accrual: Clean up types and imports.

Mostly I wanted to write the documentation about groups, and then
I kept pulling the thread.

conservancy_beancount/reports/accrual.py

@@ -70,7 +70,6 @@ import collections
 import datetime
 import enum
 import logging
-import re
 import sys
 import urllib.parse as urlparse
 
@@ -119,9 +118,7 @@ from .. import rtutil
 
 PROGNAME = 'accrual-report'
 
-CompoundAmount = TypeVar('CompoundAmount', data.Amount, core.Balance)
-PostGroups = Mapping[Optional[MetaValue], 'AccrualPostings']
-RTObject = Mapping[str, str]
+PostGroups = Mapping[Optional[str], 'AccrualPostings']
 T = TypeVar('T')
 
 logger = logging.getLogger('conservancy_beancount.reports.accrual')
@@ -213,7 +210,7 @@ class AccrualPostings(core.RelatedPostings):
             if pred(post) and 'entity' in post.meta
         )
 
-    def make_consistent(self) -> Iterator[Tuple[MetaValue, 'AccrualPostings']]:
+    def make_consistent(self) -> Iterator[Tuple[str, 'AccrualPostings']]:
         account_ok = isinstance(self.account, str)
         entity_ok = isinstance(self.entity, str)
         # `'/' in self.invoice` is just our heuristic to ensure that the
@@ -221,7 +218,10 @@ class AccrualPostings(core.RelatedPostings):
         # value like "FIXME". It can be refined if needed.
         invoice_ok = isinstance(self.invoice, str) and '/' in self.invoice
         if account_ok and entity_ok and invoice_ok:
-            yield (self.invoice, self)
+            # mypy loses track of the fact that self.invoice must be a str in
+            # this case (gated on the invoice_ok boolean) and complains it
+            # doesn't match the type of the return value.
+            yield (self.invoice, self)  # type:ignore[misc]
             return
         groups = collections.defaultdict(list)
         for post in self:
@@ -696,6 +696,14 @@ def main(arglist: Optional[Sequence[str]]=None,
     if not postings:
         logger.warning("no matching entries found to report")
         returncode |= ReturnFlag.NOTHING_TO_REPORT
+    # groups is a mapping of metadata value strings to AccrualPostings.
+    # The keys are basically arbitrary, the report classes don't rely on them,
+    # but they do help symbolize what's being grouped.
+    # For the outgoing approval report, groups maps rt-id link strings to
+    # associated accruals.
+    # For all other reports, groups starts by grouping postings together by
+    # invoice link string, then uses AccrualReport.make_consistent() to split
+    # out groups that need it.
     groups: PostGroups
     if args.report_type is None or args.report_type is ReportType.OUTGOING:
         groups = dict(AccrualPostings.group_by_first_meta_link(postings, 'rt-id'))