问题描述

我有一个这样定义的多行字符串:

I have a multi-line string defined like this:

foo = """
this is 
a multi-line string.
"""

我们将这个字符串用作我正在编写的解析器的测试输入.解析器函数接收一个 file 对象作为输入并对其进行迭代.它还直接调用 next() 方法来跳过行，所以我真的需要一个迭代器作为输入，而不是一个可迭代的.我需要一个迭代器来迭代该字符串的各个行，就像 file-object 将遍历文本文件的行一样.我当然可以这样做:

This string we used as test-input for a parser I am writing. The parser-function receives a file-object as input and iterates over it. It does also call the next() method directly to skip lines, so I really need an iterator as input, not an iterable. I need an iterator that iterates over the individual lines of that string like a file-object would over the lines of a text-file. I could of course do it like this:

lineiterator = iter(foo.splitlines())

有没有更直接的方法?在这种情况下，字符串必须遍历一次以进行拆分，然后再由解析器遍历.在我的测试用例中没关系，因为那里的字符串很短，我只是出于好奇而问.Python 为这些东西提供了很多有用且高效的内置函数，但我找不到适合这种需要的东西.

Is there a more direct way of doing this? In this scenario the string has to traversed once for the splitting, and then again by the parser. It doesn't matter in my test-case, since the string is very short there, I am just asking out of curiosity. Python has so many useful and efficient built-ins for such stuff, but I could find nothing that suits this need.

推荐答案

这里有三种可能:

foo = """
this is 
a multi-line string.
"""

def f1(foo=foo): return iter(foo.splitlines())

def f2(foo=foo):
    retval = ''
    for char in foo:
        retval += char if not char == '
' else ''
        if char == '
':
            yield retval
            retval = ''
    if retval:
        yield retval

def f3(foo=foo):
    prevnl = -1
    while True:
      nextnl = foo.find('
', prevnl + 1)
      if nextnl < 0: break
      yield foo[prevnl + 1:nextnl]
      prevnl = nextnl

if __name__ == '__main__':
  for f in f1, f2, f3:
    print list(f())

将其作为主脚本运行可确认这三个功能是等效的.使用 timeit (以及 * 100 用于 foo 以获得大量字符串以进行更精确的测量):

Running this as the main script confirms the three functions are equivalent. With timeit (and a * 100 for foo to get substantial strings for more precise measurement):

$ python -mtimeit -s'import asp' 'list(asp.f3())'
1000 loops, best of 3: 370 usec per loop
$ python -mtimeit -s'import asp' 'list(asp.f2())'
1000 loops, best of 3: 1.36 msec per loop
$ python -mtimeit -s'import asp' 'list(asp.f1())'
10000 loops, best of 3: 61.5 usec per loop

请注意，我们需要调用 list() 来确保遍历迭代器，而不仅仅是构建迭代器.

Note we need the list() call to ensure the iterators are traversed, not just built.

IOW，天真的实现快得多，甚至都不好笑:比我尝试使用 find 调用的速度快 6 倍，而后者又比较低级别的方法快 4 倍.

IOW, the naive implementation is so much faster it isn't even funny: 6 times faster than my attempt with find calls, which in turn is 4 times faster than a lower-level approach.

要记住的教训:测量总是一件好事(但必须准确)；像 splitlines 这样的字符串方法以非常快的方式实现；通过在非常低的级别编程(尤其是通过非常小片段的 += 循环)将字符串放在一起可能会很慢.

Lessons to retain: measurement is always a good thing (but must be accurate); string methods like splitlines are implemented in very fast ways; putting strings together by programming at a very low level (esp. by loops of += of very small pieces) can be quite slow.

编辑:添加了@Jacob 的建议，稍作修改以提供与其他建议相同的结果(保留一行尾随空格)，即:

Edit: added @Jacob's proposal, slightly modified to give the same results as the others (trailing blanks on a line are kept), i.e.:

from cStringIO import StringIO

def f4(foo=foo):
    stri = StringIO(foo)
    while True:
        nl = stri.readline()
        if nl != '':
            yield nl.strip('
')
        else:
            raise StopIteration

测量给出:

$ python -mtimeit -s'import asp' 'list(asp.f4())'
1000 loops, best of 3: 406 usec per loop

不如基于 .find 的方法好——仍然值得牢记，因为它可能不太容易出现小错误(任何你看到出现+1 和 -1，就像我上面的 f3 一样，应该自动触发一对一的怀疑——许多缺乏这种调整的循环也应该有这些调整——尽管我相信我的代码是也是正确的，因为我能够使用其他功能检查它的输出').

not quite as good as the .find based approach -- still, worth keeping in mind because it might be less prone to small off-by-one bugs (any loop where you see occurrences of +1 and -1, like my f3 above, should automatically trigger off-by-one suspicions -- and so should many loops which lack such tweaks and should have them -- though I believe my code is also right since I was able to check its output with other functions').

但基于拆分的方法仍然适用.

But the split-based approach still rules.

顺便说一句:f4 可能更好的样式是:

An aside: possibly better style for f4 would be:

from cStringIO import StringIO

def f4(foo=foo):
    stri = StringIO(foo)
    while True:
        nl = stri.readline()
        if nl == '': break
        yield nl.strip('
')

至少，它不那么冗长了.不幸的是，需要去除尾随 的需要禁止用 return iter(stri) 更清晰、更快速地替换 while 循环(>iter 部分在现代版本的 Python 中是多余的，我相信从 2.3 或 2.4 开始，但它也是无害的).也许也值得一试:

at least, it's a bit less verbose. The need to strip trailing s unfortunately prohibits the clearer and faster replacement of the while loop with return iter(stri) (the iter part whereof is redundant in modern versions of Python, I believe since 2.3 or 2.4, but it's also innocuous). Maybe worth trying, also:

    return itertools.imap(lambda s: s.strip('
'), stri)

或其变体——但我在这里停下来，因为它几乎是一个基于 strip 的理论练习，最简单，最快，一个.

or variations thereof -- but I'm stopping here since it's pretty much a theoretical exercise wrt the strip based, simplest and fastest, one.

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