条件判断
一般格式
if <test1>: # if test
<statements1> # Associated block
elif <test2>: # Optional elifs
<statements2>
else: # Optional else
<statements3>
if/else三元表达式
A = Y if X else Z
例子
#一般格式
>>> if choice == 'spam':
... print(1.25)
... elif choice == 'ham':
... print(1.99)
... elif choice == 'eggs':
... print(0.99)
... elif choice == 'bacon':
... print(1.10)
... else:
... print('Bad choice')
#三元表达式
>>> A = 't' if '' else 'f'
>>> A
'f'
循环
while 一般格式
while <test>: # Loop test
<statements1> # Loop body
else: # Optional else
<statements2> # Run if didn't exit loop with break
break跳出最近所在的循环（跳过整个循环语句）。
continue跳到最近所在循环的开头处（来到循环的首行）。
pass什么事也不做，只是空占位语句。
版本差异提示：Python 3.0（而不是Python 2.6）允许在可以使用表达式的任何地方使用...（三个连续的点号）来省略代码。循环else块只有当循环正常离开时才会执行（也就是没有碰到break语句）。
while 一般格式
while <test1>:
<statements1>
if <test2>: break # Exit loop now, skip else
if <test3>: continue # Go to top of loop now, to test1
else:
<statements2> # Run if we didn't hit a 'break'
while 例子
while x: # Exit when x empty
if match(x[0]):
print('Ni')
break # Exit, go around else
x = x[1:]
else:
print('Not found') # Only here if exhausted x
for 一般格式
for <target> in <object>: # Assign object items to target
<statements>
if <test>: break # Exit loop now, skip else
if <test>: continue # Go to top of loop now
else:
<statements> # If we didn't hit a 'break'
for 例子
#列表
>>> for x in ["spam", "eggs", "ham"]:
... print(x, end=' ')
...spam eggs ham
#字符串
S = "lumberjack"
>>> for x in S: print(x, end=' ') # Iterate over a string...l u m b e r j a c k
#元组
>>> T = [(1, 2), (3, 4), (5, 6)]
>>> for (a, b) in T: # Tuple assignment at work
... print(a, b)
...1 2
3 4
5 6
#字典
>>> D = {'a': 1, 'b': 2, 'c': 3}
>>> for key in D:
... print(key, '=>', D[key]) # Use dict keys iterator and index
...a => 1
c => 3
b => 2
#items返回可遍历的(键, 值) 元组数组，所以字典也可以这样遍历。
>>> for (key, value) in D.items():
... print(key, '=>', value) # Iterate over both keys and values
...a => 1
c => 3
b => 2
#分片循环
>>> S = 'abcdefghijk'
>>> for c in S[::2]:
print(c, end=' ')
...a c e g i k
# python3 扩展
>>> for (a, *b, c) in [(1, 2, 3, 4), (5, 6, 7, 8)]:
... print(a, b, c)
...1 [2, 3] 4
5 [6, 7] 8
range 迭代器
range是一个迭代器，会根据需要产生元素
>>> list(range(-5, 5))
[-5, -4, -3, -2, -1, 0, 1, 2, 3, 4]
>>> list(range(0, len(S), 2))
[0, 2, 4, 6, 8, 10]
>>> for i in range(3):
... print(i, 'Pythons')
...0 Pythons
1 Pythons
2 Pythons
# 修改列表
>>> L = [1, 2, 3, 4, 5]
>>> for i in range(len(L)): # Add one to each item in L
... L[i] += 1 # Or L[i] = L[i] + 1
...
>>> L
[2, 3, 4, 5, 6]
zip 并行迭代器
>>> L1 = [1,2,3,4]
>>> L2 = [5,6,7,8]
>>> zip(L1, L2)
<zip object at 0x026523C8>
>>> list(zip(L1, L2)) # list() required in 3.0, not 2.6
[(1, 5), (2, 6), (3, 7), (4, 8)]
>>> for (x, y) in zip(L1, L2):
... print(x, y, '--', x+y)
...1 5 -- 6
2 6 -- 8
3 7 -- 10
4 8 -- 12
通过zip构建字典
# python2.2 before
>>> keys = ['spam', 'eggs', 'toast']
>>> vals = [1, 3, 5]
>>> list(zip(keys, vals))[('spam', 1), ('eggs', 3), ('toast', 5)]
>>> D2 = {}
>>> for (k, v) in zip(keys, vals): D2[k] = v
...
>>> D2
{'toast': 5, 'eggs': 3, 'spam': 1}
#python 2.2 later
>>> keys = ['spam', 'eggs', 'toast']
>>> vals = [1, 3, 5]
>>> D3 = dict(zip(keys, vals))
>>> D3
{'toast': 5, 'eggs': 3, 'spam': 1}
enumerate函数
enumerate函数返回一个生成器，对象简而言之，这个对象有一个next方法，由下一个内置函数调用它，并且循环中每次迭代的时候它会返回一个(index,value)的元组。
>>> S = 'spam'
>>> for (offset, item) in enumerate(S):
... print(item, 'appears at offset', offset)
...s appears at offset 0
p appears at offset 1
a appears at offset 2
m appears at offset 3
