Tidy up "format" method for monthly letter usage

This did more than it said and had some unconventional behaviour:

- It modified the input data. We can avoid this by computing the
postage group on-the-fly and using "sorted" instead of "sort".

- It defined a custom, named tuple. This isn't necessary as Jinja
allows us to access elements by qualification (".") already.

We can also use the same lambda function to group and sort items,
since the sort predicate is the same one we use to group them.

app/main/views/dashboard.py

@@ -1,9 +1,7 @@
 import calendar
-from collections import namedtuple
 from datetime import datetime
 from functools import partial
 from itertools import groupby
-from operator import itemgetter
 
 from flask import (
     Response,
@@ -410,12 +408,12 @@ def get_monthly_usage_breakdown(year, monthly_usage):
 
         monthly_letters = [row for row in letters if row['month'] == month]
         letter_cost = sum(row['letter_cost'] for row in monthly_letters)
-        letter_breakdown = format_letter_details_for_month(monthly_letters)
+        letter_breakdown = get_monthly_usage_breakdown_for_letters(monthly_letters)
 
         yield {
             'month': month,
             'letter_cost': letter_cost,
-            'letter_breakdown': letter_breakdown,
+            'letter_breakdown': list(letter_breakdown),
             'sms_charged': sms_charged,
             'sms_free_allowance_used': sms_free_allowance_used,
             'sms_rate': sms_rate,
@@ -423,39 +421,30 @@ def get_monthly_usage_breakdown(year, monthly_usage):
         }
 
 
-def format_letter_details_for_month(letter_units_for_month):
-    # Format postage descriptions in letter units e.g. to 'international' not 'europe'
-    for month in letter_units_for_month:
-        for k, v in month.items():
-            if k == 'postage':
-                month[k] = get_postage_description(v)
-
-    # letter_units_for_month must be sorted before international postage values can be aggregated
+def get_monthly_usage_breakdown_for_letters(monthly_letters):
     postage_order = {'first class': 0, 'second class': 1, 'international': 2}
-    letter_units_for_month.sort(key=lambda x: (postage_order[x['postage']], x['rate']))
 
-    LetterDetails = namedtuple('LetterDetails', ['sent', 'rate', 'cost', 'postage_description'])
+    group_key = lambda row: (  # noqa: E731
+        postage_order[get_monthly_usage_postage_description(row)], row['rate']
+    )
 
-    # Aggregate the rows for international letters which have the same rate
-    result = []
-    for _key, rate_group in groupby(letter_units_for_month, key=itemgetter('postage', 'rate')):
+    rate_groups = groupby(sorted(monthly_letters, key=group_key), key=group_key)
+
+    for _key, rate_group in rate_groups:
+        # rate_group is a one-time generator so must be converted to a list for reuse
         rate_group = list(rate_group)
 
-        letter_details = LetterDetails(
-            sent=sum(x['notifications_sent'] for x in rate_group),
-            rate=rate_group[0]['rate'],
-            cost=sum(x['letter_cost'] for x in rate_group),
-            postage_description=rate_group[0]['postage']
-        )
-
-        result.append(letter_details)
-
-    return result
+        yield {
+            "sent": sum(x['notifications_sent'] for x in rate_group),
+            "rate": rate_group[0]['rate'],
+            "cost": sum(x['letter_cost'] for x in rate_group),
+            "postage_description": get_monthly_usage_postage_description(rate_group[0])
+        }
 
 
-def get_postage_description(postage):
-    if postage in ('first', 'second'):
-        return f'{postage} class'
+def get_monthly_usage_postage_description(row):
+    if row['postage'] in ('first', 'second'):
+        return f'{row["postage"]} class'
     return 'international'