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php爱好者> php文档>《python 从入门到精通》§5 控制结构

《python 从入门到精通》§5 控制结构

时间：2009-08-17 来源：oychw

§5 控制结构

2009-8-17

磁针石：xurongzhong#gmail.com

博客：oychw.cublog.cn

§5.1 关于print和import更多的东东

打印多个值：

>>> print 'Age:', 42

Age: 42

输出时会有空格分隔。为了避免空格，可以使用“+".

在脚本中，这样就不会换行：

print 'Hello,',

print 'world!'

输出Hello, world!.

import的格式：

import somemodule

from somemodule import somefunction

from somemodule import somefunction, anotherfunction, yetanotherfunction

from somemodule import *

import math as foobar

§5.2 赋值技巧

>>> x, y, z = 1, 2, 3

>>> print x, y, z

1 2 3

>>> x, y = y, x

>>> print x, y, z

2 1 3

>>> values = 1, 2, 3

>>> values

(1, 2, 3)

>>> x, y, z = values

>>> x

注意这种情况要求左右的值个数相同。Python 3.0中可以：a, b, rest* = [1, 2, 3, 4]。

x = y = somefunction()

§5.3 块

：表示缩进

§5.4 条件判断

代表False的有：False None 0 "" () [] {}，其他都为True.

>>> bool('I think, therefore I am')

True

python会自动进行这种类型转换。

name = raw_input('What is your name? ')

if name.endswith('Gumby'):

if name.startswith('Mr.'):

print 'Hello, Mr. Gumby'

elif name.startswith('Mrs.'):

print 'Hello, Mrs. Gumby'

else:

print 'Hello, Gumby'

else:

print 'Hello, stranger'

比较操作：

x == y x equals y.

x < y x is less than y.

x > y x is greater than y.

x >= y x is greater than or equal to y.

x <= y x is less than or equal to y.

x != y x is not equal to y.

x is y x and y are the same object.

x is not y x and y are different objects.

x in y x is a member of the container (e.g., sequence) y.

x not in y x is not a member of the container (e.g., sequence) y.

COMPARING INCOMPATIBLE

比较也可以嵌套：0 < age < 100

>>> x = y = [1, 2, 3]

>>> z = [1, 2, 3]

>>> x == y

True

>>> x == z

True

>>> x is y

True

>>> x is z

False

不要在基本的，不可改变的类型，比如numbers and strings中使用is，这些类型在python内部处理。

三元运算：a if b else c

断言：

>>> age = -1

>>> assert 0 < age < 100, 'The age must be realistic'

Traceback (most recent call last):

File "<stdin>", line 1, in ?

AssertionError: The age must be realistic

§5.5 循环

name = ''

while not name:

name = raw_input('Please enter your name: ')

print 'Hello, %s!' % name

for number in range(1,101):

print number

xrange一次只产生一个数，在大量循环的时候可以提高效率，一般情况没有明显效果。

与字典配合使用：

d = {'x': 1, 'y': 2, 'z': 3}

for key in d:

print key, 'corresponds to', d[key]

不过排序的为您提要自己处理。

names = ['anne', 'beth', 'george', 'damon']

ages = [12, 45, 32, 102]

for i in range(len(names)):

print names[i], 'is', ages[i], 'years old'

>>> zip(names, ages)

[('anne', 12), ('beth', 45), ('george', 32), ('damon', 102)]

for name, age in zip(names, ages):

print name, 'is', age, 'years old'

>>> zip(range(5), xrange(100000000))

[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)]

for index, string in enumerate(strings):

if 'xxx' in string:

strings[index] = '[censored]'

from math import sqrt

for n in range(99, 0, -1):

root = sqrt(n)

if root == int(root):

print n

break

from math import sqrt

for n in range(99, 81, -1):

root = sqrt(n)

if root == int(root):

print n

break

else:

print "Didn't find it!"

可见for语句也可以有else。

§5.6 List Comprehension

[x*x for x in range(10) if x % 3 == 0]

[0, 9, 36, 81]

>>> [(x, y) for x in range(3) for y in range(3)]

[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

>>> girls = ['alice', 'bernice', 'clarice']

>>> boys = ['chris', 'arnold', 'bob']

>>> [b+'+'+g for b in boys for g in girls if b[0] == g[0]]

['chris+clarice', 'arnold+alice', 'bob+bernice']

这里类似数据库里面的连接。一种更有效的方法：

girls = ['alice', 'bernice', 'clarice']

boys = ['chris', 'arnold', 'bob']

letterGirls = {}

for girl in girls:

letterGirls.setdefault(girl[0], []).append(girl)

print [b+'+'+g for b in boys for g in letterGirls[b[0]]]

§5.7 pass, del, 和 exec

pass：什么都不做

del：删除变量

>>> from math import sqrt

>>> scope = {}

>>> exec 'sqrt = 1' in scope

>>> sqrt(4)

2.0

>>> scope['sqrt']

>>> len(scope)

>>> scope.keys()

['sqrt', '__builtins__']

eval是内置的，和exec类似。exec针对陈述，eval针对表达式。Python 3.0中，raw_input被命名为input.此处尚未完全理解。