このコードでは
エラーが出ます。
setdefault
メソッドを使う
現在のバージョンの
vowels5.py
は実行時エラーになっていました。具体的
には
KeyError
でした。このエラーは存在しないキーの値にアクセスするために
起こります。
fruits
を試した結果から、面倒なエラーを心配せずに
setdefault
を何
度でも呼び出せることがわかりました。
setdefault
は存在しないキーを指
定のデフォルト値に初期化するか、または何もしません（つまり、既存のキーに
対応する既存の値はそのままになります）。
vowels5.py
でキーを使う直前
に
setdefault
を呼び出せば、キーは存在するかしないかのどちらかなので
KeyError
を確実に避けられます。いずれにせよ（
setdefault
のおかげで）プ
ログラムは動作を続け、クラッシュしなくなります。
IDLE
の編集ウィンドウで、
vowels5.py
の最初の
for
ループを次のように変
更し（
setdefault
を追加します）、
vowels6.py
として保存します。
setdefault
で
KeyError
例外
を回避できる。
1
行のコードが状況を
一変させてしまうことも
あります。
キー
No4
オブジェクト
キー
No1
オブジェクト
キー
No3
オブジェクト
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.