Lazy load form field error message translation

Follow pattern from Django form fields. Since fields are loaded at
import time, prevents calling into the translation machinery before
Django has completed its setup.

Thanks traviselam for reporting.

Reviewed-by: th3hamm0r
Reviewed-by: stefanfoulis

CHANGELOG.rst

@@ -4,10 +4,13 @@ CHANGELOG
 UNRELEASED
 ----------
 
+* Lazy load ``formfields.PhoneNumberField`` translation for ``invalid`` data.
+
 **Backwards incompatible changes**
 
 * Drop support for end-of-life Django 3.0
 
+
 5.1.0 (2021-04-02)
 ------------------
 

phonenumber_field/formfields.py

@@ -2,7 +2,8 @@
 from django.core import validators
 from django.core.exceptions import ValidationError
 from django.forms.fields import CharField
-from django.utils.translation import gettext as _
+from django.utils.text import format_lazy
+from django.utils.translation import gettext_lazy as _
 
 from phonenumber_field.phonenumber import to_python, validate_region
 from phonenumber_field.validators import validate_international_phonenumber
@@ -31,8 +32,8 @@ def __init__(self, *args, region=None, **kwargs):
                 example_number = "+12125552368"  # Ghostbusters
                 # Translators: {example_number} is an international phone number.
                 error_message = _("Enter a valid phone number (e.g. {example_number}).")
-            self.error_messages["invalid"] = error_message.format(
-                example_number=example_number
+            self.error_messages["invalid"] = format_lazy(
+                error_message, example_number=example_number
             )
 
     def to_python(self, value):

tests/test_formfields.py

@@ -1,5 +1,8 @@
+from unittest import mock
+
 from django import forms
 from django.test import SimpleTestCase
+from django.utils.functional import lazy
 
 from phonenumber_field.formfields import PhoneNumberField
 
@@ -45,3 +48,13 @@ class PhoneNumberForm(forms.Form):
         form = PhoneNumberForm({"number": ALGERIAN_PHONE_NUMBER})
         self.assertTrue(form.is_valid())
         self.assertEqual(ALGERIAN_PHONE_NUMBER, form.cleaned_data["number"])
+
+    def test_error_message_lazy(self):
+        def fail_gettext(msgid):
+            raise Exception("gettext was called unexpectedly.")
+
+        with mock.patch(
+            "phonenumber_field.formfields._",
+            side_effect=lazy(fail_gettext, str),
+        ):
+            PhoneNumberField()