write_csv ignores formatting when writing to io.StringIO()

[bskubi]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

from polars import DataFrame
import io

df = DataFrame({"c1":[1, 2], "c2":[3, 4]})
buffer = io.StringIO()
df.write_csv(buffer, separator="\t", include_header=False)
print(buffer.getvalue())

Expected output:

1      3
2      4

Wrong output:

c1,c2
1,3
2,4

Log output

No response

Issue description

I'm trying to obtain a Python string containing the output that would be written by the write_csv function.

Expected behavior

The code sample above should print a string that is tab-delimited and has no header, per the options I specify with write_csv.

Installed versions

--------Version info---------
Polars:              1.7.1
Index type:          UInt32
Platform:            Linux-6.8.0-40-generic-x86_64-with-glibc2.35
Python:              3.12.3 | packaged by conda-forge | (main, Apr 15 2024, 18:38:13) [GCC 12.3.0]

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
cloudpickle          3.0.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2024.6.1
gevent               <not installed>
great_tables         <not installed>
matplotlib           3.9.1
nest_asyncio         1.6.0
numpy                1.26.4
openpyxl             <not installed>
pandas               2.2.2
pyarrow              16.1.0
pydantic             2.8.2
pyiceberg            <not installed>
sqlalchemy           2.0.34
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>

[cmdlineluser]

Can reproduce.

This just seems to be an issue in the Python logic.

.write_csv(buf) on line 2862 doesn't take user params into consideration.

polars/py-polars/polars/dataframe/frame.py

Lines 2860 to 2871 in 9384945

	def write_csv_to_string() -> str:
	with BytesIO() as buf:
	self.write_csv(buf)
	csv_bytes = buf.getvalue()
	return csv_bytes.decode("utf8")
	should_return_buffer = False
	if file is None:
	buffer = file = BytesIO()
	should_return_buffer = True
	elif isinstance(file, StringIO):
	csv_str = write_csv_to_string()

[mcrumiller]

Maybe we should rename the parameter to source_schema? This issue seems to pop up quite often.