MAINT: Support the passing of a time rule positionally on the date_rule arg

But log a warning to the user

tests/test_algorithm.py

@@ -16,6 +16,7 @@ from collections import namedtuple
 import datetime
 from datetime import timedelta
 from textwrap import dedent
+import warnings
 from unittest import TestCase, skip
 from copy import deepcopy
 
@@ -1854,6 +1855,72 @@ def handle_data(context, data):
         )
         algo.run(self.data_portal)
 
+    def test_schedule_function_time_rule_positionally_misplaced(self):
+        """
+        Test that when a user specifies a time rule for the date_rule argument,
+        but no rule in the time_rule argument
+        (e.g. schedule_function(func, <time_rule>)), we assume that means
+        assign a time rule but no date rule
+        """
+
+        sim_params = factory.create_simulation_parameters(
+            start=pd.Timestamp('2006-01-12', tz='UTC'),
+            end=pd.Timestamp('2006-01-13', tz='UTC'),
+            data_frequency='minute'
+        )
+
+        algocode = dedent("""
+        from zipline.api import time_rules, schedule_function
+
+        def do_at_open(context, data):
+            context.done_at_open.append(context.get_datetime())
+
+        def do_at_close(context, data):
+            context.done_at_close.append(context.get_datetime())
+
+        def initialize(context):
+            context.done_at_open = []
+            context.done_at_close = []
+            schedule_function(do_at_open, time_rules.market_open())
+            schedule_function(do_at_close, time_rules.market_close())
+
+        def handle_data(algo, data):
+            pass
+        """)
+
+        with warnings.catch_warnings(record=True) as w:
+            algo = TradingAlgorithm(
+                script=algocode,
+                sim_params=sim_params,
+                env=self.env
+            )
+            algo.run(self.data_portal)
+
+        self.assertEqual(len(w), 2)
+        for i, warning in enumerate(w):
+            self.assertIsInstance(warning.message, UserWarning)
+            self.assertEqual(
+                warning.message.args[0],
+                'Got a time rule for the second positional argument '
+                'date_rule. You should use keyword argument '
+                'time_rule= when calling schedule_function without '
+                'specifying a date_rule'
+            )
+            # The warnings come from line 13 and 14 in the algocode
+            self.assertEqual(warning.lineno, 13 + i)
+
+        self.assertEqual(
+            algo.done_at_open,
+            [pd.Timestamp('2006-01-12 14:31:00', tz='UTC'),
+             pd.Timestamp('2006-01-13 14:31:00', tz='UTC')]
+        )
+
+        self.assertEqual(
+            algo.done_at_close,
+            [pd.Timestamp('2006-01-12 20:59:00', tz='UTC'),
+             pd.Timestamp('2006-01-13 20:59:00', tz='UTC')]
+        )
+
 
 class TestCapitalChanges(WithLogger,
                          WithDataPortal,

zipline/algorithm.py

@@ -100,6 +100,8 @@ from zipline.utils.events import (
     make_eventrule,
     date_rules,
     time_rules,
+    AfterOpen,
+    BeforeClose
 )
 from zipline.utils.factory import create_simulation_parameters
 from zipline.utils.math_utils import (
@@ -951,11 +953,21 @@ class TradingAlgorithm(object):
         :class:`zipline.api.date_rules`
         :class:`zipline.api.time_rules`
         """
+
+        # When the user calls schedule_function(func, <time_rule>), assume that
+        # the user meant to specify a time rule but no date rule, instead of
+        # a date rule and no time rule as the signature suggests
+        if isinstance(date_rule, (AfterOpen, BeforeClose)) and not time_rule:
+            warnings.warn('Got a time rule for the second positional argument '
+                          'date_rule. You should use keyword argument '
+                          'time_rule= when calling schedule_function without '
+                          'specifying a date_rule', stacklevel=3)
+
         date_rule = date_rule or date_rules.every_day()
-        time_rule = ((time_rule or time_rules.market_open())
+        time_rule = ((time_rule or time_rules.every_minute())
                      if self.sim_params.data_frequency == 'minute' else
                      # If we are in daily mode the time_rule is ignored.
-                     zipline.utils.events.Always())
+                     time_rules.every_minute())
 
         self.add_event(
             make_eventrule(date_rule, time_rule, half_days),

zipline/utils/events.py

@@ -689,6 +689,7 @@ class date_rules(object):
 class time_rules(object):
     market_open = AfterOpen
     market_close = BeforeClose
+    every_minute = Always
 
 
 def make_eventrule(date_rule, time_rule, half_days=True):