all Python 3Files & I/Oterm

csv.DictWriter

csv — writing

A CSV writer for dicts: the required fieldnames sets the column order, and writeheader() writes the header row.

Syntax

w = csv.DictWriter(f, fieldnames, restval='', extrasaction='raise'); w.writeheader()

The header is not written automatically — without writeheader() the file has no first row. A key missing from fieldnames raises ValueError; pass extrasaction='ignore' to drop extras silently. Missing keys, on the other hand, are not an error: restval is written instead, an empty string by default.

Examples

import csv
import io
out = io.StringIO()
w = csv.DictWriter(out, fieldnames=['name', 'age'], lineterminator='\n')
w.writeheader()
w.writerow({'name': 'Anna', 'age': 30})
w.writerow({'name': 'Bob'})
print(out.getvalue())  # → name,age / Anna,30 / Bob,

import csv
import io
w = csv.DictWriter(io.StringIO(), fieldnames=['name'])
try:
    w.writerow({'name': 'X', 'mail': 'x@y'})
except ValueError as e:
    print(e)  # → dict contains fields not in fieldnames: 'mail'

See also

Common errors

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