python编程小技巧

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python编程技巧总结：

在python的编程中会使用各种各样的语法，通过一些小技巧可以优化python代码，学习总结如下。学习参考地址：https://github.com/JeffPaine/beautiful_idiomatic_python

遍历一个序列

  1. python2:(xrange在遍历范围内创建一个迭代器，一次生成一个值，效率比range更高)

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    #bad:
    for i in [0, 1, 2, 3, 4, 5]:
        print i**2
     
    for i in range(6):
        print i**2
     
    #better:
    for i in xrange(6):
        print i**2

  2. python:(xrange更名为range，进行了优化)

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    color = ['red','green','blue','yello']
    #bad
    for i in range(len(color)):
        print(color[i])
    #better
    for co in color:
        print(co)

遍历倒序

  • python3:

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    color = ['red','green','blue','yello']
    #bad
    for i in range(len(color)-1,-1,-1):
        print(color[i])
    #better
    for co in reversed(color):
        print(co)

建立序列索引

  • python3：

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    colors = ['red', 'green', 'blue', 'yellow']
     
    #bad
    for i in range(len(colors)):
        print (i, '--->', colors[i])
     
    #better
    for i, color in enumerate(colors):
        print (i, '--->', color)

遍历两个序列

  1. python2:(用izip返回迭代)

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    names = ['raymond', 'rachel', 'matthew']
    colors = ['red', 'green', 'blue', 'yellow']
     
    #bad
    n = min(len(names), len(colors))
    for i in range(n):
        print names[i], '--->', colors[i]
     
    for name, color in zip(names, colors):
        print name, '--->', color
     
    #better
     
    for name, color in izip(names, colors):
        print name, '--->', color

  2. python3:(用zip返回迭代)

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    names = ['raymond', 'rachel', 'matthew']
    colors = ['red', 'green', 'blue', 'yellow']
     
    #bad
    n = min(len(names),len(colors))
    for  i in range(n):
        print(names[i],'--->',colors[i])
    #batter
    for name, color in zip(names,colors):
        print(name,'--->',color)

遍历排序的序列

  • python3:

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    colors = ['red','green','blue','yellow']
     
    #Forward sorted order
    for color in sorted(colors):
        print(color)
     
    #Backwards sorted order
    for color in sorted(colors,reverse=True):
        print(color)

自定义遍历序列

  • python3:

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    colors = ['red', 'green', 'blue', 'yellow']
     
    print(sorted(colors,key=len))

遍历文件遇到指定字符退出

  • python3:

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    #bad
    blocks = []
    while True:
        block = f.read(32)
        if block == '':
            break
     
        blocks.append(block)
     
    #better
    blocks = []
    if block in iter(partial(read,32),''):
        blocks.append(block)
    #iter是一个内置函数用来生产迭代器，partial的不断的读入文件中32字节，注意iter引入第二个参数，表示当读入的内容是''的时候，会触发生成器stop

函数遍历多出口问题

  • python3:

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    #bad
    def find(seq, target):
        found = False
        for i, value in enumerate(seq):
            if value == target:
                found = True
                break
        if not found:
            return -1
        return i
     
    #better(for else结构)
    def find(seq, target):
        for i, value in enumerate(seq):
            if value == target:
                break
        else:
            return -1
        return i

遍历字典

  • python3:

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    d = {'matthew': 'blue', 'rachel': 'green', 'raymond': 'red'}
     
    for k in d:
        print(k)
     
    for k in d.keys():
        if k.startswith('a'):
            del d[k]

  • 遍历键和值：

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    d = {'matthew': 'blue', 'rachel': 'green', 'raymond': 'red'}
     
    #bad
    for k in d:
        print(k, '-->', d[k])
     
    #batter
    for k,v in d.items():
        print(k,'--->',v)

构造字典

  • python3:

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    names = ['raymond', 'rachel', 'matthew']
    colors = ['red', 'green', 'blue']
     
    d = dict(zip(names, colors))
    print(d)

字典的统计

  • python3:

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    colors = ['red', 'green', 'red', 'blue', 'green', 'red']

    #bad
    d = {}
    for color in colors:
        if color not in d:
            d[color] = 0
        d[color] += 1
    print(d)

    #batter
    s = {}
    for color in colors:
        s[color] = s.get(color,0) + 1
    print(d)

对序列的值根据元素长度进行分组

  • python3:

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    names = ['raymond', 'rachel', 'matthew', 'roger',
             'betty', 'melissa', 'judith', 'charlie']
    #bad
    d = {}
    for name in names:
        key = len(name)
        if key not in d:
            d[key] = []
        d[key].append(name)

    d = {}
    for name in names:
        key = len(name)
        d.setdefault(key, []).append(name)
    print(d)

    #better
    s = {}
    for name in names:
        key = len(name)
        s.setdefault(key, []).append(name)
    print(s)

字典遍历倒序

  • python3:(使用popitem)

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    d = {'matthew': 'blue', 'rachel': 'green', 'raymond': 'red'}

    while d:
        key, value = d.popitem()
        print (key, '-->', value)

字典链接

  • python3:

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    import argparse
    import os
    import collections

    #bad
    defaults = {'color': 'red', 'user': 'guest'}
    parser = argparse.ArgumentParser()
    parser.add_argument('-u', '--user')
    parser.add_argument('-c', '--color')
    namespace = parser.parse_args([])
    command_line_args = {k:v for k, v in vars(namespace).items() if v}

    d = defaults.copy()
    d.update(os.environ)
    d.update(command_line_args)
    print(d)

    #better
    d = collections.ChainMap(command_line_args, os.environ, defaults)
    print(d)

    #关于Python中ChainMap的使用：http://blog.topspeedsnail.com/archives/9726

使用关键字提高代码清晰度

  • python3:

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    #bad
    twitter_search('@obama', False, 20, True)

    #Better
    twitter_search('@obama', retweets=False, numtweets=20, popular=True)

展开序列

  • python3:

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    p = ['Leo','Xin','30','coder']

    #bad
    fname = p[0]
    lname = p[1]
    age = p[2]
    email = p[3]

    #better
    fname,lname,age,email = p

交换变量

  • python3:

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    def fibonacci(n):
        x = 0
        y = 1
        for i in range(n):
            print(x)
            t = y
            y = x + y
            x = t

    def fibonacci(n):
        x, y = 0, 1
        for i in range(n):
            print (x)
            x, y = y, x + y

更新序列

  • python3:

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    import collections
    names = ['raymond', 'rachel', 'matthew', 'roger',
             'betty', 'melissa', 'judith', 'charlie']
    #bad
    del names[0]
    names.pop(0)
    names.insert(0, 'mark')

    #Better
    names = collections.deque(['raymond', 'rachel', 'matthew', 'roger',
                   'betty', 'melissa', 'judith', 'charlie'])
    del names[0]
    names.popleft()
    names.appendleft('mark')
    print(names)

    #利用collections模块里面的双向队列，会更简洁

使用线程锁

  • python3:

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    import threading

    lock = threading.Lock()

    #bad
    lock.acquire()
    try:
        print ('Critical section 1')
        print ('Critical section 2')
    finally:
        lock.release()

    #Better
    with lock:
        print ('Critical section 1')
        print ('Critical section 2')

修改for循环语句

  • python3:

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    result = []
    #bad
    for i in range(10):
        s = i ** 2
        result.append(s)
    print (sum(result))

    #Better
    print (sum(i**2 for i in range(10)))