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，在except 块中：告诉用户并让他们按回车。将count 设置为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：

文件不包含那么多行，按回车继续打印最大行数：

每次我按回车键。是什么导致了这种不良行为？

Answer 1

您已经用尽了文件，如果不返回 0，就无法再次读取文件再次。因此，您的 for i, k in enumerate(f, 1): 循环立即退出。这同样适用于while 1: 循环的每一次未来迭代；该文件仍在末尾，所有使用next() 的访问都将立即引发StopIteration。

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

如果你使用itertools.islice() 得到你的 1000 行会更好：

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

如果您尝试等待直到文件至少包含count 行，您必须每次重新打开文件并寻找最后记录的位置：

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:")