read specific lines from a file using python

Question

I have a file with data like:

   1xxy
   (1gmh)

[white line]
ahdkfkbbmhkkkkkyllllkkjdttyshhaggdtdyrrrutituy
[white line]  
   __________________________________________________
   Intra Chain:
   A 32
   __________________________________________________
   PAIR 1MNS HE 10 NM A ARG 33 OX1 3.22 32
   PAIR 1MNS UR 11 NM A ARG 33 OX2 3.21 12
   PAIR IMNS UK 32 NH A ASN 43 OZ1 5.21 22
   ...
   __________________________________________________

NOW i want to make it like:

   PAIR 1MNS HE 10 NM A ARG 33 OX1 3.22 32
   PAIR 1MNS UR 11 NM A ARG 33 OX2 3.21 12
   PAIR IMNS UK 32 NH A ASN 43 OZ1 5.21 22
   ...

ie remove all other characters. i tried using:

inp = open('c:/users/rox/desktop/1UMG.out','r')
for line in inp:
    if not line.strip():      # to remove excess whit lines
       continue
    else:
       z = line.strip().replace('\t',' ')
       if z.startswith('PAIR'):
          print z
inp.close()

but this code is also giving me no output. Can't figure out why z.startswith('PAIR') is not working. But up to the previous line it is going fine.

Answer 1

Looks like you are looking only at lines that start with PAIR , so why not something simple like this:

with open('data.txt') as infp:
   for line in infp:
      line = line.strip()
      if line.startswith('PAIR'):
         print(line)

will give:

PAIR 1MNS HE 10 NM A ARG 33 OX1 3.22 32
PAIR 1MNS UR 11 NM A ARG 33 OX2 3.21 12
PAIR IMNS UK 32 NH A ASN 43 OZ1 5.21 22

This output removes the leading 3 spaces, it would be trivial to add them back in if needed.

Note : using with will automatically close the file for you when you are done, or an exception is encountered.

Answer 2

除了@Levon的解释之外，由于文件对象支持迭代器协议，并且根据文件的大小，可以使用列表推导：

[l for l in open('test.txt') if l.startswith('PAIR')]