Adjust date format for "When" parameters to correspond to system locale

ChangeLog

@@ -7,6 +7,7 @@ v4.5.0
     w/ or w/o --config-folder
     - POSSIBLE ACTION REQUIRED: Use `@path/to/gcalclirc` explicitly if it stops
       reading an rc file you needed
+  * Determine date format to use based on system locale's in "When" inputs
   * Respect locally-installed certificates (ajkessel)
   * Re-add a `--noauth_local_server` to provide instructions for authenticating
     from a remote system using port forwarding

gcalcli/cli.py

@@ -37,7 +37,7 @@
 from .gcal import GoogleCalendarInterface
 from .printer import Printer, valid_color_name
 from .validators import (
-    DATETIME_INPUT_DESCRIPTION,
+    get_date_input_description,
     get_input,
     PARSABLE_DATE,
     PARSABLE_DURATION,
@@ -115,11 +115,12 @@ def run_add_prompt(parsed_args, printer):
     if parsed_args.where is None:
         parsed_args.where = get_input(printer, 'Location: ', STR_ALLOW_EMPTY)
     if parsed_args.when is None:
+        date_format_desc = get_date_input_description()
         parsed_args.when = get_input(
             printer,
             'When (? for help): ',
             PARSABLE_DATE,
-            help=f'Expected format: {DATETIME_INPUT_DESCRIPTION}',
+            help=f'Expected format: {date_format_desc}',
         )
     if parsed_args.duration is None and parsed_args.end is None:
         if parsed_args.allday:

gcalcli/utils.py

@@ -8,6 +8,7 @@
 from datetime import datetime, timedelta
 from typing import Tuple
 
+import babel
 from dateutil.parser import parse as dateutil_parse
 from dateutil.tz import tzlocal
 from parsedatetime.parsedatetime import Calendar
@@ -85,6 +86,26 @@ def get_times_from_duration(
     return start.isoformat(), stop.isoformat()
 
 
+def is_dayfirst_locale():
+    """Detect whether system locale date format has day first.
+
+    Examples:
+     - M/d/yy -> False
+     - dd/MM/yy -> True
+     - (UnknownLocaleError) -> False
+
+    Pattern syntax is documented at
+    https://babel.pocoo.org/en/latest/dates.html#pattern-syntax.
+    """
+    try:
+        locale = babel.Locale(babel.default_locale('LC_TIME'))
+    except babel.UnknownLocaleError:
+        # Couldn't detect locale, assume non-dayfirst.
+        return False
+    m = re.search(r'M|d|$', locale.date_formats['short'].pattern)
+    return m and m.group(0) == 'd'
+
+
 def get_time_from_str(when):
     """Convert a string to a time: first uses the dateutil parser, falls back
     on fuzzy matching with parsedatetime
@@ -93,7 +114,9 @@ def get_time_from_str(when):
             hour=0, minute=0, second=0, microsecond=0)
 
     try:
-        event_time = dateutil_parse(when, default=zero_oclock_today)
+        event_time = dateutil_parse(
+            when, default=zero_oclock_today, dayfirst=is_dayfirst_locale()
+        )
     except ValueError:
         struct, result = fuzzy_date_parse(when)
         if not result:

gcalcli/validators.py

@@ -2,17 +2,19 @@
 from typing import Optional
 
 from .exceptions import ValidationError
-from .utils import get_time_from_str, get_timedelta_from_str, REMINDER_REGEX
+from .utils import (
+    get_time_from_str,
+    get_timedelta_from_str,
+    REMINDER_REGEX,
+    is_dayfirst_locale,
+)
 
 # TODO: in the future, pull these from the API
 # https://developers.google.com/calendar/v3/reference/colors
 VALID_OVERRIDE_COLORS = ['lavender', 'sage', 'grape', 'flamingo',
                          'banana', 'tangerine', 'peacock', 'graphite',
                          'blueberry', 'basil', 'tomato']
 
-DATETIME_INPUT_DESCRIPTION = 'a date (e.g. 2019-12-31, tomorrow 10am, 2nd Jan, \
-Jan 4th, etc) or valid time if today'
-
 
 def get_override_color_id(color):
     return str(VALID_OVERRIDE_COLORS.index(color) + 1)
@@ -66,6 +68,13 @@ def str_to_int_validator(input_str):
         )
 
 
+def get_date_input_description():
+    dayfirst = is_dayfirst_locale()
+    sample_date = '2019-31-12' if dayfirst else '2019-12-31'
+    return f'a date (e.g. {sample_date}, tomorrow 10am, 2nd Jan, Jan 4th, etc) \
+or valid time if today'
+
+
 def parsable_date_validator(input_str):
     """
     A filter allowing any string which can be parsed
@@ -76,8 +85,9 @@ def parsable_date_validator(input_str):
         get_time_from_str(input_str)
         return input_str
     except ValueError:
+        format_desc = get_date_input_description()
         raise ValidationError(
-            f'Expected format: {DATETIME_INPUT_DESCRIPTION}. '
+            f'Expected format: {format_desc}. '
             '(Ctrl-C to exit)\n'
         )
 

pyproject.toml

@@ -27,6 +27,7 @@ classifiers = [
 ]
 dependencies = [
   "argcomplete",
+  "babel",
   "google-api-python-client>=1.4",
   "google_auth_oauthlib",
   "httplib2",