python: Avoid importing the mock module in production.

These lazy imports save a significant amount of time on Zulip's core
import process, because mock imports pbr, which in turn import
pkgresources, which is in turn incredibly slow to import.

Fixes part of #9953.

zerver/views/auth.py

@@ -1,4 +1,3 @@
-from mock import patch
 from django.forms import Form
 from django.conf import settings
 from django.core.exceptions import ValidationError
@@ -591,6 +590,13 @@ class TwoFactorLoginView(BaseTwoFactorLoginView):
         realm.uri instead of '/'.
         """
         realm_uri = self.get_user().realm.uri
+        # This mock.patch business is an unpleasant hack that we'd
+        # ideally like to remove by instead patching the upstream
+        # module to support better configurability of the
+        # LOGIN_REDIRECT_URL setting.  But until then, it works.  We
+        # import mock.patch here because mock has an expensive import
+        # process involving pbr -> pkgresources (which is really slow).
+        from mock import patch
         with patch.object(settings, 'LOGIN_REDIRECT_URL', realm_uri):
             return super().done(form_list, **kwargs)
 

zproject/backends.py

@@ -1,5 +1,4 @@
 import logging
-import mock
 from typing import Any, Dict, List, Set, Tuple, Optional, Sequence
 
 from django_auth_ldap.backend import LDAPBackend, _LDAPUser
@@ -319,7 +318,13 @@ class ZulipLDAPAuthBackend(ZulipLDAPAuthBackendBase):
     REALM_IS_NONE_ERROR = 1
 
     def __init__(self) -> None:
-        if settings.DEVELOPMENT and settings.FAKE_LDAP_MODE:  # nocoverage # We only use this in development
+        if settings.DEVELOPMENT and settings.FAKE_LDAP_MODE:  # nocoverage
+            # We only use this in development.  Importing mock inside
+            # this function is an import time optimization, which
+            # avoids the expensive import of the mock module (slow
+            # because its dependency pbr uses pkgresources, which is
+            # really slow to import.)
+            import mock
             from fakeldap import MockLDAP
 
             ldap_patcher = mock.patch('django_auth_ldap.config.ldap.initialize')