Pandas dataframe to json list format - json

Question

I have large pandas tabular dataframe to convert into JSON.
The standard .to_json() functions does not make a compact format for JSON.
How to get JSON output forma like this, using pandas only ?
{"index": [ 0, 1 ,3 ],
"col1": [ "250", "1" ,"3" ],
"col2": [ "250", "1" ,"3" ]
}
This is a much compact format form of JSON for tabular data.
(I can do a loop over the rows.... but)

Answer 1

It seems you need to_dict first and then dict to json:
df = pd.DataFrame({"index": [ 0, 1 ,3 ],
"col1": [ "250", "1" ,"3" ],
"col2": [ "250", "1" ,"3" ]
})
print (df)
col1 col2 index
0 250 250 0
1 1 1 1
2 3 3 3
print (df.to_dict(orient='list'))
{'col1': ['250', '1', '3'], 'col2': ['250', '1', '3'], 'index': [0, 1, 3]}
import json
print (json.dumps(df.to_dict(orient='list')))
{"col1": ["250", "1", "3"], "col2": ["250", "1", "3"], "index": [0, 1, 3]}
Because it is not implemented yet:
print (df.to_json(orient='list'))
ValueError: Invalid value 'list' for option 'orient'
EDIT:
If index is not column, add reset_index:
df = pd.DataFrame({"col1": [250, 1, 3],
"col2": [250, 1, 3]})
print (df)
col1 col2
0 250 250
1 1 1
2 3 3
print (df.reset_index().to_dict(orient='list'))
{'col1': [250, 1, 3], 'index': [0, 1, 2], 'col2': [250, 1, 3]}

Answer 2

You can use to_dict and json (and add the index as extra column if required via assign):
import json
df = pd.DataFrame({"col1": [250, 1, 3],
"col2": [250, 1, 3]})
json_dict = df.assign(index=df.index).to_dict(orient="list")
print(json.dumps(json_dict))
>>> '{"index": [0, 1, 2], "col1": [250, 1, 3], "col2": [250, 1, 3]}'