Python:将字符串替换为文件中的变量

Question

如果我有这样一个文件内容:

old_string
-old_string

我只想将“old_string”更改为“+new_string”，所以结果看起来像

+new_string
-old_string

我的代码给出了这个结果:

+new_string
-+new_string

这是我的代码:

    with open(filename) as f:

    s = f.read()

    if old_string not in s:

        return False

with open(filename, 'w') as f:

    s = s.replace(old_string, new_string)

    f.write(s)

    return True

我试过正则表达式，但它不会工作，因为我将正则表达式作为变量传递，这就是我到目前为止所做的:

    with open (filename, 'r' ) as f:

       content = f.read()

content_new = re.sub('(\%old_string)', r'\new_string'%(old_string,new_string), content, flags = re.M)      

Answer 1

您可以忽略开头带有连字符(“-”)的行，并替换其余部分。

下面的脚本与您的略有不同。我已发表评论以帮助您理解。这应该很容易理解。

filename ="some_file"
output_filename = "some_other_file"

old_string = "old_string"
new_string = "+new_string"

input_file_handle = open(filename,"r") # File being opened in read mode
output_file_handle = open(output_filename, "w") # File being opened in write mode

# Read in input file line by line
for line in input_file_handle:

    # Write to output file and move on to next line
    if old_string not in line:
        output_file_handle.write(line+"\n")
        continue

    # This line contains the old_string. We check if it starts with "-". 
    # If it does, write original line and move on to next line
    if line.startswith("-"):
        output_file_handle.write(line+"\n")
        continue


    # At this stage we are absolutely sure we want to replace this line's contents
    # So we write the replaced version to the new file
    output_file_handle.write(new_string+"\n")


# Close both file handles
input_file_handle.close()
output_file_handle.close()