From 7daf58ec9b734dd138fd5f7959062685927f21d6 Mon Sep 17 00:00:00 2001
From: Scott Sanderson <scoutoss@gmail.com>
Date: Mon, 2 Jun 2014 23:36:38 -0400
Subject: [PATCH] ENH: Add a class-level `instance` method on
 `TradingEnvironment`.

Adds a classmethod, `instance` on `TradingEnvironment` that returns
`zipline.finance.trading.environment`, instantiating it if necessary.

This makes it possible to initialize the default environment instance in a
less-roundabout way than creating a `SimulationParameters` object.
---
 zipline/finance/trading.py | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/zipline/finance/trading.py b/zipline/finance/trading.py
index f4ace1f5..098237c5 100644
--- a/zipline/finance/trading.py
+++ b/zipline/finance/trading.py
@@ -75,6 +75,14 @@ class NoFurtherDataError(Exception):
 
 class TradingEnvironment(object):
 
+    @classmethod
+    def instance(cls):
+        global environment
+        if not environment:
+            environment = TradingEnvironment()
+
+        return environment
+
     def __init__(
         self,
         load=None,
@@ -227,10 +235,9 @@ class SimulationParameters(object):
                  emission_rate='daily',
                  data_frequency='daily',
                  sids=None):
-        global environment
-        if not environment:
-            # This is the global environment for trading simulation.
-            environment = TradingEnvironment()
+
+        # This is the global environment for trading simulation.
+        environment = TradingEnvironment.instance()
 
         self.period_start = period_start
         self.period_end = period_end