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.
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