Re-scope expected date format defaults as builtins

This is more in line with the behavior of extending rather than
overriding these values. Help text for the command line option has also
been reworded.

Co-authored-by: Jover Lee <[email protected]>

augur/curate/format_dates.py

@@ -15,9 +15,9 @@
 from .format_dates_directives import YEAR_DIRECTIVES, YEAR_MONTH_DIRECTIVES, YEAR_MONTH_DAY_DIRECTIVES
 
 
-# Default date formats that this command should parse
+# Builtin date formats that this command should parse
 # without additional input from the user.
-DEFAULT_EXPECTED_DATE_FORMATS = [
+BUILTIN_DATE_FORMATS = [
     '%Y-%m-%d',
     '%Y-%m-XX',
     '%Y-XX-XX',
@@ -36,11 +36,13 @@ def register_parser(parent_subparsers):
 
     optional = parser.add_argument_group(title="OPTIONAL")
     optional.add_argument("--expected-date-formats", nargs="+", action="extend",
-        default=DEFAULT_EXPECTED_DATE_FORMATS,
-        help="Expected date formats that are currently in the provided date fields, " +
-             "defined by standard format codes as listed at " +
-             "https://docs.python.org/3/library/datetime.html#strftime-and-strptime-format-codes. " +
-             "If a date string matches multiple formats, it will be parsed as the first matched format in the provided order.")
+        help=f"""Custom date formats for values in the provided date fields, defined by standard
+                 format codes available at
+                 <https://docs.python.org/3/library/datetime.html#strftime-and-strptime-format-codes>.
+                 If a value matches multiple formats, it will be parsed using the first match.
+                 The following formats are builtin and automatically used:
+                 {", ".join(repr(x).replace("%", "%%") for x in BUILTIN_DATE_FORMATS)}.
+                 User-provided values are considered after the builtin formats.""")
     optional.add_argument("--failure-reporting",
         type=DataErrorMethod.argtype,
         choices=list(DataErrorMethod),
@@ -182,10 +184,14 @@ def format_date(date_string, expected_formats):
 
 
 def run(args, records):
+    expected_date_formats = BUILTIN_DATE_FORMATS
+    if args.expected_date_formats:
+        expected_date_formats.extend(args.expected_date_formats)
+
     failures = []
     failure_reporting = args.failure_reporting
     failure_suggestion = (
-        f"\nCurrent expected date formats are {args.expected_date_formats!r}. " +
+        f"\nCurrent expected date formats are {expected_date_formats!r}. " +
         "This can be updated with --expected-date-formats."
     )
     for index, record in enumerate(records):
@@ -198,7 +204,7 @@ def run(args, records):
             if date_string is None:
                 raise AugurError(f"Expected date field {field!r} not found in record {record_id!r}.")
 
-            formatted_date_string = format_date(date_string, args.expected_date_formats)
+            formatted_date_string = format_date(date_string, expected_date_formats)
             if formatted_date_string is None:
                 # Mask failed date formatting before processing error methods
                 # to ensure failures are masked even when failures are "silent"