csv does not round-trip for complex numbers

Reproduction depends on the quoting.

QUOTE_ALL

import csv
data = [1, 1j]
with open('example.csv', 'w') as f:
    writer = csv.writer(f, quoting=csv.QUOTE_ALL)
    writer.writerow(data)

with open('example.csv') as f:
    contents = f.read()
    print(contents)  # 1,1j
    reader = csv.reader(contents, quoting=csv.QUOTE_ALL)
    print(list(reader))  # [['1'], ['', ''], ['1'], ['j'], []]

In this case, we don't have our data back, but at least it does not raise.

QUOTE_NONNUMERIC

Qouting docs:

No automatic data type conversion is performed unless the QUOTE_NONNUMERIC format option is specified (in which case unquoted fields are transformed into floats)

import csv
data = [1, 1j]
with open('example.csv', 'w') as f:
    writer = csv.writer(f, quoting=csv.QUOTE_NONNUMERIC)
    writer.writerow(data)

with open('example.csv') as f:
    contents = f.read()
    print(contents)  # 1,1j
    reader = csv.reader(contents, quoting=csv.QUOTE_NONNUMERIC)
    print(list(reader))

# Traceback (most recent call last):
#   File "/Users/sobolev/Desktop/cpython/ex.py", line 11, in <module>
#     print(list(reader))
#           ^^^^^^^^^^^^
# ValueError: could not convert string to float: 'j'

In this case, it raises an error, while trying to convert 1j to float.

Current docs / tests

I cannot find any mentions of complex numbers in tests or docs for csv.

Solutions?

We can say that it works as expected: with this strange QUOTE_ALL formatting and QUOTE_NONNUMERIC exception. Add a test case for it and forget about it
We can try to quote complex as string: in this case it will be treated as "1j". I think it is much better, because it will allow users to convert this value to complex manually
Forbid writting complex numbers. However, I don't think it is a path we should go

I suggest you first fix your bug. Give csv.reader the file object, not the file contents as a string.

Those two programs are identical. You're not using QUOTE_NONNUMERIC.

@pochmann

@pochmann thanks!

I suggest you first fix your bug. Give csv.reader the file object, not the file contents as a string.

It does not really matter. See:

» ./python.exe
Python 3.12.0a0 (heads/main:ff173ed2f6, Oct 20 2022, 14:06:56) [Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import csv
>>> data = [1, 1j]
>>> with open('example.csv', 'w') as f:
...     writer = csv.writer(f, quoting=csv.QUOTE_NONNUMERIC)
...     writer.writerow(data)
... 
6
>>> with open('example.csv') as f:
...     reader = csv.reader(f, quoting=csv.QUOTE_NONNUMERIC)
...     print(list(reader))
... 
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
ValueError: could not convert string to float: '1j'

Those two programs are identical. You're not using QUOTE_NONNUMERIC.

This was a copy-paste error, updated.

Only addressing the first example, with quoting=csv.QUOTE_ALL or with default quoting, I do get the original data back as expected (on latest dev version):

with open('a.csv') as f:
    reader = csv.reader(f, quoting=csv.QUOTE_ALL)
    for r in reader:
        print(r)
# ['1', '1j']
# ['2', '2j']

So with complex numbers you would need to complex(r[1]) when reading data.

If there was no way to get the original data back, of course it would be more alarming.

@sobolevn

@sobolevn, this doesn't work for floats as well. Or did I miss something?

>>> import csv
... data = [3.14]
... with open('example.csv', 'w') as f:
...     writer = csv.writer(f, quoting=csv.QUOTE_NONNUMERIC)
...     writer.writerow(data)
...     
6
>>> with open('example.csv') as f:
...     contents = f.read()
...     print(contents)
...     reader = csv.reader(contents, quoting=csv.QUOTE_NONNUMERIC)
...     print(list(reader))
...     
3.14

Traceback (most recent call last):
  File "<python-input-1>", line 5, in <module>
    print(list(reader))
          ~~~~^^^^^^^^
ValueError: could not convert string to float: '.'

gh-155097 (PR #155099) adds a converter parameter to csv.reader which replaces the hard-coded float(). Since str(1j) is already written unquoted, complex numbers round-trip with:

csv.reader(f, quoting=csv.QUOTE_NONNUMERIC, converter=lambda i, s: complex(s))

The converter is called as converter(index, field), so converter=complex does not work — the index would be taken as the real part and the field as imag.

The QUOTE_ALL half of the report is unrelated and works as documented: in that mode the reader converts nothing, so every field is returned as a string.

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`csv` does not round-trip for `complex` numbers #98485

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csv does not round-trip for complex numbers #98485

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QUOTE_ALL

QUOTE_NONNUMERIC

Current docs / tests

Solutions?

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pochmann commented on Oct 20, 2022

pochmann commented on Oct 20, 2022

sobolevn commented on Oct 20, 2022

akulakov commented on Nov 9, 2022

skirpichev commented on Mar 25, 2025

serhiy-storchaka commented on Aug 3, 2026

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`csv` does not round-trip for `complex` numbers #98485