1. if...elif...else:
语法：
if 判断条件:
语句...
elif 判断条件:
语句...
else:
语句...
#elif语句可以有0个或多个
2. while和for循环：
while 判断条件:
语句...
for iterating_var in sequence:
statements(s)
可以在while循环体中嵌套for循环；也可以for循环中嵌套while循环。
3. for...else:for正常结束都会执行else
for...else:for正常结束（不执行break结束）都会执行else；若执行break，则不执行else
>>> for i in range(3):
... print i
... else:
... print "no break happens!"
...
0
1
2
no break happens!
>>> for i in range(5):
... print i
... break
... else:
... print "no break happend"
...
0
4. while...else:while正常结束执行else
while...else:while正常结束（不执行break结束)都会执行else；若执行break，则不执行else
>>> n=2
>>> while n>1:
... print n
... n-=1
... else:
... print "no break happend"
...
2
no break happend
>>> n=2
>>> while n>0:
... print n
... n-=1
... break
... else:
... print "no break happend"
...
2
(1) False:空、0、None、False
>>> bool([])
False
>>> bool(0)
False
>>> bool(None)
False
(2). True:1、True
>>> bool(1)
True
>>> bool(True)
True
>>> bool("s")
True
>>>
5. break、continue:
break:结束循环（彻底结束）
continue:结束本次循环，继续下一次循环
>>> for i in range(10):
... if i % 2 ==0:
... continue
... print i
...
>>> for i in range(10):
... if i%2==0:
... break
... print i
...
6. 遍历：
(1) 通过元素遍历
(2). 通过索引遍历
len()：统计长度
>>> len("abc")
3
>>> len({1:2}) #统计字典长度
1
>>>
例1： for和while遍历字符串
>>> s="abcd"
>>> for i in range(len(s)):
... print s[i]
...
>>> s
'abcd'
>>> i=0
>>> while i<len(s):
... print s[i]
... i+=1
...
例2： 遍历列表中的元组：
#coding=utf-8
a=[1,(12,3),(4,5),90]
for i in a:
if isinstance(i,(tuple,list)):
for j in i:
print j
else:
print i
7. for、while嵌套循环：
嵌套循环：在一个循环体里嵌入另一个循环
可以在while循环体中嵌套for循环；也可以for循环中嵌套while循环。
for iterating_var in sequence:
for iterating_var in sequence:
statements(s)
statements(s)
8. 直接退出多重循环：
(1). return退出多重循环：
#coding=utf-8
def test():
for i in range(5):
for j in range(5):
for k in range(5):
if i == j == k == 3:
return
else:
print i, '----', j, '----', k
test()
(2). 异常：
class getoutofloop(Exception):pass #异常
try:
for i in range(5):
for j in range(5):
for k in range(5):
if i == j == k == 3:
raise getoutofloop()
else:
print i,'----',j,'----',k
except getoutofloop:
pass
print "done"
(3). break:(多个break)
#coding=utf-8
for i in range(5):
for j in range(5):
for k in range(5):
if i == j == k == 3:
break
else:
print i,'----',j,'----',k
else:continue
break
else:continue
break