[INTERNAL] Django upgrade fixes #515

In #495 ( 06a2c20 ) we upgraded Django. This means that Django itself doesn't depend on pytz, but we still had a few instances of our code requiring it. Instead of making pytz a dependency of Promgen, we can refactor a few bits to remove the dependency.

promgen/prometheus.py

@@ -8,7 +8,6 @@
 import tempfile
 from urllib.parse import urljoin
 
-import pytz
 import yaml
 from dateutil import parser
 from django.core.exceptions import ValidationError
@@ -264,6 +263,9 @@ def silence(*, labels, duration=None, **kwargs):
     Post a silence message to Alert Manager
     Duration should be sent in a format like 1m 2h 1d etc
     """
+    # We active a timezone here, because the frontend (browser)
+    # will be POSTing date times without timezones
+    timezone.activate(util.setting("timezone", "UTC"))
 
     if duration:
         start = timezone.now()
@@ -278,9 +280,11 @@ def silence(*, labels, duration=None, **kwargs):
         kwargs["startsAt"] = start.isoformat()
         kwargs["endsAt"] = end.isoformat()
     else:
-        local_timezone = pytz.timezone(util.setting("timezone", "UTC"))
         for key in ["startsAt", "endsAt"]:
-            kwargs[key] = local_timezone.localize(parser.parse(kwargs[key])).isoformat()
+            dt = parser.parse(kwargs[key])
+            if timezone.is_naive(dt):
+                dt = timezone.make_aware(dt)
+            kwargs[key] = dt.isoformat()
 
     kwargs["matchers"] = [
         {

promgen/templatetags/promgen.py

@@ -10,12 +10,10 @@
 import yaml
 from django import template
 from django.urls import reverse
+from django.utils import timezone
 from django.utils.html import format_html
 from django.utils.safestring import mark_safe
 from django.utils.translation import gettext as _
-from pytz import timezone
-
-from promgen import util
 
 register = template.Library()
 
@@ -98,9 +96,16 @@ def pretty_yaml(data):
 
 @register.filter()
 def strftime(timestamp, fmt):
-    tz = util.setting("timezone", "UTC")
+    """
+    Convert a timestamp to a specific format.
+
+    Normally we would use the built in django `time` filters, but sometimes we
+    get a numeric value as a timestamp (from Prometheus results) so we want a
+    single place where we can convert any type of value to a timestamp.
+    """
     if isinstance(timestamp, int) or isinstance(timestamp, float):
-        return timezone(tz).localize(datetime.fromtimestamp(timestamp)).strftime(fmt)
+        dt = datetime.fromtimestamp(timestamp, timezone.utc)
+        return timezone.localtime(dt).strftime(fmt)
     return timestamp