v1.1.0 datatable hidden_columns/exporting loses hidden headers when export_header='names'

[laoxu0212]

Hi there,

hidden_columns in 1.1.0 has really helped me a lot, however, here comes the question with exporting:

After using hidden_columns to hide some of the columns, I need to export data(no matter what is displayed after hiding several columns or the whole table).

It seems that the data of hidden columns will be exported anyway but the headers of them will not if I set export_headers='names'. It just leaves several data without headers.

I think the "ids" or the "names" should be same for the headers but the content, therefore, could I believe that there might be something wrong?

Here is a demo code to show that problem:

`import dash
import dash_table

app = dash.Dash(name)

app.layout = dash_table.DataTable(
id='table',
columns=[{"name": i, "id": i} for i in ['a', 'b', 'c', 'd']],
data=[{'a':1, 'b':1, 'c':1, 'd':1}, {'a':1, 'b':1, 'c':1, 'd':1},{'a':1, 'b':1, 'c':1, 'd':1},{'a':1, 'b':1, 'c':1, 'd':1}],
hidden_columns=['d'],
export_format='csv',
export_headers='names'
)

if name == 'main':
app.run_server(debug=True)`

And the csv looks like:

[Marc-Andre-Rivet]

Fixing the default behavior to use visible columns for both data / headers.

A new prop export_columns will be added to allow exporting all the columns if desired. Value can be all or visible.

[laoxu0212]

Thanks for your enhancement on #592
I think I might not have clarified the bug:

Currently all the columns will be exported.
If you choose export_headers='names' , only visible columns' headers will be shown.
This leaves some columns in CSV without headers.

If you choose export_headers='ids', all columns' headers will be shown.

"export_columns" mentioned in #592 (comment) is a great enhancement. But I'm afraid that if we don't set this property, we may still see some data columns without headers.

Here we got:
columns=[{"name": 'column-' + i, "id": 'column-' + i} for i in ['a', 'b', 'c', 'd']],
data=[{'column-a': 'a', 'column-b': 'b', 'column-c': 'c', 'column-d': 'd'}] * 3,
hidden_columns=['column-c'],
export_format='csv',
export_headers='names'

But here we got:
columns=[{"name": 'column-' + i, "id": 'column-' + i} for i in ['a', 'b', 'c', 'd']],
data=[{'column-a': 'a', 'column-b': 'b', 'column-c': 'c', 'column-d': 'd'}] * 3,
hidden_columns=['column-c'],
export_format='csv',
export_headers='ids'

At least I believe leaving columns without hears in exported file is not correct.

[Marc-Andre-Rivet]

@laoxu0212 The changes in #592 should cover your current use case and be available in the v1.4.0 release. Sorry for not making that clear in my previous comment. With the fix, the columns and headers will match as expected, whether there are hidden columns or not. The additional prop is there to decide whether you want to export the hidden columns or not as both scenarios are considered valid.