refactor: no mutating countries/timezones choices (ietf-tools#9037)

* refactor: don't mutate countries/timezones

* refactor: capitalize/make tuples to discourage mutation

Author: jennifer-richards

ietf/meeting/forms.py

@@ -22,7 +22,7 @@
 from ietf.doc.models import Document, State, NewRevisionDocEvent
 from ietf.group.models import Group
 from ietf.group.utils import groups_managed_by
-from ietf.meeting.models import Session, Meeting, Schedule, countries, timezones, TimeSlot, Room
+from ietf.meeting.models import Session, Meeting, Schedule, COUNTRIES, TIMEZONES, TimeSlot, Room
 from ietf.meeting.helpers import get_next_interim_number, make_materials_directories
 from ietf.meeting.helpers import is_interim_meeting_approved, get_next_agenda_name
 from ietf.message.models import Message
@@ -38,11 +38,6 @@
 from ietf.utils.validators import ( validate_file_size, validate_mime_type,
     validate_file_extension, validate_no_html_frame)
 
-# need to insert empty option for use in ChoiceField
-# countries.insert(0, ('', '-'*9 ))
-countries.insert(0, ('', '-' * 9))
-timezones.insert(0, ('', '-' * 9))
-
 # -------------------------------------------------
 # Helpers
 # -------------------------------------------------
@@ -140,12 +135,12 @@ class InterimMeetingModelForm(forms.ModelForm):
     approved = forms.BooleanField(required=False)
     city = forms.CharField(max_length=255, required=False)
     city.widget.attrs['placeholder'] = "City"
-    country = forms.ChoiceField(choices=countries, required=False)
+    country = forms.ChoiceField(choices=COUNTRIES, required=False)
     country.widget.attrs['class'] = "select2-field"
     country.widget.attrs['data-max-entries'] = 1
     country.widget.attrs['data-placeholder'] = "Country"
     country.widget.attrs['data-minimum-input-length'] = 0
-    time_zone = forms.ChoiceField(choices=timezones)
+    time_zone = forms.ChoiceField(choices=TIMEZONES)
     time_zone.widget.attrs['class'] = "select2-field"
     time_zone.widget.attrs['data-max-entries'] = 1
     time_zone.widget.attrs['data-minimum-input-length'] = 0

ietf/meeting/models.py

@@ -49,15 +49,20 @@
 )
 from ietf.utils.fields import MissingOkImageField
 
-countries = list(pytz.country_names.items())
-countries.sort(key=lambda x: x[1])
+# Set up countries / timezones, including an empty choice for fields
+EMPTY_CHOICE = ("", "-" * 9)
+COUNTRIES = (EMPTY_CHOICE,) + tuple(
+    sorted(pytz.country_names.items(), key=lambda x: x[1])
+)
 
-timezones = []
-for name in pytz.common_timezones:
-    tzfn = os.path.join(settings.TZDATA_ICS_PATH, name + ".ics")
-    if not os.path.islink(tzfn):
-        timezones.append((name, name))
-timezones.sort()
+_tzdata_ics_path = Path(settings.TZDATA_ICS_PATH)
+TIMEZONES = (EMPTY_CHOICE,) + tuple(
+    sorted(
+        (name, name)
+        for name in pytz.common_timezones
+        if not (_tzdata_ics_path / f"{name}.ics").is_symlink()
+    )
+)
 
 
 class Meeting(models.Model):
@@ -72,11 +77,11 @@ class Meeting(models.Model):
     days = models.IntegerField(default=7, null=False, validators=[MinValueValidator(1)],
         help_text="The number of days the meeting lasts")
     city = models.CharField(blank=True, max_length=255)
-    country = models.CharField(blank=True, max_length=2, choices=countries)
+    country = models.CharField(blank=True, max_length=2, choices=COUNTRIES)
     # We can't derive time-zone from country, as there are some that have
     # more than one timezone, and the pytz module doesn't provide timezone
     # lookup information for all relevant city/country combinations.
-    time_zone = models.CharField(max_length=255, choices=timezones, default='UTC')
+    time_zone = models.CharField(max_length=255, choices=TIMEZONES, default='UTC')
     idsubmit_cutoff_day_offset_00 = models.IntegerField(blank=True,
         default=settings.IDSUBMIT_DEFAULT_CUTOFF_DAY_OFFSET_00,
         help_text = "The number of days before the meeting start date when the submission of -00 drafts will be closed.")