don't allow --num-envs >1 with no --env (#4203)

* don't allow --num-envs >1 with no --env

* changelog

* PR feedback

com.unity.ml-agents/CHANGELOG.md

 ### Minor Changes
 
 ### Bug Fixes
+`mlagents-learn` will now raise an error immediately if `--num-envs` is greater than 1 without setting the `--env`
+argument. (#4203)
 
 ## [1.2.0-preview] - 2020-07-15
 

ml-agents/mlagents/trainers/settings.py

     env_path: Optional[str] = parser.get_default("env_path")
     env_args: Optional[List[str]] = parser.get_default("env_args")
     base_port: int = parser.get_default("base_port")
-    num_envs: int = parser.get_default("num_envs")
+    num_envs: int = attr.ib(default=parser.get_default("num_envs"))
+
+    @num_envs.validator
+    def validate_num_envs(self, attribute, value):
+        if value > 1 and self.env_path is None:
+            raise ValueError("num_envs must be 1 if env_path is not set.")
 
 
 @attr.s(auto_attribs=True)

ml-agents/mlagents/trainers/tests/test_settings.py

     RewardSignalType,
     RewardSignalSettings,
     CuriositySettings,
+    EnvironmentSettings,
     EnvironmentParameterSettings,
     ConstantSettings,
     UniformSettings,
     check_dict_is_at_least(second_export, dict_export)
     # Check that the two exports are the same
     assert dict_export == second_export
+
+
+def test_environment_settings():
+    # default args
+    EnvironmentSettings()
+
+    # 1 env is OK if no env_path
+    EnvironmentSettings(num_envs=1)
+
+    # multiple envs is OK if env_path is set
+    EnvironmentSettings(num_envs=42, env_path="/foo/bar.exe")
+
+    # Multiple environments with no env_path is an error
+    with pytest.raises(ValueError):
+        EnvironmentSettings(num_envs=2)