Find number of columns in csv file

Question

My program needs to read csv files which may have 1,2 or 3 columns, and it needs to modify its behaviour accordingly. Is there a simple way to check the number of columns withou

Answer 1

You can use itertools.tee

itertools.tee(iterable[, n=2])
Return n independent iterators from a single iterable.

eg.

reader1, reader2 = itertools.tee(csv.reader(f, delimiter=d))
columns = len(next(reader1))
del reader1
for row in reader2:
    ...

Note that it's important to delete the reference to reader1 when you are finished with it - otherwise tee will have to store all the rows in memory in case you ever call next(reader1) again

Answer 2

I would rebuild it as follows ( if the file is not too big ):

import csv
f = 'testfile.csv'
d = '\t'

reader = list(csv.reader(f,delimiter=d))
fields = len( reader[0] )
for row in reader:
    if fields == 1:
        pass
    elif fields == 2:
        pass
    elif fields == 3:
        pass
    else:
        raise CSVError("Too many columns in input file.")

Answer 3

I would suggest a simple way like this:

with open('./testfile.csv', 'r') as csv:
     first_line = csv.readline()
     your_data = csv.readlines()

ncol = first_line.count(',') + 1 

Answer 4

What happens if the user provides you with a CSV file with fewer columns? Are default values used instead?

If so, why not extend the row with null values instead?

reader = csv.reader(f,delimiter=d)
for row in reader:
    row += [None] * (3 - len(row))
    try:
        foo, bar, baz = row
    except ValueError:
        # Too many values to unpack: too many columns in the CSV
        raise CSVError("Too many columns in input file.")

Now bar and baz will at least be None and the exception handler will take care of any rows longer than 3 items.

Answer 5

This seems to work as well:

import csv

datafilename = 'testfile.csv'
d = '\t'
f = open(datafilename,'r')

reader = csv.reader(f,delimiter=d)
ncol = len(next(reader)) # Read first line and count columns
f.seek(0)              # go back to beginning of file
for row in reader:
    pass #do stuff