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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.

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