在文本文件中的行之间读取

Question

First of all, the contents of my example text file looks like this:

Some Data
Nothing important
Start here
This is important
Grab this line too
And this ono too
End here
Text goes on, but isn't important
Next text
Blaah

And now, I want to read in the text file, and I only want to grab lines between " Start here " and " End here ".

So my Python code looks like:

filename = 'example_file.txt'

with open(filename, 'r') as input:
   for line in input: # First loop breaks at specific line
       if 'Start here' in line:
           break

   for line_1 in input: # Second loop grabs all lines
       print line_1.strip()

   for line_2 in input: # Third loop breaks at specific line
       if 'End here' in line_2:
           break

But it doesn't work.

Here my output, when I run it:

This is important
Grab this line too
And this on too
End here
Text goes on, but isn't important
Next text
Blaah

As you can see, my script doesn't break at End here . The program begins at the correct line, but its doesn't break at the correct line.

What's wrong?

Answer 1

It's the second loop that needs the break...

for line_1 in input:
    if 'End here' in line_1:
        break
    print line_1.strip()

Answer 2

Your problem is that you should be checking for 'End Here' in your second loop, as the second and third one don't run at the same time. In fact, the third loop won't even run.

With that in mind, this code will work:

filename = 'mydata.txt'

with open(filename, 'r') as f:
    for line in f:
        if 'Start here' in line:
            break

    for line_1 in f:
        if 'End here' in line:
            break
        else:
            print line.strip()

However, there are still some optimizations we can make:

• Variables on for loops are local to that for loop only, so we can reuse the name;
• any code after break won't run anyways, so we can get rid of the else ;
• open uses read-mode by default.

With this in mind, your final code would look like this:

filename = 'mydata.txt'

with open(filename) as f:
    for line in f:
        if 'Start here' in line:
            break

    for line in f:
        if 'End here' in line:
            break
        print line.strip()

Run that, and you'll get the desired output:

This is important
Grab this line too
And this ono too

Answer 3

You can use regular expressions ( re module) with the re.DOTALL option so that newlines are considered as regular characters.

import re

source = """Some Data
Nothing important
Start here
This is important
Grab this line too
And this ono too
End here
Text goes on, but isn't important
Next text
Blaah"""

# or else:
# source = open(filename, 'r').read() # or similar

result = re.search("Start here(.*)End here", source, re.DOTALL).group(1).strip()

print result

> This is important
> Grab this line too
> And this ono too

Why it works:

• re.search looks for the pattern in some string;
• Parentheses separates the matches in groups . The first group is the whole pattern, the second group is the parentheses. Groups can be sequenced and nested;
• .* means "any char, any number of times". It is required to take everything between the two hard-coded markers (namely Start Here and End here );
• re.DOTALL is the secret: it will treat newline chars as if they were regular string chars. Dot is the symbol for "any char", so "dot all" means "treat any char as a regular char, even new-line chars".
• group(1) means you want the second (zero-based index) group, which is the one inside the parentheses.

Answer 4

You can read all lines first and enumerate it:

filename = 'example_file.txt'

useful_content = []
with open(filename, 'r') as input:
    all_lines = input.readlines()  # read all lines
    for idx in range(len(all_lines)):  # iterate all lines
    if 'Start here' in all_lines[idx]:
        useful_content.append(all_lines[idx].strip())
        idx = idx + 1
        # found start of useful contents, continue iterate till it ends
        while 'End here' not in all_lines[idx]:
            useful_content.append(all_lines[idx].strip())
            idx = idx + 1
        break
for line in useful_content:
    print(line)