python - 处理异常时奇怪的嵌套循环行为

Question

目标:如果计数大于实际行数，则在 except block 中:告诉用户并让他们按回车键。设置 count 等于文件中的总行数并重试循环。

count = 10000
with open('mobydick_ch1.txt') as f:

        while 1:
            lines = []
            try:
                for i in range(count):
                    lines.append(next(f))  # iterate through file and append each line in range
                    break
            except StopIteration:
                if not input("File does not contain that many lines, press enter to continue printing maximum lines:"):
                    for i, k in enumerate(f, 1):
                        count = i

        f.close()  # close file

        # format output. enumerate lines, start at 1
        # http://stackoverflow.com/questions/4440516/in-python-is-there-an-elegant-
        # way-to-print-a-list-in-a-custom-format-without-ex
    print(''.join('Line {0}: {1}'.format(*k) for k in enumerate(lines, 1)))

我目前得到:

File does not contain that many lines, press enter to continue printing maximum lines:

每次我按回车键。是什么导致了这种不良行为？

Answer 1

你已经耗尽了文件，你不能再次读取文件而不返回0。结果你的for i, k in enumerate(f, 1) : 循环立即退出。这同样适用于 while 1: 循环的每个 future 迭代；文件仍在末尾，所有使用 next() 的访问都会立即引发 StopIteration。

你已经知道你读了多少行，只需设置count = len(lines)。无需为了设置 count 而再次读取整个文件。

如果用itertools.islice()就更好了得到你的 1000 行:

from itertools import islice

count = 10000
with open('mobydick_ch1.txt') as f:
    lines = list(islice(f, count))  # list of up to count lines
if len(lines) < count:
    input("File does not contain that many lines, press enter to continue printing maximum lines:")
    count = len(lines)  # set count to actual number of lines

如果您试图等待直到文件包含至少 count 行，您将不得不每次重新打开文件并寻找最后记录的地点:

lines = []
pos = 0
while len(lines) < count:
    with open('mobydick_ch1.txt') as f:
        f.seek(pos)
        lines.extend(islice(f, count - len(lines)))
        pos = f.tell()
    if len(lines) < count:
        input("File does not contain that many lines, press enter to continue printing maximum lines:")