How to read specific blocks of data file within certain keywords - python-3.x

Question

I have a text data file that looks as shown below:
BEGIN_CYCLE
..
start_data
2d_data1
end_data
..
..
END_CYCLE
BEGIN_CYCLE
..
start_data
2d_data2
end_data
BEGIN_CYCLE
..
start_data
2d_data3
end_data
...
END_CYCLE
and so on
I am only interested in data blocks that start with start_data and end with end_data keywords, AND fall between BEGIN_CYCLE and a matching END_CYCLE keywords. In the above example, I want to read 2d_data1 and 2d_data3. Notice that although 2d_data2 starts with start_data and ends with end_data keywords, it is NOT bound by BEGIN_CYCLE and a matching END_CYCLE keyword. It only has a BEGIN_CYCLE and no matching END_CYCLE keyword. Of course I can have any number of begin and end cycles, and not just 3. My code below still reads 2d_data2 and actually skips over 2d_data3, and reads subsequent data blocks correctly. I do not know why exactly this is happening.
indexes = []
with open(file) as f:
lines = f.readlines()
for i, line in enumerate(lines):
if line.startswith('BEGIN_CYCLE'):
s = i
elif line.startswith('END_CYCLE'):
e = i
indexes.append((s, e))
else:
pass
temp_list = [list(range(*idx)) for idx in indexes]
indexes = [item for sublist in temp_list for item in sublist]
data = []
with open(file) as f:
for i, line in enumerate(f):
if 'start_data' in line and i in indexes:
chunk = []
for line in f:
if not line.startswith('end_data'):
chunk.append(''.join(line.strip().split()))
else:
break
data.append(chunk)
My thought process is to first identify valid test cycles (those with begin_cycle and end_cycle keywords) which explains the first part of the code. Then within these bounds, I am searching for start_data and end_data keywords and appending lines of data into chunks which I eventually collect in a list of data. The problem with my code is that 2d_data2 is read and not ignored. In fact, the code works fine whenever the test file always has matching BEGIN_CYCLE and END_CYCLE keywords. However, as soon as there is one or more instances of missing END_CYCLE keywords, then instead of ignoring any data block under that cycle, it includes it. Any help or alternative solution is appreciated. Thanks.

Answer 1

Below works exactly like I wanted. However, I don't like the idea of opening the file each time I loop over the indexes. I cannot think of a fix for this.
indexes = []
with open(file) as f:
lines = f.readlines()
for i, line in enumerate(lines):
if line.startswith('BEGIN_CYCLE'):
s = i
elif line.startswith('END_CYCLE'):
e = i
indexes.append((s, e))
else:
pass
data = []
for idx in indexes:
with open(file) as f:
for line in itertools.islice(f, idx[0], idx[1]):
if line.startswith('start_data'):
chunk = []
for line in f:
if not line.startswith('end_data'):
chunk.append(''.join(line.strip().split()))
else:
break
data.append(chunk)