views:

239

answers:

4

I have a multi-line string defined like this:

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

This string us 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())

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.

A: 

I suppose you could roll your own:

def parse(string):
    retval = ''
    for char in string:
        retval += char if not char == '\n' else ''
        if char == '\n':
            yield retval
            retval = ''
    if retval:
        yield retval

I'm not sure how efficient this implementation is, but that will only iterate over your string once.

Mmm, generators.

Edit:

Of course you'll also want to add in whatever type of parsing actions you want to take, but that's pretty simple.

Wayne Werner
Pretty inefficient for long lines (the `+=` part has worst-case `O(N squared)` performance, though several implementation tricks try to lower that when feasible).
Alex Martelli
Yeah - I've just been learning about that recently. Would it be faster to append to a list of chars and then ''.join(chars) them? Or is that an experiment I should undertake myself? ;)
Wayne Werner
@Wayne, please do measure yourself, it's instructive -- and be sure to try both short lines like in the OP's example, and long ones!-)
Alex Martelli
For short strings ( < ~40 chars) the += is actually quicker, but hits worst case quickly. For longer strings, the `.join` method actually looks like O(N) complexity. Since I couldn't find the particular comparison made on SO yet, I started a question http://stackoverflow.com/questions/3055477/how-slow-is-pythons-string-concatenation-vs-str-join (that surprisingly received more answers than just my own!)
Wayne Werner
A: 

If I read Modules/cStringIO.c correctly, this should be quite efficient (although somewhat verbose):

from cStringIO import StringIO

def iterbuf(buf):
    stri = StringIO(buf)
    while True:
        nl = stri.readline()
        if nl != '':
            yield nl.strip()
        else:
            raise StopIteration
Jacob Oscarson
+6  A: 

Here are three possibilities:

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 == '\n' else ''
        if char == '\n':
            yield retval
            retval = ''
    if retval:
        yield retval

def f3(foo=foo):
    prevnl = -1
    while True:
      nextnl = foo.find('\n', 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())

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

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

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.

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.

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('\n')
        else:
            raise StopIteration

Measuring gives:

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

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').

But the split-based approach still rules.

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('\n')

at least, it's a bit less verbose. The need to strip trailing \ns 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('\n'), stri)

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

Alex Martelli
Also, `(line[:-1] for line in cStringIO.StringIO(foo))` is pretty fast; almost as fast as the naive implementation, but not quite.
Matt Anderson
Thank you for this great answer. I guess the main lesson here (as I am new to python) is to make using `timeit` a habit.
Space_C0wb0y
@Space, yep, timeit is good, any time you care about performance (be sure to use it carefully, e.g. in this case see my note about needing a `list` call to actually time all the relevant parts!-).
Alex Martelli
A: 

I'm not sure what you mean by "then again by the parser". After the splitting has been done, there's no further traversal of the string, only a traversal of the list of split strings. This will probably actually be the fastest way to accomplish this, so long as the size of your string isn't absolutely huge. The fact that python uses immutable strings means that you must always create a new string, so this has to be done at some point anyway.

If your string is very large, the disadvantage is in memory usage: you'll have the original string and a list of split strings in memory at the same time, doubling the memory required. An iterator approach can save you this, building a string as needed, though it still pays the "splitting" penalty. However, if your string is that large, you generally want to avoid even the unsplit string being in memory. It would be better just to read the string from a file, which already allows you to iterate through it as lines.

However if you do have a huge string in memory already, one approach would be to use StringIO, which presents a file-like interface to a string, including allowing iterating by line (internally using .find to find the next newline). You then get:

import StringIO
s = StringIO.StringIO(myString)
for line in s:
    do_something_with(line)
Brian