BUG: Reset position price (etc.) after closing position

Fixes https://github.com/kernc/backtesting.py/issues/27

backtesting/_util.py

@@ -5,6 +5,13 @@ import numpy as np
 import pandas as pd
 
 
+def try_(lazy_func, default=None, exception=Exception):
+    try:
+        return lazy_func()
+    except exception:
+        return default
+
+
 def _as_str(value):
     if isinstance(value, (Number, str)):
         return str(value)

backtesting/backtesting.py

@@ -406,7 +406,7 @@ class Position:
         Profit (positive) or loss (negative) of current position,
         in percent of position open price.
         """
-        return self.pl / (self.open_price * abs(self.size))
+        return self.pl / (self.open_price * abs(self.size) or 1)
 
     @property
     def is_long(self):
@@ -514,6 +514,8 @@ class _Broker:
 
         self._cash += pl
         self._position = 0
+        self._position_open_price = 0
+        self._position_open_i = None
 
     @property
     def equity(self):

backtesting/test/_test.py

@@ -25,7 +25,9 @@ from backtesting.lib import (
     plot_heatmaps
 )
 from backtesting.test import GOOG, EURUSD, SMA
-from backtesting._util import _Indicator, _as_str, _Array
+from backtesting._util import _Indicator, _as_str, _Array, try_
+
+SHORT_DATA = GOOG.iloc[:20]  # Short data for fast tests with no indicator lag
 
 
 @contextmanager
@@ -297,6 +299,41 @@ class TestBacktest(TestCase):
                 self.assertEqual(stats['_strategy'].__class__, strategy)
 
 
+class TestStrategy(TestCase):
+    def test_position(self):
+        def coroutine(self):
+            yield self.buy()
+
+            assert self.position
+            assert self.position.is_long
+            assert not self.position.is_short
+            assert self.position.size > 0
+            assert self.position.pl
+            assert self.position.pl_pct
+            assert self.position.open_price > 0
+            assert self.position.open_time
+
+            yield self.position.close()
+
+            assert not self.position
+            assert not self.position.is_long
+            assert not self.position.is_short
+            assert not self.position.size
+            assert not self.position.pl
+            assert not self.position.pl_pct
+            assert not self.position.open_price
+            assert not self.position.open_time
+
+        class S(Strategy):
+            def init(self):
+                self.step = coroutine(self)
+
+            def next(self):
+                try_(self.step.__next__, None, StopIteration)
+
+        Backtest(SHORT_DATA, S).run()
+
+
 class TestOptimize(TestCase):
     def test_optimize(self):
         bt = Backtest(GOOG.iloc[:100], SmaCross)