Reduce unset script_env variable to warning (#5105)

conda_build/build.py

@@ -1919,9 +1919,12 @@ def bundle_conda(output, metadata: MetaData, env, stats, **kw):
                 val = var.split("=", 1)[1]
                 var = var.split("=", 1)[0]
             elif var not in os.environ:
-                raise ValueError(
-                    f"env var '{var}' specified in script_env, but is not set."
+                warnings.warn(
+                    "The environment variable '%s' specified in script_env is undefined."
+                    % var,
+                    UserWarning,
                 )
+                val = ""
             else:
                 val = os.environ[var]
             env_output[var] = val

conda_build/environ.py

@@ -491,7 +491,9 @@ def meta_vars(meta: MetaData, skip_build_id=False):
             value = os.getenv(var_name)
         if value is None:
             warnings.warn(
-                "The environment variable '%s' is undefined." % var_name, UserWarning
+                "The environment variable '%s' specified in script_env is undefined."
+                % var_name,
+                UserWarning,
             )
         else:
             d[var_name] = value

news/5105-script-env-warn

@@ -0,0 +1,20 @@
+### Enhancements
+
+* Relax script_env error in outputs when variable referenced in script_env is not defined.
+  This unifies current behavior with the top-level build. (#5105)
+
+### Bug fixes
+
+* <news item>
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* <news item>

tests/test-recipes/split-packages/_build_script_missing_var/meta.yaml

@@ -0,0 +1,9 @@
+package:
+  name: test_build_script_in_output
+  version: 1.0
+
+outputs:
+  - name: test_1
+    build:
+      script_env:
+        - TEST_FN_DOESNT_EXIST

tests/test_subpackages.py

@@ -335,6 +335,16 @@ def test_build_script_and_script_env(testing_config):
     api.build(recipe, config=testing_config)
 
 
+@pytest.mark.sanity
+def test_build_script_and_script_env_warn_empty_script_env(testing_config):
+    recipe = os.path.join(subpackage_dir, "_build_script_missing_var")
+    with pytest.warns(
+        UserWarning,
+        match="The environment variable 'TEST_FN_DOESNT_EXIST' specified in script_env is undefined",
+    ):
+        api.build(recipe, config=testing_config)
+
+
 @pytest.mark.sanity
 @pytest.mark.skipif(sys.platform != "darwin", reason="only implemented for mac")
 def test_strong_run_exports_from_build_applies_to_host(testing_config):