python - 嵌套循环迭代单个文件

Question

我想删除文件中的某些特定行。我要删除的部分包含在两行之间(也将被删除)，名为 STARTING_LINE 和 CLOSING_LINE。如果文件末尾之前没有结束行，则操作应该停止。

例子:

...blabla...
[Start] <-- # STARTING_LINE
This is the body that I want to delete
[End] <-- # CLOSING_LINE
...blabla...

我想出了三种不同的方法来实现同一件事(加上下面 tdelaney 的回答提供的一种方法)，但我想知道哪种方法最好。请注意，我不是在寻找主观意见:我想知道我应该选择一种方法而不是另一种方法的真正原因。

1。很多 if 条件(只有一个 for 循环):

def delete_lines(filename):
    with open(filename, 'r+') as my_file:
        text = ''
        found_start = False
        found_end = False

        for line in my_file:
            if not found_start and line.strip() == STARTING_LINE.strip():
                found_start = True
            elif found_start and not found_end:
                if line.strip() == CLOSING_LINE.strip():
                    found_end = True
                continue
            else:
                print(line)
                text += line

        # Go to the top and write the new text
        my_file.seek(0)
        my_file.truncate()
        my_file.write(text)

2。在打开的文件上嵌套 for 循环:

def delete_lines(filename):
    with open(filename, 'r+') as my_file:
        text = ''
        for line in my_file:
            if line.strip() == STARTING_LINE.strip():
                # Skip lines until we reach the end of the function
                # Note: the next `for` loop iterates on the following lines, not
                # on the entire my_file (i.e. it is not starting from the first
                # line). This will allow us to avoid manually handling the
                # StopIteration exception.
                found_end = False
                for function_line in my_file:
                    if function_line.strip() == CLOSING_LINE.strip():
                        print("stop")
                        found_end = True
                        break
                if not found_end:
                    print("There is no closing line. Stopping")
                    return False
            else:
                text += line

        # Go to the top and write the new text
        my_file.seek(0)
        my_file.truncate()
        my_file.write(text)

3。 while True 和 next()(有 StopIteration 异常)

def delete_lines(filename):
    with open(filename, 'r+') as my_file:
        text = ''
        for line in my_file:
            if line.strip() == STARTING_LINE.strip():
                # Skip lines until we reach the end of the function
                while True:
                    try:
                        line = next(my_file)
                        if line.strip() == CLOSING_LINE.strip():
                            print("stop")
                            break
                    except StopIteration as ex:
                        print("There is no closing line.")
            else:
                text += line

        # Go to the top and write the new text
        my_file.seek(0)
        my_file.truncate()
        my_file.write(text)

4。 itertools(来自tdelaney的回答):

def delete_lines_iter(filename):
    with open(filename, 'r+') as wrfile:
        with open(filename, 'r') as rdfile:
            # write everything before startline
            wrfile.writelines(itertools.takewhile(lambda l: l.strip() != STARTING_LINE.strip(), rdfile))
            # drop everything before stopline.. and the stopline itself
            try:
                next(itertools.dropwhile(lambda l: l.strip() != CLOSING_LINE.strip(), rdfile))
            except StopIteration:
                pass
            # include everything after
            wrfile.writelines(rdfile)
        wrfile.truncate()

看来这四种实现的效果是一样的。所以……

问题:我应该使用哪一个？哪一个是最 Pythonic 的？哪个最有效？

有更好的解决方案吗？

---

编辑:我尝试使用timeit 评估大文件上的方法。为了在每次迭代中都有相同的文件，我删除了每个代码的编写部分；这意味着评估主要考虑阅读(和文件打开)任务。

t_if = timeit.Timer("delete_lines_if('test.txt')", "from __main__ import delete_lines_if")
t_for = timeit.Timer("delete_lines_for('test.txt')", "from __main__ import delete_lines_for")
t_while = timeit.Timer("delete_lines_while('test.txt')", "from __main__ import delete_lines_while")
t_iter = timeit.Timer("delete_lines_iter('test.txt')", "from __main__ import delete_lines_iter")

print(t_if.repeat(3, 4000))
print(t_for.repeat(3, 4000))
print(t_while.repeat(3, 4000))
print(t_iter.repeat(3, 4000))

结果:

# Using IF statements:
[13.85873354100022, 13.858520206999856, 13.851908310999988]
# Using nested FOR:
[13.22578497800032, 13.178281234999758, 13.155530822999935]
# Using while:
[13.254994718000034, 13.193942980999964, 13.20395484699975]
# Using itertools:
[10.547019549000197, 10.506679693000024, 10.512742852999963]

Answer 1

您可以使用 itertools 让它看起来很漂亮。我会对时间比较感兴趣。

import itertools
def delete_lines(filename):
    with open(filename, 'r+') as wrfile:
        with open(filename, 'r') as rdfile:
            # write everything before startline
            wrfile.writelines(itertools.takewhile(lambda l: l.strip() != STARTING_LINE.strip(), rdfile))
            # drop everything before stopline.. and the stopline itself
            next(itertools.dropwhile(lambda l: l.strip() != CLOSING_LINE.strip(), rdfile))
            # include everything after 
            wrfile.writelines(rdfile)
        wrfile.truncate()