Python if vs try-except

问题描述

我想知道为什么 try-except 比下面程序中的 if 慢.

I was wondering why the try-except is slower than the if in the program below.

def tryway():
    try:
        while True:
            alist.pop()
    except IndexError:
        pass

def ifway():
    while True:
        if alist == []:
            break
        else:
            alist.pop()
if __name__=='__main__':
    from timeit import Timer
    alist = range(1000)
    print "Testing Try"
    tr = Timer("tryway()","from __main__ import tryway")
    print tr.timeit()
    print "Testing If"
    ir = Timer("ifway()","from __main__ import ifway")
    print ir.timeit()

我得到的结果很有趣.

Testing Try
2.91111302376
Testing If
0.30621099472

谁能解释一下为什么 try 这么慢?

Can anyone shed some light why the try is so much slower?

推荐答案

您只设置了一次 alist.第一次调用tryway"会清除它，然后每次连续调用什么都不做.

You're setting alist only once. The first call to "tryway" clears it, then every successive call does nothing.

def tryway():
    alist = range(1000)
    try:
        while True:
            alist.pop()
    except IndexError:
        pass

def ifway():
    alist = range(1000)
    while True:
        if alist == []:
            break
        else:
            alist.pop()
if __name__=='__main__':
    from timeit import Timer
    print "Testing Try"
    tr = Timer("tryway()","from __main__ import tryway")
    print tr.timeit(10000)
    print "Testing If"
    ir = Timer("ifway()","from __main__ import ifway")
    print ir.timeit(10000)

>>> Testing Try
>>> 2.09539294243
>>> Testing If
>>> 2.84440898895

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