[Resolve #870] Bugfix in _call_sceptre_handler

Before this, the _call_sceptre_handler method makes multiple calls to
os.getcwd() in determining paths to add and then later paths to remove.
In the event that the sceptre_handler code then makes a call to
os.chdir, the second call ends up removing the wrong path.

This patch ensures that the paths removed are the same ones that are
added.

sceptre/template.py

@@ -148,18 +148,20 @@ def _call_sceptre_handler(self):
         :raises: IOError
         :raises: TemplateSceptreHandlerError
         """
+
         # Get relative path as list between current working directory and where
         # the template is
         # NB: this is a horrible hack...
         relpath = os.path.relpath(self.path, os.getcwd()).split(os.path.sep)
-        relpaths_to_add = [
-            os.path.sep.join(relpath[:i+1])
+        paths_to_add = [
+            os.path.join(os.getcwd(), os.path.sep.join(relpath[:i+1]))
             for i in range(len(relpath[:-1]))
         ]
+
         # Add any directory between the current working directory and where
         # the template is to the python path
-        for directory in relpaths_to_add:
-            sys.path.append(os.path.join(os.getcwd(), directory))
+        for path in paths_to_add:
+            sys.path.append(path)
         self.logger.debug(
             "%s - Getting CloudFormation from %s", self.name, self.path
         )
@@ -179,8 +181,10 @@ def _call_sceptre_handler(self):
                 )
             else:
                 raise e
-        for directory in relpaths_to_add:
-            sys.path.remove(os.path.join(os.getcwd(), directory))
+
+        for path in paths_to_add:
+            sys.path.remove(path)
+
         return body
 
     def upload_to_s3(self):

tests/fixtures/templates/chdir.py

@@ -0,0 +1,12 @@
+# -*- coding: utf-8 -*-
+
+from troposphere import Template
+
+from os import chdir
+
+def sceptre_handler(sceptre_user_data):
+    t = Template()
+
+    chdir("..")
+
+    return t.to_json()

tests/test_template.py

@@ -210,6 +210,21 @@ def test_body_with_generic_template(self):
             expected_output_dict = json.loads(f.read())
         assert output_dict == expected_output_dict
 
+    def test_body_with_chdir_template(self):
+        self.template.sceptre_user_data = None
+        self.template.name = "chdir"
+        current_dir = os.getcwd()
+        self.template.path = os.path.join(
+            os.getcwd(),
+            "tests/fixtures/templates/chdir.py"
+        )
+        try:
+            json.loads(self.template.body)
+        except ValueError:
+            assert False
+        finally:
+            os.chdir(current_dir)
+
     def test_body_with_missing_file(self):
         self.template.path = "incorrect/template/path.py"
         with pytest.raises(IOError):