# csv.QUOTE\_STRINGS should be able to read what it has written **URL:** \ **Category:** Ideas\ **Created:** [May 7, 2025, 3:16am UTC](https://discuss.python.org/t/csv-quote-strings-should-be-able-to-read-what-it-has-written/91035 "2025-05-07T03:16:54Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![andy-maier](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/andy-maier/32/26327_2.png) [@andy-maier](https://discuss.python.org/u/andy-maier)\ **Post date:** [May 7, 2025, 3:16am UTC](https://discuss.python.org/t/csv-quote-strings-should-be-able-to-read-what-it-has-written/91035/1 "2025-05-07T03:16:54Z")
The current behavior of csv.QUOTE\_STRINGS (as of Python 3.13, with the bug fix for [CSV reader should support QUOTE\_NOTNULL and QUOTE\_STRINGS · Issue #113732 · python/cpython · GitHub](https://github.com/python/cpython/issues/113732)) is that it cannot read all data types it has written. Example Python data to be written: ```python field_names = ['str', 'int', 'float', 'bool', 'none', 'list', 'dict', 'set'] field_rows = [ { 'str': 'foo', 'int': 42, 'float': 3.14, 'bool': True, 'none': None, 'list': ['a', 'b'], 'dict': {'a': 1, 'b': 2}, 'set': {'a', 'b'}, }, { 'str': '', 'int': 0, 'float': 0.0, 'bool': False, 'none': None, 'list': ['a'], 'dict': {'a': 1}, 'set': {'a'}, }, { 'str': '', 'int': 0, 'float': 0.0, 'bool': False, 'none': None, 'list': [], 'dict': {}, 'set': {}, }, ``` Resulting .csv file written with `csv.DictWriter` with `delimiter=",", quotechar='"', quoting=csv.QUOTE_STRINGS`: ```python "str","int","float","bool","none","list","dict","set" "foo",42,3.14,True,,"['a', 'b']","{'a': 1, 'b': 2}","{'a', 'b'}" "",0,0.0,False,,['a'],{'a': 1},{'a'} "",0,0.0,False,,[],{},{} ``` The format in this .csv file is quite nice because: - It distinguishes boolean values from strings - It distinguishes None from empty strings and thus, it provides the ability to (mostly) re-establish the original Python types when reading it. (The only minor issue is that empty dict and empty set cannot be distinguished) However, reading that .csv file with: ```python with open(csvfile_name, 'r') as csvfile: rows = list(csv.DictReader( csvfile, lineterminator="\n", delimiter=",", quotechar='"', quoting=csv.QUOTE_STRINGS)) ``` results in this failure (pytest output): ```python self = def __next__ (self): if self.line_num == 0: # Used only for its side effect. self.fieldnames > row = next(self.reader) E ValueError: could not convert string to float: 'True' /opt/homebrew/Cellar/python@3.13/3.13.0_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/csv.py:178: ValueError ``` Even though `True` is not quoted in the .csv file, it is interpreted as a string, and because it was not quoted, it is attempted to be converted to float, following the csv.QUOTE\_STRINGS rule to interpret an unquoted empty string as `None` and to convert all other non-quoted fields to float. A similar problem exists for lists and dicts and sets that end up unquoted in the .csv file. In addition, int and float values can be distinguished in the .csv file by the presence of a dot, but both int and float values are converted to float when reading. I propose that the behavior of csv.QUOTE\_STRINGS when reading is changed to the following: - convert unquoted empty strings to `None` - convert unquoted boolean values to bool - convert unquoted lists to list - convert unquoted dicts (including empty dict) to dict - convert unquoted (non-empty) sets to set - convert unquoted values that match ints, to int - convert unquoted values that match floats, to float - convert other unquoted values to str - convert quoted values to str The user could then check for strings that represent dicts or lists or sets and convert them. Doing that in the csv module would go too far IMO, because there may be intentional string typed values with such content. I think this is change would be reasonably backwards compatible, because it changes only a situation that previously lead to an exception, and I think it is unlikely that this exception could have been recovered by users. The distinction between float and int would now cause int to be returned where float was returned previously, but that would probably work in most cases, unless the user code checks for types. If this change is considered too backwards incompatible, a new quote constant with the proposed reading behavior (and the writing behavior of `csv.QUOTE_STRING`) could be introduced. BTW, I tried the other `csv.QUOTE_*` constants, but none of them came close to writing the .csv file as distinguishable as `csv.QUOTE_STRINGS`. Please comment on this proposal, and let me know whether this could be brought forward as an issue on [GitHub · Where software is built](https://github.com/python/cpython/issues). ---
**Author:** ![mcepl](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/mcepl/32/24095_2.png) [@mcepl](https://discuss.python.org/u/mcepl)\ **Post date:** [May 7, 2025, 11:59am UTC](https://discuss.python.org/t/csv-quote-strings-should-be-able-to-read-what-it-has-written/91035/2 "2025-05-07T11:59:17Z")
This is not primarily a Pythonic problem, but general problem of the CSV format. Is there any prior work on this problem (libraries, papers, anything)? Matěj ---
**Author:** ![pf\_moore](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/pf_moore/32/35_2.png) [@pf\_moore](https://discuss.python.org/u/pf_moore)\ **Post date:** [May 7, 2025, 12:07pm UTC](https://discuss.python.org/t/csv-quote-strings-should-be-able-to-read-what-it-has-written/91035/3 "2025-05-07T12:07:47Z")
As @mcepl noted, CSV isn’t designed as a lossless format capable of round-tripping arbitrary language data types. If you want to propose a new Python-specific custom dialect that supports round tripping of a selected set of data types, that’s fine. But changing any of the existing dialects or behaviours is, IMO, unacceptable - they are all defined the way they are to support working with existing variants of the CSV format which exist in real-world data. ---
**Author:** ![storchaka](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/storchaka/32/217_2.png) [@storchaka](https://discuss.python.org/u/storchaka)\ **Post date:** [August 3, 2026, 8:23am UTC](https://discuss.python.org/t/csv-quote-strings-should-be-able-to-read-what-it-has-written/91035/4 "2026-08-03T08:23:52Z")
[Add converter and formatter parameters to csv.reader and csv.writer · Issue #155097 · python/cpython · GitHub](https://github.com/python/cpython/issues/155097) adds a `converter` parameter to `csv.reader` which replaces the hard-coded `float()`.