feat!: emit warning that StrictUndefined will be enabled by default in the future

copier/_main.py

@@ -710,6 +710,14 @@ def jinja_env(self) -> SandboxedEnvironment:
                     f"Unsupported envops.undefined value specified: {undefined_class}.\n"
                     'Supported values are "jinja2.Undefined" and "jinja2.StrictUndefined".'
                 )
+        else:
+            warnings.warn(
+                "Copier does not detect undefined variables in templates unless `undefined: jinja2.StrictUndefined` is specified. "
+                "Consider enabling StrictUndefined to find undefined variables in your templates. "
+                "This may be enabled by default in the future."
+                "To silence this warning, set `undefined: jinja2.Undefined` in your template's _envops.",
+                FutureWarning,
+            )
         try:
             env = SandboxedEnvironment(loader=loader, extensions=extensions, **envops)
         except ModuleNotFoundError as error:

tests/test_dynamic_file_structures.py

@@ -20,6 +20,7 @@ def test_folder_loop(tmp_path_factory: pytest.TempPathFactory) -> None:
     )
     with warnings.catch_warnings():
         warnings.simplefilter("error")
+        warnings.filterwarnings("ignore", message=".*jinja2.StrictUndefined.*")
         copier.run_copy(
             str(src),
             dst,
@@ -56,6 +57,7 @@ def test_nested_folder_loop(tmp_path_factory: pytest.TempPathFactory) -> None:
     )
     with warnings.catch_warnings():
         warnings.simplefilter("error")
+        warnings.filterwarnings("ignore", message=".*jinja2.StrictUndefined.*")
         copier.run_copy(
             str(src),
             dst,
@@ -95,6 +97,7 @@ def test_file_loop(tmp_path_factory: pytest.TempPathFactory) -> None:
     )
     with warnings.catch_warnings():
         warnings.simplefilter("error")
+        warnings.filterwarnings("ignore", message=".*jinja2.StrictUndefined.*")
         copier.run_copy(
             str(src),
             dst,
@@ -153,6 +156,7 @@ def test_folder_loop_dict_items(tmp_path_factory: pytest.TempPathFactory) -> Non
 
     with warnings.catch_warnings():
         warnings.simplefilter("error")
+        warnings.filterwarnings("ignore", message=".*jinja2.StrictUndefined.*")
 
         copier.run_copy(
             str(src),

tests/test_jinja2_undefined.py

@@ -1,3 +1,5 @@
+from pathlib import Path
+
 import pytest
 from jinja2.exceptions import UndefinedError
 
@@ -80,3 +82,39 @@ def test_strictundefined_undefined_variable(
 
     with pytest.raises(UndefinedError, match="'undefined_variable' is undefined"):
         copier.run_copy(str(src), dst)
+
+
+def test_futurewarning(tmp_path: Path) -> None:
+    with pytest.warns(
+        FutureWarning,
+        match="Copier does not detect undefined variables in templates unless `undefined: jinja2.StrictUndefined` is specified.",
+    ):
+        copier.run_copy("./tests/demo_config_empty", tmp_path)
+
+
+# fail on warnings
+@pytest.mark.filterwarnings("error")
+def test_silenced_warning(
+    tmp_path_factory: pytest.TempPathFactory,
+) -> None:
+    src, dst = map(tmp_path_factory.mktemp, ("src", "dst"))
+
+    build_file_tree(
+        {
+            (src / ".copier-answers.yml.jinja"): (
+                """\
+                # Changes here will be overwritten by Copier
+                {{ _copier_answers|to_nice_yaml }}
+                """
+            ),
+            (src / "copier.yaml"): (
+                """\
+                _envops:
+                    undefined: jinja2.Undefined
+                """
+            ),
+            (src / "test.jinja"): "{{ undefined_variable }}",
+        }
+    )
+
+    copier.run_copy(str(src), dst)

tests/test_normal_jinja2.py

@@ -24,6 +24,7 @@ def test_normal_jinja2(tmp_path_factory: pytest.TempPathFactory) -> None:
                     comment_end_string: "#}"
                     lstrip_blocks: true
                     trim_blocks: true
+                    undefined: jinja2.Undefined
                 name: Guybrush
                 todo: Become a pirate
                 """
@@ -60,6 +61,7 @@ def test_to_not_keep_trailing_newlines_in_jinja2(
                 _templates_suffix: .jinja
                 _envops:
                     keep_trailing_newline: false
+                    undefined: jinja2.Undefined
                 data: foo
                 """
             ),

tests/test_output.py

@@ -111,6 +111,8 @@ def test_messages_with_inline_text(
                 _message_after_copy: Project {{ project_name }} successfully created
                 _message_before_update: Updating on {{ _copier_conf.os }}
                 _message_after_update: Project {{ project_name }} successfully updated
+                _envops:
+                    undefined: jinja2.Undefined
                 """
             ),
             (src / "{{ _copier_conf.answers_file }}.jinja"): (
@@ -231,6 +233,8 @@ def test_messages_with_included_text(
                 _message_after_copy: "{% include 'message_after_copy.md.jinja' %}"
                 _message_before_update: "{% include 'message_before_update.md.jinja' %}"
                 _message_after_update: "{% include 'message_after_update.md.jinja' %}"
+                _envops:
+                    undefined: jinja2.Undefined
                 """
             ),
             (src / "message_before_copy.md.jinja"): (

tests/test_templated_prompt.py

@@ -17,7 +17,6 @@
 
 from .helpers import (
     BRACKET_ENVOPS,
-    BRACKET_ENVOPS_JSON,
     COPIER_PATH,
     SUFFIX_TMPL,
     Keyboard,
@@ -227,12 +226,15 @@ def test_templated_prompt_custom_envops(
 
 def test_templated_prompt_builtins(tmp_path_factory: pytest.TempPathFactory) -> None:
     src, dst = map(tmp_path_factory.mktemp, ("src", "dst"))
+    envops = BRACKET_ENVOPS.copy()
+    # suppress FutureWarning about StrictUndefined
+    envops["undefined"] = "jinja2.Undefined"
     build_file_tree(
         {
             (src / "copier.yaml"): (
                 f"""\
                 _templates_suffix: {SUFFIX_TMPL}
-                _envops: {BRACKET_ENVOPS_JSON}
+                _envops: {json.dumps(envops)}
                 question1:
                     default: "[[ now() ]]"
                 question2: