ENH: Verify params passed to get_datetime

zipline/algorithm.py

@@ -15,7 +15,7 @@
 from copy import copy
 import operator as op
 import warnings
-
+from datetime import tzinfo
 import logbook
 import pytz
 import pandas as pd
@@ -95,7 +95,8 @@ from zipline.utils.api_support import (
     ZiplineAPI,
     disallowed_in_before_trading_start)
 
-from zipline.utils.input_validation import ensure_upper_case, error_keywords
+from zipline.utils.input_validation import ensure_upper_case, error_keywords, \
+    expect_types, optional, coerce_string
 from zipline.utils.cache import CachedObject, Expired
 from zipline.utils.calendars import get_calendar
 
@@ -1477,8 +1478,11 @@ class TradingAlgorithm(object):
         self.performance_needs_update = True
 
     @api_method
+    @preprocess(tz=coerce_string(pytz.timezone))
+    @expect_types(tz=optional(tzinfo))
     def get_datetime(self, tz=None):
-        """Returns the current simulation datetime.
+        """
+        Returns the current simulation datetime.
 
         Parameters
         ----------
@@ -1492,14 +1496,9 @@ class TradingAlgorithm(object):
         """
         dt = self.datetime
         assert dt.tzinfo == pytz.utc, "Algorithm should have a utc datetime"
-
         if tz is not None:
-            # Convert to the given timezone passed as a string or tzinfo.
-            if isinstance(tz, string_types):
-                tz = pytz.timezone(tz)
             dt = dt.astimezone(tz)
-
-        return dt  # datetime.datetime objects are immutable.
+        return dt
 
     def update_dividends(self, dividend_frame):
         """