FileSystemStorage no longer accepts reverse_lazy as base_url

[tpazderka]

Changes introduced with commit [fb9d8f06] fixing #22717 causes FileSystemStorage to evaluate the content of base_url uppon initialization.
This causes issues if the base_url is defined as reverse_lazy as it is resolved and can cause ImportError.

I think that the laziness of the base_url should be preserved. Either the check for the trailing slash can me moved to path generation or the __init__ can be updated to expect reverse_lazy as base_url.

[Tim Graham]

Could you provide a test for tests/file_storage/tests.py that demonstrates your use case?

[tpazderka]

Patch for tests/file_storage/tests.py

[tpazderka]

Not exactly our usecase as our failure is on ImportError which seems hard to test, but I hope that this patch shows the issue we are having.

The basic idea is that reverse_lazy should remain lazy as long as possible. Resolving the base_url on init() defeats the purpose of using reverse_lazy there.

[Tim Graham]

Should we skip that check if base_url is lazy or do you see some alternative?

[Claude Paroz]

We should explore making base_url a descriptor, that might help delaying the check.

[tpazderka]

I have created a pull request ​https://github.com/django/django/pull/6989 which omitts the check if the base_url is not of string_types.

[Tim Graham <timograham@…>]

In b820b61:

Fixed #26896 -- Allowed a lazy base_url for FileSystemStorage.

[Tim Graham <timograham@…>]

In d61dbc2:

[1.10.x] Fixed #26896 -- Allowed a lazy base_url for FileSystemStorage.

Backport of b820b6108a7d3f11ec18774d16d657f4f63fe9fa from master