feature/enabling environment variables unset action

conans/client/tools/env.py

@@ -32,20 +32,29 @@ def run_environment(conanfile):
 @contextmanager
 def environment_append(env_vars):
     """
-    :param env_vars: List of simple environment vars. {name: value, name2: value2} => e.g.: MYVAR=1
+    :param env_vars: List (dict) of simple environment vars. {name: value, name2: value2} => e.g.: MYVAR=1
                      The values can also be lists of appendable environment vars. {name: [value, value2]}
                       => e.g. PATH=/path/1:/path/2
+                     If the value is set to None, then that environment variable is unset.
     :return: None
     """
+    unset_vars = []
+    for key in env_vars.keys():
+        if env_vars[key] is None:
+            unset_vars.append(key)
+    for var in unset_vars:
+        env_vars.pop(var, None)
     for name, value in env_vars.items():
         if isinstance(value, list):
             env_vars[name] = os.pathsep.join(value)
             old = os.environ.get(name)
             if old:
                 env_vars[name] += os.pathsep + old
-    if env_vars:
+    if env_vars or unset_vars:
         old_env = dict(os.environ)
         os.environ.update(env_vars)
+        for var in unset_vars:
+            os.environ.pop(var)
         try:
             yield
         finally:

conans/test/unittests/client/tools/test_env.py

@@ -0,0 +1,48 @@
+# coding=utf-8
+
+import os
+import unittest
+import mock
+
+from conans.client.tools import env
+
+
+class ToolsEnvTest(unittest.TestCase):
+    def test_environment_append_variables(self):
+        with mock.patch.dict('os.environ', {}),\
+             env.environment_append({'env_var1': 'value',
+                                     'env_var2': 'value2'}):
+                self.assertEqual(os.environ['env_var1'], 'value')
+                self.assertEqual(os.environ['env_var2'], 'value2')
+
+    def test_environment_append_variables_without_values(self):
+        with mock.patch.dict('os.environ',
+                             {'env_var1': 'value',
+                              'env_var2': 'value2'}),\
+             env.environment_append({}):
+                self.assertEqual(os.environ['env_var1'], 'value')
+                self.assertEqual(os.environ['env_var2'], 'value2')
+
+    def test_environment_append_overwriting(self):
+        with mock.patch.dict('os.environ', {'env_var1': 'value'}),\
+             env.environment_append({'env_var1': 'new_value'}):
+                self.assertEqual(os.environ['env_var1'], 'new_value')
+
+    def test_environment_append_list(self):
+        with mock.patch.dict('os.environ', {}),\
+             env.environment_append({'env_var1': ['value1', 'value2']}):
+                self.assertEqual(os.environ['env_var1'], 'value1' +
+                                 os.pathsep + 'value2')
+
+    def test_environment_append_unsetting_some_variables(self):
+        with mock.patch.dict('os.environ', {'env_var1': 'value'}),\
+             env.environment_append({'env_var1': None, 'env_var2': 'value2'}):
+                self.assertNotIn('env_var1', os.environ)
+                self.assertEqual(os.environ['env_var2'], 'value2')
+
+    def test_environment_append_unsetting_all_variables(self):
+        with mock.patch.dict('os.environ',
+                             {'env_var1': 'value',
+                              'env_var2': 'value2'}),\
+             env.environment_append({'env_var1': None}):
+                self.assertNotIn('env_var1', os.environ)