Data loss of microseconds on MySQL and SQLite

[Adam Johnson]

Django provides django.db.backends.utils.typecast_time as a converter function for MySQL and SQLite. This function uses floating point logic which unfortunately causes loss of precision on some values, for example:

In [1]: from django.db.backends.utils import typecast_time

In [2]: typecast_time('12:45:30.126631')  # okay
Out[2]: datetime.time(12, 45, 30, 126631)

In [3]: typecast_time('12:45:30.126630')   # woops
Out[3]: datetime.time(12, 45, 30, 126629)

This will probably only cause loss of a single microsecond, but it's annoying that the database doesn't roundtrip values.

[Łukasz Więcek]

​https://github.com/django/django/pull/6450

[Tim Graham]

Should this be backported? Typically that's what we do for data loss issues.

[Adam Johnson]

I'm hoping so, we've had to patchy.patch this in

[Tim Graham <timograham@…>]

In d3c87a24:

Fixed #26498 -- Fixed TimeField microseconds round-tripping on MySQL and SQLite.

Thanks adamchainz for the report and review.

[Tim Graham <timograham@…>]

In 0a411b22:

[1.8.x] Fixed #26498 -- Fixed TimeField microseconds round-tripping on MySQL and SQLite.

Thanks adamchainz for the report and review.

Backport of d3c87a2425b30400c3e6ea76585a9a537b6d0386 from master

[Tim Graham <timograham@…>]

In 51400cb:

[1.9.x] Fixed #26498 -- Fixed TimeField microseconds round-tripping on MySQL and SQLite.

Thanks adamchainz for the report and review.

Backport of d3c87a2425b30400c3e6ea76585a9a537b6d0386 from master