Add a custom template filter for formatting displaying lists of items in emails.

We do this operation several times in the digest e-mails.

(imported from commit 7b906bad582c628d4461637dc7af374df349e09a)

zerver/lib/digest.py

@@ -18,12 +18,6 @@ from zerver.models import UserProfile, UserMessage, Recipient, Stream, \
 # 4. Interesting stream traffic, as determined by the longest and most
 #    diversely comment upon topics.
 
-def and_n_others(things, limit):
-    # A helper for the commonly appended "and N other(s)" string, with
-    # the appropriate pluralization.
-    return " and %d other%s" % (len(things) - limit,
-                                "" if len(things) == limit + 1 else "s")
-
 def gather_hot_conversations(user_profile, stream_messages):
     # Gather stream conversations of 2 types:
     # 1. long conversations

zerver/templatetags/app_filters.py

@@ -0,0 +1,38 @@
+from django.template import Library
+
+register = Library()
+
+def and_n_others(values, limit):
+    # A helper for the commonly appended "and N other(s)" string, with
+    # the appropriate pluralization.
+    return " and %d other%s" % (len(values) - limit,
+                                "" if len(values) == limit + 1 else "s")
+
+@register.filter(name='display_list', is_safe=True)
+def display_list(values, display_limit):
+    """
+    Given a list of values, return a string nicely formatting those values,
+    summarizing when you have more than `display_limit`. Eg, for a
+    `display_limit` of 3 we get the following possible cases:
+
+    Jessica
+    Jessica and Waseem
+    Jessica, Waseem, and Tim
+    Jessica, Waseem, Tim, and 1 other
+    Jessica, Waseem, Tim, and 2 others
+    """
+    if len(values) == 1:
+        # One value, show it.
+        display_string = "%s" % (values[0],)
+    elif len(values) <= display_limit:
+        # Fewer than `display_limit` values, show all of them.
+        display_string = ", ".join(
+            "%s" % (value,) for value in values[:-1])
+        display_string += " and %s" % (values[-1],)
+    else:
+        # More than `display_limit` values, only mention a few.
+        display_string = ", ".join(
+            "%s" % (value,) for value in values[:display_limit])
+        display_string += and_n_others(values, display_limit)
+
+    return display_string