If you try to convert datetime objects to seconds since epoch and back
it will not work since the microseconds get lost:

>>> dt = datetime(2008, 5, 1, 13, 35, 41, 567777)
>>> seconds = mktime(dt.timetuple())
>>> datetime.fromtimestamp(seconds) == dt
False

Current fix is to do
>>> seconds += (dt.microsecond / 1000000.0)
>>> datetime.fromtimestamp(seconds) == dt
True

That's expected as mktime is just a thin wrapper over libc mktime() and
it does not expect microseconds. Changing time.mktime doesn't seems an
option, so the best alternative is to implement a method in datetime
type. Is there a real demand for C code implementing this to justify it?

Attached a patch which adds a .totimetuple(...) method to
datetime.datetime and tests for it.

The intention is that the dt.totimetuple(...) method is equivalent to:
mktime(dt.timetuple()) + (dt.microsecond / 1000000.0)

Patch adding documentation for datetime.totimestamp(...).

I think the name is not good, should be "toepoch" or something like that.

datetime has fromtimestamp already, so using totimestamp keeps naming
consistency (see toordinal and fromordinal).