Fixes error swallowing in set_environ

AUTHORS.rst

@@ -183,3 +183,4 @@ Patches and Suggestions
 - Shmuel Amar (`@shmuelamar <https://github.com/shmuelamar>`_)
 - Gary Wu (`@garywu <https://github.com/garywu>`_)
 - Ryan Pineo (`@ryanpineo <https://github.com/ryanpineo>`_)
+- Matt Liu <liumatt@gmail.com> (`@mlcrazy <https://github.com/mlcrazy>`_)

requests/utils.py

@@ -612,18 +612,18 @@ def set_environ(env_name, value):
     the environment variable 'env_name'.
 
     If 'value' is None, do nothing"""
-    if value is not None:
+    value_changed = value is not None
+    if value_changed:
         old_value = os.environ.get(env_name)
         os.environ[env_name] = value
     try:
         yield
     finally:
-        if value is None:
-            return
-        if old_value is None:
-            del os.environ[env_name]
-        else:
-            os.environ[env_name] = old_value
+        if value_changed:
+            if old_value is None:
+                del os.environ[env_name]
+            else:
+                os.environ[env_name] = old_value
 
 
 def should_bypass_proxies(url, no_proxy):

tests/test_utils.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
 import os
+import copy
 from io import BytesIO
 
 import pytest
@@ -17,7 +18,7 @@ from requests.utils import (
     requote_uri, select_proxy, should_bypass_proxies, super_len,
     to_key_val_list, to_native_string,
     unquote_header_value, unquote_unreserved,
-    urldefragauth, add_dict_to_cookiejar)
+    urldefragauth, add_dict_to_cookiejar, set_environ)
 from requests._internal_utils import unicode_is_ascii
 
 from .compat import StringIO, cStringIO
@@ -651,4 +652,29 @@ def test_should_bypass_proxies_win_registry(url, expected, override,
     monkeypatch.setenv('NO_PROXY', '')
     monkeypatch.setattr(winreg, 'OpenKey', OpenKey)
     monkeypatch.setattr(winreg, 'QueryValueEx', QueryValueEx)
-    assert should_bypass_proxies(url, no_proxy=None) == expected
+
+
+@pytest.mark.parametrize(
+    'env_name, value', (
+            ('no_proxy', '192.168.0.0/24,127.0.0.1,localhost.localdomain'),
+            ('no_proxy', None),
+            ('a_new_key', '192.168.0.0/24,127.0.0.1,localhost.localdomain'),
+            ('a_new_key', None),
+    ))
+def test_set_environ(env_name, value):
+    """Tests set_environ will set environ values and will restore the environ."""
+    environ_copy = copy.deepcopy(os.environ)
+    with set_environ(env_name, value):
+        assert os.environ.get(env_name) == value
+
+    assert os.environ == environ_copy
+
+
+def test_set_environ_raises_exception():
+    """Tests set_environ will raise exceptions in context when the
+    value parameter is None."""
+    with pytest.raises(Exception) as exception:
+        with set_environ('test1', None):
+            raise Exception('Expected exception')
+
+    assert 'Expected exception' in str(exception.value)