Python中infile对象两次循环仅执行一次的问题排查

你遇到的这个问题完全和文件读取的「指针机制」有关！先看看你写的代码（我帮你整理了正确的缩进格式）：

infile = open("australianFatalities.csv", "r")
for line in infile:
    words = line.split(",")
    crash_id = words[0]
    crash_state = words[1]
    if crash_state == "NSW":
        print(crash_id)

for line in infile:
    words = line.split(",")
    crash_id = words[0]
    crash_state = words[1]
    if crash_state == "NSW":
        print(crash_state)

你会发现第一段循环能正常输出crash_id，但第二段直接跳过；哪怕调换两段顺序，也只有先跑的那段能执行——这是因为文件对象就像一个带「读取指针」的游标：

• 第一次循环时，指针从文件最开头出发，每读一行就往后挪一步，直到把所有内容读完，指针停在了文件的末尾位置。
• 第二次循环的时候，指针已经在末尾了，没有更多内容可以读取，所以循环直接不执行。

给你几个简单易懂的解决办法： #

1. 每次循环前重新打开文件

最简单的方式，每次需要遍历的时候重新打开文件，这样指针会自动回到开头：

# 第一段循环
with open("australianFatalities.csv", "r") as infile:
    for line in infile:
        words = line.split(",")
        crash_id = words[0]
        crash_state = words[1]
        if crash_state == "NSW":
            print(crash_id)

# 第二段循环，重新打开文件
with open("australianFatalities.csv", "r") as infile:
    for line in infile:
        words = line.split(",")
        crash_id = words[0]
        crash_state = words[1]
        if crash_state == "NSW":
            print(crash_state)

（用with语句会自动帮你关闭文件，比手动调用close()更安全哦）

2. 先把文件内容全部读到列表里

把所有行先存到一个列表中，之后可以反复遍历这个列表：

# 一次性把文件所有行读到列表里
with open("australianFatalities.csv", "r") as infile:
    all_lines = list(infile)

# 第一段循环，遍历列表
for line in all_lines:
    words = line.split(",")
    crash_id = words[0]
    crash_state = words[1]
    if crash_state == "NSW":
        print(crash_id)

# 第二段循环，再次遍历同一个列表
for line in all_lines:
    words = line.split(",")
    crash_id = words[0]
    crash_state = words[1]
    if crash_state == "NSW":
        print(crash_state)

3. 用seek()把指针移回开头

如果不想重新打开或者存列表，可以用infile.seek(0)把指针强制移回文件开头，不过新手可以先试试前两种更直观的方法：

infile = open("australianFatalities.csv", "r")

# 第一段循环
for line in infile:
    words = line.split(",")
    crash_id = words[0]
    crash_state = words[1]
    if crash_state == "NSW":
        print(crash_id)

# 把指针移回文件开头
infile.seek(0)

# 第二段循环
for line in infile:
    words = line.split(",")
    crash_id = words[0]
    crash_state = words[1]
    if crash_state == "NSW":
        print(crash_state)

infile.close()

这样处理后，两段循环就能正常执行啦！

内容的提问来源于stack exchange，提问作者PlanetAlexanderProjects