优化循环
循环之外能做的事不要放在循环内，比如下面的优化可以快一倍：
a = range(10000) size_a = len(a) %timeit -n 1000 for i in a: k = len(a) %timeit -n 1000 for i in a: k = size_a 1000 loops, best of 3: 569 µs per loop 1000 loops, best of 3: 256 µs per loop
优化包含多个判断表达式的顺序
对于and，应该把满足条件少的放在前面，对于or，把满足条件多的放在前面。如：
a = range(2000) %timeit -n 100 [i for i in a if 10 < i < 20 or 1000 < i < 2000] %timeit -n 100 [i for i in a if 1000 < i < 2000 or 100 < i < 20] %timeit -n 100 [i for i in a if i % 2 == 0 and i > 1900] %timeit -n 100 [i for i in a if i > 1900 and i % 2 == 0] 100 loops, best of 3: 287 µs per loop 100 loops, best of 3: 214 µs per loop 100 loops, best of 3: 128 µs per loop 100 loops, best of 3: 56.1 µs per loop
使用join合并迭代器中的字符串
In [1]: %%timeit ...: s = '' ...: for i in a: ...: s += i ...: 10000 loops, best of 3: 59.8 µs per loop In [2]: %%timeit s = ''.join(a) ...: 100000 loops, best of 3: 11.8 µs per loop
join对于累加的方式，有大约5倍的提升。
选择合适的格式化字符方式
s1, s2 = 'ax', 'bx'
%timeit -n 100000 'abc%s%s' % (s1, s2)
%timeit -n 100000 'abc{0}{1}'.format(s1, s2)
%timeit -n 100000 'abc' + s1 + s2
100000 loops, best of 3: 183 ns per loop
100000 loops, best of 3: 169 ns per loop
100000 loops, best of 3: 103 ns per loop
三种情况中，%的方式是最慢的，但是三者的差距并不大（都非常快）。(个人觉得%的可读性最好)
不借助中间变量交换两个变量的值
In [3]: %%timeit -n 10000 a,b=1,2 ....: c=a;a=b;b=c; ....: 10000 loops, best of 3: 172 ns per loop In [4]: %%timeit -n 10000 a,b=1,2 a,b=b,a ....: 10000 loops, best of 3: 86 ns per loop
使用a,b=b,a而不是c=a;a=b;b=c;来交换a,b的值，可以快1倍以上。
使用if is
a = range(10000) %timeit -n 100 [i for i in a if i == True] %timeit -n 100 [i for i in a if i is True] 100 loops, best of 3: 531 µs per loop 100 loops, best of 3: 362 µs per loop
使用 if is True 比 if == True 将近快一倍。
使用级联比较x < y < z
x, y, z = 1,2,3 %timeit -n 1000000 if x < y < z:pass %timeit -n 1000000 if x < y and y < z:pass 1000000 loops, best of 3: 101 ns per loop 1000000 loops, best of 3: 121 ns per loop
x < y < z效率略高，而且可读性更好。
while 1 比 while True 更快
def while_1(): n = 100000 while 1: n -= 1 if n <= 0: break def while_true(): n = 100000 while True: n -= 1 if n <= 0: break m, n = 1000000, 1000000 %timeit -n 100 while_1() %timeit -n 100 while_true() 100 loops, best of 3: 3.69 ms per loop 100 loops, best of 3: 5.61 ms per loop
