nbgrader collect: allow collection *only* of submissions before due date

- Because by default, collect will fetch *all* assignments, even
  though which are past the deadline - which thus could get zero
  points, while one submitted right before the deadline would get
  points.  Of course it depends on how grading is set up.

- Controlled with the "nbgrader collect --before-duedate".  In the
  future there could be a "--before=TIMESTAMP" option.

- If there are no assignments submitted before the due date, take the
  latest assignment, even after the due date.

- There could be partial credit for things after the due date, so the
  it makes sense to take something better after the due date than
  worse before.  But, nbgrader can only store one submission and the
  evaluation is separate from the collection, so we can't support that
  easily yet.

- This initial patch contributed by @aleksanderilin under the nbgrader
  license.

Author: aleksanderilin

nbgrader/apps/collectapp.py

@@ -20,6 +20,10 @@
         {'ExchangeCollect' : {'update': True}},
         "Update existing submissions with ones that have newer timestamps."
     ),
+    'before_duedate': (
+        {'ExchangeCollect' : {'before_duedate': True}},
+        "Collect the last submission before due date or the last submission if no submission before due date."
+    ),
 })
 
 class CollectApp(NbGrader):

nbgrader/exchange/collect.py

@@ -4,10 +4,12 @@
 import sys
 from collections import defaultdict
 from textwrap import dedent
+import datetime
 
 from traitlets import Bool
 
 from .exchange import Exchange
+from ..api import Gradebook, MissingEntry
 from ..utils import check_mode, parse_utc
 
 # pwd is for matching unix names with student ide, so we shouldn't import it on
@@ -31,6 +33,11 @@ class ExchangeCollect(Exchange):
         help="Update existing submissions with ones that have newer timestamps."
     ).tag(config=True)
 
+    before_duedate = Bool(
+        False,
+        help="Collect the last submission before due date or the last submission if no submission before due date."
+    ).tag(config=True)
+
     check_owner = Bool(
         default_value=True,
         help="Whether to cross-check the student_id with the UNIX-owner of the submitted directory."
@@ -63,7 +70,24 @@ def init_src(self):
         pattern = os.path.join(self.inbound_path, '{}+{}+*'.format(student_id, self.coursedir.assignment_id))
         records = [self._path_to_record(f) for f in glob.glob(pattern)]
         usergroups = groupby(records, lambda item: item['username'])
-        self.src_records = [self._sort_by_timestamp(v)[0] for v in usergroups.values()]
+
+        with Gradebook(self.coursedir.db_url, self.coursedir.course_id) as gb:
+            try:
+                assignment = gb.find_assignment(self.coursedir.assignment_id)
+                self.duedate = assignment.duedate
+            except MissingEntry:
+                self.duedate = None
+        if self.duedate is None or not self.before_duedate:
+            self.src_records = [self._sort_by_timestamp(v)[0] for v in usergroups.values()]
+        else:
+            self.src_records = []
+            for v in usergroups.values():
+                records = self._sort_by_timestamp(v)
+                records_before_duedate = [record for record in records if record['timestamp'] <= self.duedate]
+                if records_before_duedate:
+                    self.src_records.append(records_before_duedate[0])
+                else:
+                    self.src_records.append(records[0])
 
     def init_dest(self):
         pass
@@ -108,6 +132,10 @@ def copy_files(self):
                 if self.update and (existing_timestamp is None or new_timestamp > existing_timestamp):
                     copy = True
                     updating = True
+                elif self.before_duedate and existing_timestamp != new_timestamp:
+                    copy = True
+                    updating = True
+
             else:
                 copy = True