Feature Request: add to df.write_ndjson(json_columns: list[str]), for columns to be decoded and written out as JSON

[DeflateAwning]

Description

We store a column as geojson in a string in a dataframe. It would be really nice to be able to write this column out as json in the outputted JSON. Very niche use case, but all the alternative hacks are pretty awful.

[cmdlineluser]

Is the problem that you have something like:

df = pl.select(a=1,b=2,c=pl.lit('{"foo":[1, 2, 3],"bar":[4,5,6]}'))

df.select(pl.col.c.str.json_decode()).write_ndjson()

# '{"c":{"foo":[1,2,3],"bar":[4,5,6]}}\n'

But you need the data written without the outer column name label?

# {"foo":[1,2,3],"bar":[4,5,6]}\n'

[DeflateAwning]

Here are some examples that hopefully help better explain my issue with that.

The issue is that null values are outputed as "geometry":{"type":null,"coordinates":null,"features":null} instead of just null.

# Issue #17054
import polars as pl
import json
import tempfile

big_geojson_obj = {"type":"FeatureCollection","features":[{"id":"baddba6f1276e861263d05d9cbecff74","type":"Feature","properties":{"lineColor":"#ffa000","lineWidth":2,"fillColor":"#ffe082","fillOpacity":0.1},"geometry":{"coordinates":[[[-114.42286807,55.199035275],[-118.90384586,53.413681626],[-115.7853142,51.95024781],[-111.63559015,53.23660491],[-114.42286807,55.199035275]]],"type":"Polygon"}}]}

df = pl.DataFrame({
    "id": [1, 2, 3, 4],
    "name": ["Location1", "Location2", "LocationWithLongGeom", "LocationWithNullGeom"],
    "geometry": [
        '{"type":"Point","coordinates":[102.0,0.5]}',
        '{"type":"Point","coordinates":[103.0,1.0]}',
        json.dumps(big_geojson_obj),
        None
    ]
})

print("================ START BASIC WAY ================")
# Just output the column as a String (not what I want)
with tempfile.NamedTemporaryFile(suffix=".ndjson") as f:
	df.write_ndjson(f.name)
	f.seek(0)
	print(f.read().decode())
print("================ END BASIC WAY ================")

print("================ START DEMO GOAL ================")
# Obviously this way is very slow
for row in df.iter_rows(named=True):
	row_out = row.copy()
	row_out["geometry"] = json.loads(row_out["geometry"]) if row_out["geometry"] is not None else None
	print(json.dumps(row_out))  # fill write would happen here
print("================ END DEMO GOAL ================")

print("================ START SUGGESTION 1 ================")
# This is the previous suggestion.
# The issue is that null values are outputed as `"geometry":{"type":null,"coordinates":null,"features":null}` instead of just `null`.
df1 = df.with_columns(pl.col('geometry').str.json_decode())
with tempfile.NamedTemporaryFile(suffix=".ndjson") as f:
	df1.write_ndjson(f.name)
	f.seek(0)
	print(f.read().decode())
print("================ END SUGGESTION 1 ================")

================ START BASIC WAY ================
{"id":1,"name":"Location1","geometry":"{\"type\":\"Point\",\"coordinates\":[102.0,0.5]}"}
{"id":2,"name":"Location2","geometry":"{\"type\":\"Point\",\"coordinates\":[103.0,1.0]}"}
{"id":3,"name":"LocationWithLongGeom","geometry":"{\"type\": \"FeatureCollection\", \"features\": [{\"id\": \"baddba6f1276e861263d05d9cbecff74\", \"type\": \"Feature\", \"properties\": {\"lineColor\": \"#ffa000\", \"lineWidth\": 2, \"fillColor\": \"#ffe082\", \"fillOpacity\": 0.1}, \"geometry\": {\"coordinates\": [[[-114.42286807, 55.199035275], [-118.90384586, 53.413681626], [-115.7853142, 51.95024781], [-111.63559015, 53.23660491], [-114.42286807, 55.199035275]]], \"type\": \"Polygon\"}}]}"}
{"id":4,"name":"LocationWithNullGeom","geometry":null}

================ END BASIC WAY ================
================ START DEMO GOAL ================
{"id": 1, "name": "Location1", "geometry": {"type": "Point", "coordinates": [102.0, 0.5]}}
{"id": 2, "name": "Location2", "geometry": {"type": "Point", "coordinates": [103.0, 1.0]}}
{"id": 3, "name": "LocationWithLongGeom", "geometry": {"type": "FeatureCollection", "features": [{"id": "baddba6f1276e861263d05d9cbecff74", "type": "Feature", "properties": {"lineColor": "#ffa000", "lineWidth": 2, "fillColor": "#ffe082", "fillOpacity": 0.1}, "geometry": {"coordinates": [[[-114.42286807, 55.199035275], [-118.90384586, 53.413681626], [-115.7853142, 51.95024781], [-111.63559015, 53.23660491], [-114.42286807, 55.199035275]]], "type": "Polygon"}}]}}
{"id": 4, "name": "LocationWithNullGeom", "geometry": null}
================ END DEMO GOAL ================
================ START SUGGESTION 1 ================
{"id":1,"name":"Location1","geometry":{"type":"Point","coordinates":[102.0,0.5],"features":null}}
{"id":2,"name":"Location2","geometry":{"type":"Point","coordinates":[103.0,1.0],"features":null}}
{"id":3,"name":"LocationWithLongGeom","geometry":{"type":"FeatureCollection","coordinates":null,"features":[{"id":"baddba6f1276e861263d05d9cbecff74","type":"Feature","properties":{"lineColor":"#ffa000","lineWidth":2,"fillColor":"#ffe082","fillOpacity":0.1},"geometry":{"coordinates":[[[-114.42286807,55.199035275],[-118.90384586,53.413681626],[-115.7853142,51.95024781],[-111.63559015,53.23660491],[-114.42286807,55.199035275]]],"type":"Polygon"}}]}}
{"id":4,"name":"LocationWithNullGeom","geometry":{"type":null,"coordinates":null,"features":null}}

================ END SUGGESTION 1 ================

[cjackal]

The issue is that null values are outputed as "geometry":{"type":null,"coordinates":null,"features":null} instead of just null.

Well this is one of the oldest feature request in polars project, #3462. Your geometry column has dtype Struct[Utf8, Utf8] and currently polars Struct dtype has no validity buffer, so any null in Struct[Utf8,Utf8] is represented as {null, null}.

For an workaround one must venture outside polars (e.g. move to pyarrow and manually replace {null, null} with null), which is precisely what geojson spec requires...

My personal thinking is that #3462 should be marked P-high and 1.0 todo list, current implementation of Struct dtype in polars makes it hard to inter-op with other dataframe libraries due to its inconsistency with standard arrow spec.