Python Cookbook Notes Chapter1

Andimeo — Fri, 27 Apr 2012 13:27:00 GMT

1.2 Swapping Values Without Using aTemporary Variable

利用元组赋值的特性：a, b, c = b, c, a

Tuples are often surrounded withparentheses, as in (b, c, a), but the parentheses are not necessary, exceptwhere the commas would otherwise have some other meaning (e.g., in a functioncall). The commas are what create a tuple, by packing the values that are thetuple's items.

1.3 Constructing a Dictionary WithoutExcessive Quoting

利用python的可变参数：*args 和 **kwds

*args是一个list，**kwds是一个dict，如果两个一起出现，那么**kwds必须在最后，其余有名称的变量必须在最前

1.4 Getting a Value from a Dictionary

使用d.get(‘key’, ‘not found’)来解决，第二个参数未指定时，返回None

1.5 Adding an Entry to a Dictionary

使用somedict.setdefault(somekey, []).append(somevalue)来解决。

当enry是mutable的时候setdefault函数很有用，它令d[key]=value，然后返回value。

当entry是immutable的时候，setdefault则不是很有用

1.6 Associating Multiple Values with EachKey in a Dictionary

有两种方法：

1 d1 = {}
2 d1.setdefault(key,[]).append(value)
3 d2 = {}
4 d2.setdefault(key,{})[value] = 1

第一种不会自动去重，第二种可以自动去除重复的元素。

1.7 Dispatching Using a Dictionary

利用字典，实现其他语言中switch、case或select的作用。 1 animals = []

2 number_of_felines = 0
3 def deal_with_a_cat( ):
4    global number_of_felines
5    print "meow"
6    animals.append('feline')
7    number_of_felines += 1
8 def deal_with_a_dog( ):
9    print "bark"
10    animals.append('canine')
11 def deal_with_a_bear( ):
12    print "watch out for the *HUG*!"
13    animals.append('ursine')
14 tokenDict = {
15    "cat": deal_with_a_cat,
16    "dog": deal_with_a_dog,
17    "bear": deal_with_a_bear,
18 }
19 # Simulate, say, somewords read from a file
20 words = ["cat","bear", "cat", "dog"]
21 for word in words:
22    # Look up the function to call for each word, then call it
23    functionToCall = tokenDict[word]
24    functionToCall( )
25    # You could also do it in one step, tokenDict[word]( )

Andimeo 2012-04-27 21:27 发表评论

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Python Cookbook Notes Chapter1