python - 尝试 'a+' 同时读取和附加但没有用

Question

这些是 contacts.txt 文件的内容:

foo 69

bar 70

baz 71

我想删除“foo 69”，这就是我所做的:

with open('contacts.txt','a+') as f:
    for line in f:
        with open('contacts.txt','a+') as f:
            if "foo" in line:
                line.replace("foo", "")

它什么也没做。

Answer 1

正确的做法是先完整读取内容，修改后再写回文件。

这种方法也很干净且可读。

#first read everything
with open('file_name','r') as f:
        content =  f.read()

#now perform modifications
content = content.replace('foo 69','')

#now write back to the file
with open('file_name','w') as f:
        f.write(content)

现在，我已经评论了您在代码中遇到的一些问题:

with open('file_name','a+') as f:
    for line in f:#here you are iterating through the content of the file
        # at each iteration line will equal foo 69, then bar 70 and then bar 71...

        # Now, there is no reason to open the file here again, I guess you opened
        # it to write again, but your mode is set to `a` which will append contents
        # not overwrite them
        with open('contacts.txt','a+') as f:
            if "foo" in line:
                line.replace("foo", "") #here the modified content is lost
                # because you're not storing them anywhere

编辑 - 如评论中所述，如果您的文件很大并且您不想阅读所有内容。

那么最好的方法是逐行读取内容，然后将内容写入另一个文件，不包括要删除的行。

to_replace = 'foo 69\n' #note \n is neccessary here
with open('input.txt','r') as input_file:
    with open('ouput.txt','w') as output:
        for line in input_file:
            if line!=to_replace:
                output.write(line)


#Now, let's say you want to delete all the contents of the input_file
#just open it in write mode and close without doing anything
input_file = open('input_file.txt','w')
input_file.close()

# If you want to delete the entire input_file and rename output_file to
# original file_name then you can do this in case of linux OS using subprocess
subprocess.call(['mv', 'output_file.txt', 'input_file.txt'])

这是非常有效的内存，因为在任何时间点内存中只有一行内容。 input_file 只是指向文件的指针，迭代 - for line in input_file 不会读取整个文件并开始一个接一个地迭代内容。