Built in Python function to skip row of data

Question

I have a script that reads in a large data file 3GB I don't need all of the data and would like to skip particular rows of data if a condition is meet. Is there a Python function to skip a row of data in a data file and continue reading the file? I checked the 3.2 docs but only found a function that skips chunks of data.

EDIT

reading in data like this

def read_file(F):      #Function that reads data froma file  
                   #and extracts specific data columns
X = []
Y = []                # Creats Data Lists
Z = []


N =   11912639  # number of lines to be read
f = open(F)           #Opens file
f.readline()          # Strips Header
nlines = islice(f, N) #slices file to only read N lines


for line in nlines:             #Loop Strips empty lines as well as replaces tabs with   space
    if line !='':
        line = line.strip()
        line = line.replace('\t',' ')
        columns = line.split()
        x = columns[0]             # assigns variable to columns
        y = columns[1]
        z = columns[2]
        X.append(x)
        Y.append(y)                #appends data in list
        Z.append(z)

What I was thinking of doing is putting an if statement in the above code something like

if x > somevalue:
    skipline
else:
    continue 

Answer 1

If rows in your files correspond to lines, then just use a list comprehension:

with open(path) as input_file:
    contents = [row for row in input_file if not unwanted(row)]

Similar constructs may be possible if your reading the file from some lazy reader other than the default line-by-line textfile reader.

Replace [] with () if you want to read the file lazily.

Answer 2

If I understand your sample code right, what you're looking for is something like this:

for line in nlines:
    line = line.strip()
    if line == '': 
        continue # skip empty lines

    line = line.replace('\t',' ')
    x, y, z = line.split()

    if x > somevalue: 
        continue # skip line if x > somevalue

    X.append(x)
    Y.append(y)
    Z.append(z)

Answer 3

You could do it manually yourself.

for line in file:
    if 'foo' not in line:  
        print 'profiting'