Python 连续读取文件

Question

我有一个正在从文件中读取的 Python 脚本。第一个命令计算行数。第二个打印第二行，尽管第二个不工作。

lv_file = open("filename.txt", "rw+")

# count the number of lines =================================
lv_cnt = 0
for row in lv_file.xreadlines():
    lv_cnt = lv_cnt + 1

# print the second line =====================================
la_lines = la_file.readlines()
print la_lines[2]

lv_file.close()

当我这样写它时它可以工作，但我不明白为什么我必须关闭文件并重新打开它才能让它工作。我是否滥用了某种功能？

lv_file = open("filename.txt", "rw+")

# count the number of lines =================================
lv_cnt = 0
for row in lv_file.xreadlines():
    lv_cnt = lv_cnt + 1

lv_file.close()

lv_file = open("filename.txt", "rw+")

# print the second line =====================================
la_lines = la_file.readlines()
print la_lines[2]

lv_file.close()

Answer 1

文件对象是一个迭代器。一旦你遍历了所有的行，迭代器就用完了，进一步的读取将无济于事。

为避免关闭并重新打开文件，您可以使用 seek 倒回到开头：

lv_file.seek(0)

Answer 2

你要的是file.seek():

示例：（基于您的代码）

lv_file = open("filename.txt", "rw+")

# count the number of lines =================================
lv_cnt = 0
for row in lv_file.xreadlines():
    lv_cnt = lv_cnt + 1

lv_file.seek(0)  # reset file pointer

# print the second line =====================================
la_lines = la_file.readlines()
print la_lines[2]

lv_file.close()

这会将文件指针重置回其起始位置。

pydoc file.seek:

seek(offset, whence=SEEK_SET) Change the stream position to the given byte offset. offset is interpreted relative to the position indicated by whence. Values for whence are:

SEEK_SET or 0 – start of the stream (the default); offset should be zero or positive SEEK_CUR or 1 – current stream position; offset may be negative SEEK_END or 2 – end of the stream; offset is usually negative Return the new absolute position.

New in version 2.7: The SEEK_* constants

更新： 一种更好的计算次数的方法。迭代文件中的行数并且只关心第二行：

def nth_line_and_count(filename, n):
    """Return the nth line in a file (zero index) and the no. of lines"""

    count = 0

    with open(filename, "r") as f:
        for i, line in enumerate(f):
            count += 1
            if i == n:
                value = line

    return count, value

nlines, line = nth_line_and_count("filename.txt", 1)

Answer 3

由于 xreadlines() 保留指向它发送给您的最后一行的指针，因此当您执行

la_lines = la_file.readlines()

它基本上记住了它给你的最后一行的索引。当您关闭文件然后打开它时，它会创建一个新的迭代器，并再次指向第 0 行。

Python 连续读取文件

Python Consecutive Reads From File

python

input

readlines