interative shell dependent config

changelog/947.feature.rst

@@ -0,0 +1 @@
+allow injecting config value inside the ini file dependent of the fact that we're connected to an interactive shell or not  - by :user:`gaborbernat`

doc/config.rst

@@ -557,6 +557,17 @@ the above example is roughly equivalent to
 .. _`command positional substitution`:
 .. _`positional substitution`:
 
+interactive shell substitution
+++++++++++++++++++++++++++++++
+
+It's possible to inject a config value only when tox is running in interactive shell (standard input):
+
+    {tty:ON_VALUE:OFF_VALUE}
+
+The first value is the value to inject when the interactive terminal is available,
+the second value is the value to use when it's not. The later on is optional. A good use case
+for this is e.g. passing in the ``--pdb`` flag for pytest.
+
 substitutions for positional arguments in commands
 ++++++++++++++++++++++++++++++++++++++++++++++++++
 

src/tox/config.py

@@ -1430,23 +1430,27 @@ def _replace_match(self, match):
 
         if sub_type == "env":
             return self._replace_env(match)
+        if sub_type == "tty":
+            if is_interactive():
+                return match.group("substitution_value")
+            return match.group("default_value")
         if sub_type is not None:
             raise tox.exception.ConfigError(
                 "No support for the {} substitution type".format(sub_type)
             )
         return self._replace_substitution(match)
 
     def _replace_env(self, match):
-        envkey = match.group("substitution_value")
-        if not envkey:
+        key = match.group("substitution_value")
+        if not key:
             raise tox.exception.ConfigError("env: requires an environment variable name")
         default = match.group("default_value")
-        envvalue = self.reader.get_environ_value(envkey)
-        if envvalue is not None:
-            return envvalue
+        value = self.reader.get_environ_value(key)
+        if value is not None:
+            return value
         if default is not None:
             return default
-        raise tox.exception.MissingSubstitution(envkey)
+        raise tox.exception.MissingSubstitution(key)
 
     def _substitute_from_other_section(self, key):
         if key.startswith("[") and "]" in key:
@@ -1475,6 +1479,10 @@ def _replace_substitution(self, match):
         return str(val)
 
 
+def is_interactive():
+    return sys.stdin.isatty()
+
+
 class _ArgvlistReader:
     @classmethod
     def getargvlist(cls, reader, value, replace=True):

tests/unit/test_config.py

@@ -2724,3 +2724,29 @@ def test_config_bad_config_type_specified(monkeypatch, tmpdir, capsys):
     msg = "\n".join(notes) + "\n"
     assert err == msg
     assert "ERROR:" not in out
+
+
+def test_interactive_na(newconfig, monkeypatch):
+    monkeypatch.setattr(tox.config, "is_interactive", lambda: False)
+    config = newconfig(
+        """
+        [testenv:py]
+        setenv = A = {tty:X:Y}
+        """
+    )
+    assert config.envconfigs["py"].setenv["A"] == "Y"
+
+
+def test_interactive_available(newconfig, monkeypatch):
+    monkeypatch.setattr(tox.config, "is_interactive", lambda: True)
+    config = newconfig(
+        """
+        [testenv:py]
+        setenv = A = {tty:X:Y}
+        """
+    )
+    assert config.envconfigs["py"].setenv["A"] == "X"
+
+
+def test_interactive():
+    tox.config.is_interactive()