csv.DictWriter emits strange errors if fieldnames is an iterator

[bendotc]

BPO	32547
Nosy	@rhettinger, @bendotc, @iritkatriel, @dignissimus
PRs	gh-76728: Coerce DictReader and DictWriter fieldnames argument to a list #32225
Files	sam_ezeh.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-01-14.00:41:44.336>
labels = ['type-feature', 'library', '3.11']
title = 'csv.DictWriter emits strange errors if fieldnames is an iterator'
updated_at = <Date 2022-04-01.08:41:33.520>
user = 'https://github.com/bendotc'

bugs.python.org fields:

activity = <Date 2022-04-01.08:41:33.520>
actor = 'sam_ezeh'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-01-14.00:41:44.336>
creator = 'bendotc'
dependencies = []
files = ['50711']
hgrepos = []
issue_num = 32547
keywords = ['patch']
message_count = 4.0
messages = ['309904', '309907', '415573', '416473']
nosy_count = 4.0
nosy_names = ['rhettinger', 'bendotc', 'iritkatriel', 'sam_ezeh']
pr_nums = ['32225']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32547'
versions = ['Python 3.11']

[bendotc]

If I pass an iterator to csv.DictWriter as the fieldname field, then DictWriter consumes that iterator pretty quickly, emitting strange errors such as the following when trying to write the headers.

>>> import csv
>>> fields = iter(["a", "b", "c", "d"])
>>> f = open('test.csv', 'w')
>>> writer = csv.DictWriter(f, fieldnames=fields)
>>> writer.writeheader()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.4/csv.py", line 142, in writeheader
    self.writerow(header)
  File "/usr/lib/python3.4/csv.py", line 153, in writerow
    return self.writer.writerow(self._dict_to_list(rowdict))
  File "/usr/lib/python3.4/csv.py", line 149, in _dict_to_list
    + ", ".join([repr(x) for x in wrong_fields]))
ValueError: dict contains fields not in fieldnames: 'c', 'a'

This is because writeheader and _dict_to_list repeatedly iterate over self.fieldnames. It seems like this could be solved by making a list of fieldnames in the constructor.

[rhettinger]

This isn't a bug. The docs specify that *fieldnames* is a sequence, "The fieldnames parameter is a sequence of keys that identify the order in which values in the dictionary passed to the writerow() method are written to the csvfile."

We could make this a feature request. but as far as I can tell there isn't a real use case, nor has one ever arisen in the decade long history of the module. Adding the list(fieldnames) behavior could be done without a backwards compatibility issue, but it does incur an overhead that would be paid by all existing working code.

[iritkatriel]

Perhaps we could raise an exception with a clearer error message when fieldnames not a sequence?

[dignissimus]

I've submitted a patch that introduces code that raises TypeError at construction if fieldnames is not a sequence

[furkanonder]

@iritkatriel The issue seems to have been resolved. Can we close the issue?