如何保持行之间的缩进？

Question

我正在用Python写一个新应用程序，我需要保持行与行之间的缩进。

我有一个文件里的代码：

line 1
   __LINE TO CHANGE__
line 3

我想要得到的是：

line 1
   added code line a
   added code line b
line 3

我试过一些方法，比如添加\n、\n\r，使用""" """字符串，或者string("")，但我只得到了这两个结果：

line 1
    added code line a added code line b
line 3


line 1
    added code line a
added code line b
line 3

我正在使用replace()函数来更改一行。

谢谢

编辑：

我的代码：

Reading from file and put into variable named code:
line 1
    __LINE TO CHANGE__
line 3

text = "added code line a"
text += "added code line b" 

available_pos = ["__LINE TO CHANGE__"]
text_to_add = [text]

code = code.replace(available_pos, text_to_add)

Answer 1

如果你不知道用来缩进的具体字符是什么（比如空格、制表符等等），或者缩进用了多少个字符，那么你需要从你从文件中读取的原始行里获取这些信息，然后用这些信息来开头你新的一行。我将借用并补充Padraic的回答：

with open("file.txt") as f:
    lines = f.readlines()
    for ind, line in enumerate(lines):
        num_of_chars = len(line) - len(line.lstrip()) # Get the preceding characters.
        new_line = line[:num_of_chars] + "added code line a" # Add the same to the new line.
        lines[ind] = new_line

Answer 2

另一个例子是关于HTML标签的，这里有三个部分需要从一个文件template.html中读取，并放入一个叫template的变量里。

<div>
   <div>
        ___MENU___
    </div>
</div>
<div>
    ___CONTENT___
</div>
<div>
    ___BOTTOM___
</div>

接下来，我用代码来修改这三个部分，代码是这样的：

html = "<p>"
hrml += "   HELLO WORLD"   
html += "</p>"

然后再用同样的方法替换其他两个文本。

available_pos = ["___MENU___", "___CONTENT___", "___BOTTOM___"]
text_to_add = [html, html1, html2]

template = template.replace(available_pos, text_to_add)

Answer 3

with open("in.txt") as f:
    lines = f.readlines() # get all lines
    lines[1] = "    added code line a\n    added code line b\n" #reassign second line in lines 
    with open("in.txt","w") as f1: # overwrite the file with updated content
        f1.writelines(lines)

" added code line a\n added code line b\n" 这个代码会在两行上写入内容，每行前面都有四个空格的缩进。

如果你想根据特定条件来替换内容，可以使用 enumerate 和 if 检查。

with open("in.txt") as f:
    lines = f.readlines()
    for ind, line in enumerate(lines):
        if "whatever" in line:
           lines[ind] = "   added code line a\n   added code line b\n"
    with open("in.txt","w") as f1:
        f1.writelines(lines)

Answer 4

import re

INDENT_RE = re.compile(r'^\s*$')

def matching_indent(line, pattern):
    """
    Returns indent if line matches pattern, else returns None.
    """
    if line.endswith(pattern):
        indent = line[:-len(pattern)]
        if INDENT_RE.match(indent):
            return indent
    return None

def replace_line(lines, pattern, replacements):
    for line in lines:
        indent = matching_indent(line, pattern)
        if indent is None:
            yield line
        else:
            for replacement in replacements:
                yield indent + replacement

你可以这样使用它：

code = '''line 1
    __LINE TO CHANGE__
line 3'''

print('\n'.join(replace_line(
    code.split('\n'),                           # one string per line
    '__LINE TO CHANGE__',                       # the string to replace
    ["added code line a", "added code line b"]  # the strings to replace with
)))

输出结果：

line 1
    added code line a
    added code line b
line 3

你也可以用这个来处理文件，方法类似于：

with open("input") as f:
    print(''.join(replace_line(f, 'some pattern\n', ['foo\n', 'bar\n'])))

注意，这里我在模式和替换的末尾加了一个 '\n'。如果你打算把这个和 readlines 的输出一起使用（每行的末尾都有一个 \n），那么你可能需要调整这个函数，让它能处理这些换行符。