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 = '	' 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 sp…

Accepted Answer

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

Find number of columns in csv file

Solution

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