《Python 第五章》条件，循环和其他语句

(print, import, 赋值语句)
pass, del, exec语句的用法。
有些自以为已经掌握的知识点，还隐藏着一些让人惊讶的特性。
* 使用逗号输出 *

>>> 1, 2, 3
(1, 2, 3)
>>> print 1, 2, 3   # print 参数并不能像我们预期那样构成一个元组
1 2 3
>>> print (1, 2, 3)
(1, 2, 3)
>>> 'age', 42
('age', 42)
>>> name = 'Gumby'
>>> mr = 'Mr'
>>> greeting = 'Hello'
>>> print name, mr, greeting
Gumby Mr Hello
>>> import math as abc  #别名
>>> abc.sqrt(4)
2.0
>>> from math import sqrt as bcd # 为函数提供别名
>>> bcd(4)
2.0

# 序列解包

>>> 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
1
# popitem 将键值作为元组返回
>>> scoundrel = {'name': 'Robin', 'girlfriend': 'Marion'}
>>> key, value = scoundrel.popitem()
>>> key
'girlfriend'
>>> value
'Marion'

* 链式赋值 *

x = y = somefunction()

* 增量赋值 *

>>> x = 2
>>> x += 1
>>> x *= 2
>>> x
6
>>> fnord = 'foo'
>>> fnord += 'bar'
>>> fnord *= 2
>>> fnord
'foobarfoobar'

* 语句块：缩排的乐趣 *
在 Python中，冒号 (:) 用来标识语句块的开始
* 条件和条件语句布尔值*
下面的值在作为布尔表达式的时候，会被解释器看作假 (false)
False, None, 0, "", (), [], {}

>>> bool("")
False
>>> bool(42)
True
 if 的使用
num = input('Enter a number : ')
if num > 0:
    print 'positive'
elif num < 0:
    print 'negative'
else :
    print 'zero'

更复杂的条件
*1, 比较运算符 x == y, x <= y, x is y, x is not y, x in y, x not in y
在Python中比较运算和赋值运算一样是可以连接的。

a if b else c

* 断言 *

>>> age = 10
>>> assert 0 < age < 100
>>> assert 0 < age < 10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError，
 * while 循环 *
---while1.py---
x = 1
while x <= 100:
    print x
    x += 1
---while2.py ---
name = ''
while not name.strip():
    name = raw_input('Please enter your name: ')
print 'Hello, %s!' % name

* for 循环 * 比while更简洁，应该尽量多用！

>>> numbers = range(10)
>>> for i in numbers : print i

* 循环遍历字典元素 *

>>> d = {'x' : 1, 'y' : 2, 'z' : 3}
>>> for key in d : print key, 'corresponds to', d[key]
... 
y corresponds to 2
x corresponds to 1
z corresponds to 3
>>> for key, value in d.items(): print key, 'corresponds to', value
... 
y corresponds to 2
x corresponds to 1
z corresponds to 3

* 一些迭代工具 *
(1) 并行迭代

>>> names = ['anne', 'beth', 'george', 'damon']
>>> ages = [12, 45, 32, 102]
>>> for i in range(len(name)): print names[i], ages[i]
anne 12
beth 45
george 32
damon 102
>>> zip(names , ages)
[('anne', 12), ('beth', 45), ('george', 32), ('damon', 102)]
>>> for name, age in zip(name, ages): print name, age
anne 12
beth 45
george 32
damon 102

(2) 编号迭代

for index, string in enumerate(strings):
	if 'xxx' in string:
		strings[index] = '[censored]'

这个函数可以在提供索引的地方迭代索引-值对
(3) 翻转和排序迭代

>>> sorted([4,3, 6, 2, 6, 3, 8])
[2, 3, 3, 4, 6, 6, 8]
>>> sorted('Hello, world')
[' ', ',', 'H', 'd', 'e', 'l', 'l', 'l', 'o', 'o', 'r', 'w']
>>> list(reversed('Hello,world'))
['d', 'l', 'r', 'o', 'w', ',', 'o', 'l', 'l', 'e', 'H']
>>> ''.join(reversed('Hello,world'))
'dlrow,olleH'