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merged 4 commits into from
Apr 19, 2024
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Avoid using floating points during timestamp-datetime conversions #591

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e92f68e
avoid using float to convert timestamp from and to datetime
hakanakyurek Apr 18, 2024
12d9cc0
removed floor division in from_datetime
hakanakyurek Apr 19, 2024
64c0e67
removed rounding for sub-second precision in to_datetime
hakanakyurek Apr 19, 2024
34ab273
1e3/10**3 --> 1000
hakanakyurek Apr 19, 2024
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6 changes: 4 additions & 2 deletions msgpack/ext.py
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Original file line number Diff line number Diff line change
Expand Up @@ -157,12 +157,14 @@ def to_datetime(self):
:rtype: `datetime.datetime`
"""
utc = datetime.timezone.utc
return datetime.datetime.fromtimestamp(0, utc) + datetime.timedelta(seconds=self.to_unix())
return datetime.datetime.fromtimestamp(0, utc) + datetime.timedelta(
seconds=self.seconds, microseconds=round(self.nanoseconds / 1e3)
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Just use 1000 instead of 1e3?

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I prefer // over round.

Consider "12:34:56.78". It is "12:34" or "12:34:56", not "12:35" nor "12:34:57".

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Just use 1000 instead of 1e3?

same as the previous comment, I just though "10**x" or "1ex" was the convention that was being used.

I prefer // over round.

Consider "12:34:56.78". It is "12:34" or "12:34:56", not "12:35" nor "12:34:57".

I'm not actually knowledgeable about whether there should be a round or not. The idea I had with the round is when working with a timestamp with nanosecond precision, lets say, "100,000,900" nanoseconds, I thought it should round to "100,001" microseconds, whereas "100,000,100" nanoseconds should be "100,000" microseconds.

With the case "12:34:56.78", I'm not sure whether it should "12:34:56" or "12:34:57", if rounding isn't the norm in these cases, we should definitely remove it and the tests related to it.

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The idea I had with the round is when working with a timestamp.

The year of 2024-10-01 is 2025?
The day of 2024-10-01 15:00:00 is 2024-10-02?
The hour of 15:34 is 16?

Despite what you explain it in natural language, "round down" is consistent in programming.

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https://github.com/golang/go/blob/a63907808d14679c723e566cb83acc76fc8cafc2/src/time/time.go#L1214-L1244

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https://docs.oracle.com/en/java/javase/22/docs/api/java.base/java/time/Instant.html#toEpochMilli()

If this instant has greater than millisecond precision, then the conversion will drop any excess precision information as though the amount in nanoseconds was subject to integer division by one million.

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hmm i see, thanks a lot it makes more sense. Then i'll remove the rounding and tests related to it.

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2023-12-31 23:59:59.999999500 -- year 2023, date 2023-12-31.

After round half by micros:

2024-01-01 00:00:00.000000 -- year 2024, date 2024-01-01.

This is why most languages uses consistent "round down" for subseconds.
Consistency is important to avoid bugs. Every programmer should love consistency.

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yup of course, I didn't look at it like that when I was proposing the change. Again it makes more sense to round down instead as it would be more consistant as you said. Thanks for the explanation!

)

@staticmethod
def from_datetime(dt):
"""Create a Timestamp from datetime with tzinfo.

:rtype: Timestamp
"""
return Timestamp.from_unix(dt.timestamp())
return Timestamp(seconds=int(dt.timestamp() // 1), nanoseconds=dt.microsecond * 10**3)
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That x // 1 feels strange.

Considering you use int() after that anyway, why do you need that?

Also: just use 1000 instead of 10**3?

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That x // 1 feels strange.

Considering you use int() after that anyway, why do you need that?

Yes, i overlooked that part completely, we dont need the floor division at all. I'll remove it.

Also: just use 1000 instead of 10**3?

we can of course use 1000 instead, its just that in the code that i checked, i either saw "10**x" or "1ex" and wanted follow that convention.

18 changes: 18 additions & 0 deletions test/test_timestamp.py
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Expand Up @@ -86,6 +86,24 @@ def test_timestamp_datetime():
utc = datetime.timezone.utc
assert t.to_datetime() == datetime.datetime(1970, 1, 1, 0, 0, 42, 0, tzinfo=utc)

ts = datetime.datetime(2024, 4, 16, 8, 43, 9, 420317, tzinfo=utc)
ts2 = datetime.datetime(2024, 4, 16, 8, 43, 9, 420318, tzinfo=utc)

assert Timestamp.from_datetime(ts2).nanoseconds - Timestamp.from_datetime(ts).nanoseconds == 1e3
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Suggested change
assert Timestamp.from_datetime(ts2).nanoseconds - Timestamp.from_datetime(ts).nanoseconds == 1e3
assert Timestamp.from_datetime(ts2).nanoseconds - Timestamp.from_datetime(ts).nanoseconds == 1000


ts3 = datetime.datetime(2024, 4, 16, 8, 43, 9, 4256)
ts4 = datetime.datetime(2024, 4, 16, 8, 43, 9, 4257)
assert (
Timestamp.from_datetime(ts4).nanoseconds - Timestamp.from_datetime(ts3).nanoseconds == 1e3
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Suggested change
Timestamp.from_datetime(ts4).nanoseconds - Timestamp.from_datetime(ts3).nanoseconds == 1e3
Timestamp.from_datetime(ts4).nanoseconds - Timestamp.from_datetime(ts3).nanoseconds == 1000

)

assert Timestamp.from_datetime(ts).to_datetime() == ts

t2 = Timestamp(1713256989, 420318123)
t3 = Timestamp(1713256989, 420318499)
t4 = Timestamp(1713256989, 420318501)
assert t2.to_datetime() == t3.to_datetime() != t4.to_datetime()
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Maybe just using 2 asserts would be simpler.

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definitely, we dont necessarily need to compare with t2 now i think of it. But these were to test the rounding specifically, if rounding is wrong way to go, these tests arent necessarily needed imo.



def test_unpack_datetime():
t = Timestamp(42, 14)
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