Python文件缩减-第2部分

Question

这是我上一个问题(Python中使用随机化的文本文件缩减)的后续问题。正在修改运行多个缩减，但只有第一个输出文件包含缩减，以下3个文件为零大小。这一定是我没有看到的明显的东西……

#!/usr/bin/env python
import random
import sys
from itertools import chain, groupby
def choose_random(iterator, fraction, random=random.random):
    """Lazy analog of:
        L = list(iterator)
        k = int(len(L) * fraction + .5) or 1 # get at least one
        result = random.sample(L, k)
    Note: this function doesn't randomize the order of elements
          that would require to keep selected elements in memory
          and number of output elements is not exactly k
    """
    # always yield at least one item if input is not empty
    item = next(iterator)
    it = (x for x in chain([item], iterator) if random() < fraction)
    for x in chain([next(it, item)], it):
        yield x
def getkey(line):
    return line.split("t")[3] # 4th column
reductions = [0.25, 0.50, 0.75, 1]
filename = "foo"
outfile = [open("-".join([x, filename]), "w") for x in map(str, reductions)]
try:
    with open(filename, "r") as f:
        for ln, k in enumerate(map(float, reductions)):
            for key, group in groupby(f, key=getkey):
                outfile[ln].writelines(choose_random(group, fraction=k))
finally:
    for f in outfile:
        f.close()

---

输出如下所示(0.25-foo文件包含正确的缩减，其余为空):

-rw-r--r--  1 staff  staff   53326048 Mar 27 03:42 0.25-foo
-rw-r--r--  1 staff  staff          0 Mar 27 03:42 0.5-foo
-rw-r--r--  1 staff  staff          0 Mar 27 03:42 0.75-foo
-rw-r--r--  1 staff  staff          0 Mar 27 03:42 1-foo

Answer 1

您打开foo一次，但尝试遍历它四次。在第一次缩减结束时，就到了文件的末尾。要么重新打开:

try:
    for ln, k in enumerate(map(float, reductions)):
        with open(filename, "r") as f:
            for key, group in groupby(f, key=getkey):
                outfile[ln].writelines(choose_random(group, fraction=k))
finally:
    for f in outfile:
        f.close()

或每次缩减后倒带:

try:
    with open(filename, "r") as f:
        for ln, k in enumerate(map(float, reductions)):
            for key, group in groupby(f, key=getkey):
                outfile[ln].writelines(choose_random(group, fraction=k))
            f.seek(0)
finally:
    for f in outfile:
        f.close()

我会同时打开两个文件:

reductions = [0.25, 0.50, 0.75, 1.0]
filename = "foo"
for fraction in reductions:
    with open(filename, "r") as f, open('%s-%s' % (fraction, filename), 'w') as outfile:
        for key, group in groupby(f, key=getkey):
            outfile.writelines(choose_random(group, fraction=fraction))