Question

I wrote a function that serializes a list of dictionaries as a CSV file using the csv module, with code like this:

data = csv.DictWriter(out_f, fieldnames)
data.writerows(dictrows)

However, I sometimes want to write out to a file only a subset of each dictionary’s keys. If I pass as fieldnames a subset of the keys that each dictionary has, I get the error:

"dict contains fields not in fieldnames"

How can I make it so that DictRows will write just a subset of the fields I specify to CSV, ignoring those fields that are in the dictionary but not in fieldnames?

Answer 1

Simplest and most direct approach is to pass extrasaction='ignore' when you initialize your DictWriter instance, as documented here:

If the dictionary passed to the
writerow() method contains a key not
found in fieldnames, the optional
extrasaction parameter indicates what
action to take. If it is set to
'raise' a ValueError is raised. If it
is set to 'ignore', extra values in
the dictionary are ignored.

It also works on writerows, which, internally, just calls writerow repeatedly.