Improve tests for env variable expansion functionality

This includes mocking the environment variables instead of modifying the
global environment during unit tests, and adding extra tests for the
underlying expansion function in `utils.py`.

Author: xanarin

tests/test_config.py

@@ -884,10 +884,10 @@ def test_alias_volumes_set(self):
         assert v.host_path == '/host/bar'
         assert v.options == ['z', 'ro']
 
-    def test_volumes_with_env_vars_simple(self):
+    def test_volumes_with_env_vars_simple(self, monkeypatch):
         '''volume definitions can contain environment variables'''
-        os.environ["TEST_VOL_PATH"] = "/bar/baz"
-        os.environ["TEST_VOL_PATH2"] = "/moo/doo"
+        monkeypatch.setenv("TEST_VOL_PATH", "/bar/baz")
+        monkeypatch.setenv("TEST_VOL_PATH2", "/moo/doo")
         with open('.scuba.yml', 'w') as f:
             f.write(r'''
                 image: na
@@ -905,11 +905,11 @@ def test_volumes_with_env_vars_simple(self):
         assert v.host_path == '/moo/doo/foo'
         assert v.options == []
 
-    def test_volumes_with_env_vars_complex(self):
+    def test_volumes_with_env_vars_complex(self, monkeypatch):
         '''complex volume definitions can contain environment variables'''
-        os.environ["TEST_HOME"] = "/home/testuser"
-        os.environ["TEST_TMP"] = "/tmp"
-        os.environ["TEST_MAIL"] = "/var/spool/mail/testuser"
+        monkeypatch.setenv("TEST_HOME", "/home/testuser")
+        monkeypatch.setenv("TEST_TMP", "/tmp")
+        monkeypatch.setenv("TEST_MAIL", "/var/spool/mail/testuser")
 
         with open('.scuba.yml', 'w') as f:
             f.write(r'''
@@ -945,10 +945,10 @@ def test_volumes_with_env_vars_complex(self):
         assert v.host_path == "/var/spool/mail/testuser"
         assert v.options == ['z', 'ro']
 
-    def test_volumes_with_invalid_env_vars(self):
+    def test_volumes_with_invalid_env_vars(self, monkeypatch):
         '''Volume definitions cannot include unset env vars'''
         # Ensure that the entry does not exist in the environment
-        os.environ.pop("TEST_VAR1", None)
+        monkeypatch.delenv("TEST_VAR1", raising=False)
         with open('.scuba.yml', 'w') as f:
             f.write(r'''
                 image: na

tests/test_utils.py

@@ -128,3 +128,26 @@ def test_writeln():
         scuba.utils.writeln(s, 'hello')
         scuba.utils.writeln(s, 'goodbye')
         assert s.getvalue() == 'hello\ngoodbye\n'
+
+def test_expand_env_vars(monkeypatch):
+    monkeypatch.setenv("MY_VAR", "my favorite variable")
+    assert scuba.utils.expand_env_vars("This is $MY_VAR") == \
+            "This is my favorite variable"
+    assert scuba.utils.expand_env_vars("What is ${MY_VAR}?") == \
+            "What is my favorite variable?"
+
+def test_expand_missing_env_vars(monkeypatch):
+    monkeypatch.delenv("MY_VAR", raising=False)
+    # Verify that a KeyError is raised for unset env variables
+    with pytest.raises(KeyError) as kerr:
+        scuba.utils.expand_env_vars("Where is ${MY_VAR}?")
+    assert kerr.value.args[0] == "MY_VAR"
+
+
+def test_expand_env_vars_dollars():
+    # Verify that a ValueError is raised for bare, unescaped '$' characters
+    with pytest.raises(ValueError):
+        scuba.utils.expand_env_vars("Just a lonely $")
+
+    # Verify that it is possible to get '$' characters in an expanded string
+    assert scuba.utils.expand_env_vars(r"Just a lonely $$") == "Just a lonely $"