File(allow_none=True) is not possible

[ddorian]

Hi team,

Is there interest in making File(allow_none=True)?
This would allow to send None to file and it being validated as valid.
The use case is to delete the file when editing a resource (set it to None to delete it).
I can make pull request.

[sloria]

I think adding allow_none to File makes sense for API consistency with other fields 👍 PR welcome!

[ddorian]

An issue would be when you upload files using multipart/form-data.
There is no None/null value in that scenario possible.
Maybe an empty string for that case?

[sloria]

Oh, right, that's probably why File doesn't support allow_none in the first place 🤔
I'm not sure an empty string makes sense as a null value for this case.

@uncle-lv any thoughts on this?

[uncle-lv]

An issue would be when you upload files using multipart/form-data. There is no None/null value in that scenario possible. Maybe an empty string for that case?

Yes, we have handled it.

[uncle-lv]

There is an interesting thing.

If no file is uploaded, the content is empty:

-----------------------------267432055312135418634201536635
Content-Disposition: form-data; name="file"


-----------------------------267432055312135418634201536635--

If an empty file is uploaded(such as an empty text file), the content is also empty, but there is a filename:

-----------------------------497031844591475693105063701
Content-Disposition: form-data; name="file"; filename="empty.txt"
Content-Type: text/plain


-----------------------------497031844591475693105063701--

It seems that we cannot judge whether a file is uploaded by content.
Maybe filename is a better choice?

[uncle-lv]

If no filename, data will be deserialized as a str whatever content-type is.
If filename exists, data will be deserialized as a FileStorage whatever content-type is.

I think we can handle it according to the following rules:
no filename and empty content(an empty str) → None/null(to be compatible with swagger)
filename exists and empty content(an empty FileStorage) → Not None/null and a valid file

[sloria]

@uncle-lv seems like a sensible proposal to me. is it straightforward to validate against filename within the field?

[uncle-lv]

@uncle-lv seems like a sensible proposal to me. is it straightforward to validate against filename within the field?

No need to validate filename. If no filename, the value will be a str, otherwise it'll be a FileStorage.

[sloria]

oh i see. that certainly makes it simpler then.

PRs welcome!

[uncle-lv]

In the end-to-end case, it's difficult to whether determine the file is None or missing.

Browser(Firefox/Chrome) will send send empty data with empty filename when no file is selected in HTML.

-----------------------------267432055312135418634201536635
Content-Disposition: form-data; name="file"; filename=""
Content-Type: application/octet-stream


-----------------------------267432055312135418634201536635--

Swagger will send send empty data without filename when no file is selected.

-----------------------------267432055312135418634201536635
Content-Disposition: form-data; name="file"


-----------------------------267432055312135418634201536635--

Axios will send a string with value "null" when the file field of FormData is set null.

-----------------------------267432055312135418634201536635
Content-Disposition: form-data; name="file"

null
-----------------------------267432055312135418634201536635--

I have no idea to handle it.🤯

[ddorian]

My idea was to use it on a REST API, so I only needed to support 1 way.

Or you can support all ways if considering to be used from a browser.