load a file for conjunctive normal form into a list of lists

Question

I have written some code to load a cnf from a file which store the cnf according to the standard described here.

the file is:

c  simple_v3_c2.cnf      // lines bigining by c are comments
c  
p cnf 3 2                // the line bigining by p is the description of the pb
1 -3 0                   // folowing lines are formulation of the pb, with 0 as ending caractere
2 3 -1 0

I want to load it into [[1, -3][2,3,-1]]

The code I have written works, but it seems ugly to me. I would be interested in having some feedback on it. (I am new to python).

def loadCnfFile(fileName='example.cnf'):
""" retourne une liste de listes d'entiers decrivants la forme normale conjonctive"""
cnf=[]
cnfFile = open(fileName, 'r')
for line in cnfFile:
    if line[0]!="c" and line[0]!="p":
        l=line.split("0")[0].strip().split(" ")
        m=[]
        for k in l:
            m.append(int(k))
        cnf.append(m)
cnfFile.close()
return cnf

Thanks !

Answer 1

using list comprehension:

In [66]: with open("example.cnf") as f:
        print [map(int,line.split("0")[0].split()) for line in f if line and \
                            not (line.startswith("c") or line.startswith("p"))]
   ....:     
[[1, -3], [2, 3, -1]]

or:

with open("example.cnf") as f:
         x= lambda y,c:y.startswith(c)
         print [map(int,line.split("0")[0].split()) for line in f if line and \
                                not any(x(line,z) for z in ("c","p"))]
   ....:     
[[1, -3], [2, 3, -1]]

Answer 2

I guess the best feedback on your code would be to rewrite it in a more "pythonic" manner. For example:

def cnf_lines(path):
    """Yields cnf lines as lists from the file."""

    with open(path) as fp:
        for line in fp:
            if not line.startswith(('c', 'p')):
                items = map(int, line.split())
                yield items[:-1]

Key points:

• PEP-8 conformance (no camelCase in python please)
• context managers (with) for file operations
• generators (yield) instead of accumulating lists

NB: this code is intentionally simplified and does not fully support the specs you linked to.

Answer 3

Ashwini'S code is correct and appealing to an experienced programmer (thank you), but for someone new to python (what you seem to be) maybe a plain for-loop is easier to understand:

result = []
with open("example.cnf") as f:
    for line in f:
        if not (line.startswith("c") or line.startswith("p")):
            result.append([int(x) for x in line.rstrip("0").rstrip("0\n").split()])