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 without "consuming" a row before the iterator runs? The following code is the most elegant I could manage, but I would prefer to run the check before the for loop starts:

import csv f = 'testfile.csv' d = '\t'  reader = csv.reader(f,delimiter=d) for row in reader:     if reader.line_num == 1: fields = len(row)     if len(row) != fields:         raise CSVError("Number of fields should be %s: %s" % (fields,str(row)))     if fields == 1:         pass     elif fields == 2:         pass     elif fields == 3:         pass     else:         raise CSVError("Too many columns in input file.") 

Edit: I should have included more information about my data. If there is only one field, it must contain a name in scientific notation. If there are two fields, the first must contain a name, and the second a linking code. If there are three fields, the additional field contains a flag which specifies whether the name is currently valid. Therefore if any row has 1, 2 or 3 columns, all must have the same.

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

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