Allow escaping of {} in defaults (tox-dev#1696)

Fixes tox-dev#1502

Author: jayvdb

CONTRIBUTORS

@@ -48,6 +48,7 @@ Jake Windle
 Jannis Leidel
 Joachim Brandon LeBlanc
 Johannes Christ
+John Mark Vandenberg
 Jon Dufresne
 Josh Smeaton
 Josh Snyder

docs/changelog/1502.feature.rst

@@ -0,0 +1 @@
+Allow \{ and \} in default of {env:key:default}. - by :user:`jayvdb`

src/tox/config/__init__.py

@@ -1731,7 +1731,7 @@ class Replacer:
         (?<!\\)[{]
         (?:(?P<sub_type>[^[:{}]+):)?    # optional sub_type for special rules
         (?P<substitution_value>(?:\[[^,{}]*\])?[^:,{}]*)  # substitution key
-        (?::(?P<default_value>[^{}]*))?   # default value
+        (?::(?P<default_value>([^{}]|\\{|\\})*))?   # default value
         [}]
         """,
         re.VERBOSE,

tests/unit/config/test_config.py

@@ -453,6 +453,17 @@ def test_command_env_substitution_global(self, newconfig):
         envconfig = config.envconfigs["python"]
         assert envconfig.commands == [["echo", "bar"]]
 
+    def test_command_env_substitution_default_escape(self, newconfig):
+        """Ensure literal { and } in default of {env:key:default} values."""
+        config = newconfig(
+            r"""
+            [testenv]
+            commands = echo {env:FOO:\{bar\}}
+        """,
+        )
+        envconfig = config.envconfigs["python"]
+        assert envconfig.commands == [["echo", "{bar}"]]
+
     def test_regression_issue595(self, newconfig):
         config = newconfig(
             """