Importing data from a text file using python

Python's struct.unpack is probably the quickest way to split fixed-length fields. Here's a function that will lazily read your file and return tuples of numbers that match your criteria:

import struct

def parsefile(filename):
    with open(filename) as myfile:
        for line in myfile:
            line = line.rstrip('\n')
            fields = struct.unpack('11s11s8s8s5s', line)
            if 'OW' in fields[1]:
                yield (int(fields[3]), int(fields[4]))

Usage:

if __name__ == '__main__':
    for field in parsefile('file.txt'):
        print field

Test data:

1234567890a1234567890a123456781234567812345
something  maybe OW d 111111118888888855555
aaaaa      bbbbb      1234    1212121233333
other thinganother OW 121212  6666666644444

Output:

(88888888, 55555)
(66666666, 44444)

In Python you can extract a substring at known positions using a slice - this is normally done with the list[start:end] syntax. However you can also create slice objects that you can use later to do the indexing.

So you can do something like this:

columns = [slice(11,22), slice(30,38), slice(38,44)]

myfile = open('some/file/path')
for line in myfile:
    fields = [line[column].strip() for column in columns]
    if "OW" in fields[0]:
        value1 = int(fields[1])
        value12 = int(fields[2]) 
        ....

Separating out the slices into a list makes it easy to change the code if the data format changes, or you need to do stuff with the other fields.

Here's a function which might help you:

def rows(f, columnSizes):
    while True:
        row = {}
        for (key, size) in columnSizes:
            value = f.read(size)
            if len(value) < size: # EOF
                return
            row[key] = value
        yield row

for an example of how it's used:

from StringIO import StringIO

sample = StringIO("""aaabbbccc
d  e  f  
g  h  i  
""")

for row in rows(sample, [('first', 3),
                         ('second', 3),
                         ('third', 4)]):
    print repr(row)

Note that unlike the other answers, this example is not line-delimited (it uses the file purely as a provider of bytes, not an iterator of lines), since you specifically mentioned that the fields were not separated, I assumed that the rows might not be either; the newline is taken into account specifically.

You can test if one string is a substring of another with the 'in' operator. For example,

>>> 'OW' in 'hello'
False
>>> 'OW' in 'helOWlo'
True

So in this case, you might do

if 'OW' in row['third']:
    stuff()

but you can obviously test any field for any value as you see fit.

entries = ((float(line[30:38]), float(line[38:43])) for line in myfile if "OW" in line[11:22])

for num1, num2 in entries:
  # whatever

entries = []
with open('my_file.txt', 'r') as f:
  for line in f.read().splitlines()
    line = line.split()
    if line[1].find('OW') >= 0
      entries.append( ( int(line[-2]) , int(line[-1]) ) )

entries is an array containing tuples of the last two entries

edit: oops