add testcase and helper function

2 files changed, 14 insertions, 2 deletions

diff --git a/test_uritemplate.py b/test_uritemplate.py
index d23a7ed..756a9a6 100644
--- a/test_uritemplate.py
+++ b/test_uritemplate.py
@@ -491,6 +491,12 @@ class TestURITemplate(RFCTemplateExamples('RFCMeta', (TestCase,), {})):
         d[u] += 1
         self.assertEqual(d, {t: 2})
 
+    def test_no_mutate(self):
+        args = {}
+        t = URITemplate('')
+        t.expand(args, key=1)
+        self.assertEqual(args, {})
+
 
 class TestURIVariable(TestCase):
     def setUp(self):
diff --git a/uritemplate/template.py b/uritemplate/template.py
index 0d45214..363e8ea 100644
--- a/uritemplate/template.py
+++ b/uritemplate/template.py
@@ -21,6 +21,10 @@ from uritemplate.variable import URIVariable
 template_re = re.compile('{([^\}]+)}')
 
 
+def _update(kwargs, var_dict):
+    kwargs.update((k, v) for k, v in var_dict.items() if k not in kwargs)
+
+
 class URITemplate(object):
 
     """This parses the template and will be used to expand it.
@@ -121,7 +125,8 @@ class URITemplate(object):
                   ``val2`` will be used instead of ``val1``.
 
         """
-        kwargs.update(var_dict or {})
+        if var_dict:
+            _update(kwargs, var_dict)
 
         return self._expand(kwargs, False)
 
@@ -141,6 +146,7 @@ class URITemplate(object):
             t.partial()  # => URITemplate('https://api.github.com{/end}')
 
         """
-        kwargs.update(var_dict or {})
+        if var_dict:
+            _update(kwargs, var_dict)
 
         return URITemplate(self._expand(kwargs, True))