Naive datetime is interpreted as UTC when set last_modified

[serhiy-storchaka]

When set the last_modified property of StreamResponse the naive datetime object is interpreted as UTC. But naive datetime objects are usually in local timezone. For example datetime.now() returns naive datetime objects in local timezone. It provokes bugs in user code.

For example:

$ cat test_last_modified2.py
from aiohttp.web import StreamResponse
from datetime import datetime, timezone

resp = StreamResponse()
dt = datetime.now(timezone.utc)
dt2 = datetime.now()
resp.last_modified = dt
print(resp.last_modified)
print(repr(resp.last_modified))
print(resp.headers["Last-Modified"])
resp.last_modified = dt2
print(resp.last_modified)
print(repr(resp.last_modified))
print(resp.headers["Last-Modified"])

$ python test_last_modified2.py
2020-12-02 10:06:47+00:00
datetime.datetime(2020, 12, 2, 10, 6, 47, tzinfo=datetime.timezone.utc)
Wed, 02 Dec 2020 10:06:47 GMT
2020-12-02 12:06:47+00:00
datetime.datetime(2020, 12, 2, 12, 6, 47, tzinfo=datetime.timezone.utc)
Wed, 02 Dec 2020 12:06:47 GMT

It would be better either raise an error if naive datetime object is passed or interpret it as local time.

[greshilov]

I can pick up this two issues from Serhiy, since I worked recently with those parts of code in my ETag PR (#5298).

[asvetlov]

Happy to review.
@greshilov I know I should review yours #5298 for ETags.
Now sure if I can do it on workdays but don't worry please -- I'll find time for the review on the weekend at least.