Question

I have a list of rules for a given input file for my function. If any of them are violated in the file given, I want my program to return an error message and quit.

• the first line should start with a ‘#’ symbol (indicating a header line)
• every line should have exactly 10 columns
• column 2 (counting from 0) should be either a + or – symbol
• column 8 should be a comma-separated list of integers
• column 9 should be a comma-separated list of integers, with exactly the same number of integers as column 8.

To attempt to do this I have written the following code:

Edited steveha’s answer into code

with open(infile, 'r') as fp:
line = fp.readline().strip()
if not line.startswith('#'):
    print ('First line does not start with #')
    sys.exit(1)
n = 1
for line in fp.readlines():
    d = '(\d+,\d+)'
    n += 1
    cols = line.strip().split()
    i = search(d, line)
    if len(cols) != 10:
        print('Lenth not equal to 10')
        sys.exit(1)
    if cols[2] != '+' or '-':
        print('Column 2 is not a + or - symbol')
        sys.exit(1)
    if i and cols[8] != i.group(1):
        print('Column 8 is not a comma-separated list of integers')
        sys.exit(1)
    if i and cols[9] != i.group(1) and len(cols[9]) != len(cols[8]):
        print('Column 9 in not a comma-separated list of integers with the exact same number of integers in column 8')
        sys.exit(1)

Yet, when I run this, not all the conditionals appear to be functional. I am trying to do this correctly?

Answer 1

This line is wrong:

if cols[2] != '+' or '-':

This would be correct:

if not (cols[2] == '+' or cols[2] =='-'):

I would suggest this instead:

if cols[2] not in ('+', '-'):

Also, I suggest you not call fp.readlines(). The file object fp works as an iterator, and when you iterate it you get one line at a time. So your loop would then become:

for line in fp:

Also, it looks like n is keeping track of line number? In that case, there is an idiomatic Python way you can do it, like so:

for n, line in enumerate(fp, 1):

enumerate() takes an iterator and returns the next value from the iterator together with an incrementing count. By default the count starts at 0, but you can optionally specify a starting number, as I did here to make it start at 1.

And it is best practice in Python to use the with statement to open files, so I suggest you do this:

with open(infile, 'r') as fp:
    line = fp.readline().strip()
    if not line.startswith('#'):
        print ('First line does not start with #')
        sys.exit(1)
    for line in fp:
        # process lines here

The code you are showing does not fully make sense to me. This line:

i = search(d, line)

You must have already done a from re import search command. I actually recommend just doing import re and then explicitly calling re.search() but I guess that is a matter of preference. Anyway, this sets i to the match group result from re.search() (or to None if the match fails). But later on in the code you are testing r rather than i, and you never set r in any code we see here so I am not sure what that will do. Personally I use m as the variable name for a match group.

Your regular expression just matches a pair of positive integers. Nothing there counts how many integers there are. len(cols[8]) is checking how many characters in cols[8].

You are calling a string method function .split(''), which is not correct. On my system it raises an exception: ValueError: empty separator Just call .split() to split on white space; I’ll assume that the comma-separated integers lists must not have any white space.

Finally, please consider the guidelines in PEP 8. Your variable FirstLine is capitalized like a class name rather than a variable name; that didn’t exactly confuse me, but it was sort of distracting. Most of the Python community follows PEP 8.

http://www.python.org/dev/peps/pep-0008/

Taking all of the above into account, I simply re-wrote your code:

import sys

def parse_list_of_int(s):
    try:
        return [int(x) for x in s.split(',')]
    except Exception:
        return None

with open("test.txt", 'r') as f:
    # read line 1
    line = f.readline().strip()
    if not line.startswith('#'):
        print ('First line does not start with #')
        sys.exit(1)

    # need to start enumerate() at 2 because we pulled line 1 out above
    for i, line in enumerate(f, 2):
        cols = line.strip().split()
        if len(cols) != 10:
            print('line {0}: Length not equal to 10'.format(i))
            sys.exit(1)
        if cols[2] not in ('+', '-'):
            print('line {0}: Column 2 is not a + or - symbol'.format(i))
            sys.exit(1)
        lst8 = parse_list_of_int(cols[8])
        if lst8 is None:
            print('line {0}: Column 8 is not a comma-separated list of integers').format(i)
            sys.exit(1)
        lst9 = parse_list_of_int(cols[9])
        if lst9 is None:
            print('line {0}: Column 9 is not a comma-separated list of integers'.format(i))
            sys.exit(1)
        if len(lst8) != len(lst9):
            print('line {0}: Column 8 and column 9 do not have same number of integers'.format(i))
            sys.exit(1)

print('No problems!')
sys.exit(0)

I wrote a simple function to parse out the list of integers, build a Python list, and return it. Then the code can actually check properly whether the two lists are the same length.