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====== Ora qui appunto sul tempo e python... ======

Parliamo della conversione tra Unix Timestamp e Date time con Timezone, in Django e in SQL.

Output di un codice di test nella piattaforma ''my-iot-framework'':

<code>
---
Testing timestamps datetime conversions..
UNIX TIMESTAMP ts = 1
---
[UNAWARE] fromtimestamp = dtunaware = datetime.datetime(1970, 1, 1, 1, 0, 1)
[UNAWARE] dtunaware.strftime("%s") = '1'
[UNAWARE] defaultfilters.date(dtunaware, "U") = '1'
[UNAWARE] EXTRACT EPOCH dtunaware_metrics[0]["ts"] = 3601
---
[AWARE] fromtimestamp.replace(tzinfo=UTC): dtaware = datetime.datetime(1970, 1, 1, 1, 0, 1, tzinfo=<UTC>)
[AWARE] dtaware.strftime("%s") = '1'
[AWARE] defaultfilters.date(dtaware, "U") = '3601'
[AWARE] EXTRACT EPOCH dtaware_metrics[0]["ts"] = 7201
---
[UTC] utcfromtimestamp: utcdt = datetime.datetime(1970, 1, 1, 0, 0, 1)
[UTC] utcdt.strftime("%s") = '-3599'
[UTC] defaultfilters.date(utcdt, "U") = '-3599'
[UTC] EXTRACT EPOCH utcdt_metrics[0]["ts"] = 1

</code>

Codice per riprodurre questo output nella shell Django ''manage.py shell'':

<code>
from datetime import datetime
import pytz

from django.template import defaultfilters
from django.db.models.functions import Extract

from web.models import Board
from web.models.metrics import get_board_metric_model

ts = 1
dtunaware = datetime.fromtimestamp(ts)
dtaware = datetime.fromtimestamp(ts).replace(tzinfo=pytz.UTC)
utcdt = datetime.utcfromtimestamp(ts)

board = Board.objects.first()
m = get_board_metric_model(board)
m.objects.create(time=utcdt, data={})
m.objects.create(time=dtaware, data={})
m.objects.create(time=dtunaware, data={})

dtunaware_metrics = m.objects.filter(time=dtunaware).annotate(ts=Extract("time", "epoch")).values()
dtaware_metrics = m.objects.filter(time=dtaware).annotate(ts=Extract("time", "epoch")).values()
utcdt_metrics = m.objects.filter(time=utcdt).annotate(ts=Extract("time", "epoch")).values()


print("---")
print(f"Testing timestamps datetime conversions..")
print(f"UNIX TIMESTAMP {ts = }")

print("---")
print(f"[UNAWARE] fromtimestamp = {dtunaware = }")
print(f'[UNAWARE] {dtunaware.strftime("%s") = }')
print(f'[UNAWARE] {defaultfilters.date(dtunaware, "U") = }')
print(f'[UNAWARE] EXTRACT EPOCH {dtunaware_metrics[0]["ts"] = }')

print("---")
print(f"[AWARE] fromtimestamp.replace(tzinfo=UTC): {dtaware = }")
print(f'[AWARE] {dtaware.strftime("%s") = }')
print(f'[AWARE] {defaultfilters.date(dtaware, "U") = }')
print(f'[AWARE] EXTRACT EPOCH {dtaware_metrics[0]["ts"] = }')

print("---")
print(f"[UTC] utcfromtimestamp: {utcdt = }")
print(f'[UTC] {utcdt.strftime("%s") = }')
print(f'[UTC] {defaultfilters.date(utcdt, "U") = }')
print(f'[UTC] EXTRACT EPOCH {utcdt_metrics[0]["ts"] = }')
</code>

Benedette siano le f-strings per il debug!