fix: defer snapshot default extension import (#734)

* test: add coverage for bug #719

* fix: defer snapshot default extension import

Fixes unable to use pytest's `pythonpath` option with this project's
`--snapshot-default-extension` option. Does cause extension import
errors to raise later than CLI argument parsing, and therefore emit on
stdout, instead of stderr.

---------

Co-authored-by: Noah Negin-Ulster <noah.negin-ulster@tophatmonocle.com>

src/syrupy/__init__.py

@@ -41,11 +41,6 @@ def __import_extension(value: Optional[str]) -> Any:
         raise argparse.ArgumentTypeError(e)
 
 
-def __default_extension_option(value: Optional[str]) -> Any:
-    __import_extension(value)
-    return value
-
-
 def pytest_addoption(parser: Any) -> None:
     """
     Exposes snapshot plugin configuration to pytest.
@@ -78,7 +73,6 @@ def pytest_addoption(parser: Any) -> None:
     # all pytest options to be serializable.
     group.addoption(
         "--snapshot-default-extension",
-        type=__default_extension_option,
         default=None,
         dest="default_extension",
         help="Specify the default snapshot extension",

src/syrupy/utils.py

@@ -48,11 +48,14 @@ def import_module_member(path: str) -> Any:
             )
         )
     try:
-        return getattr(import_module(module_name), module_member_name)
+        module = import_module(module_name)
     except ModuleNotFoundError:
         raise FailedToLoadModuleMember(
             gettext("Module '{}' does not exist.").format(module_name)
         )
+
+    try:
+        return getattr(module, module_member_name)
     except AttributeError:
         raise FailedToLoadModuleMember(
             gettext("Member '{}' not found in module '{}'.").format(

tests/integration/test_snapshot_option_extension.py

@@ -37,12 +37,7 @@ def test_snapshot_default_extension_option_failure(testfile):
         "--snapshot-default-extension",
         "syrupy.extensions.amber.DoesNotExistExtension",
     )
-    result.stderr.re_match_lines(
-        (
-            r".*error: argument --snapshot-default-extension"
-            r": Member 'DoesNotExistExtension' not found.*",
-        )
-    )
+    result.stdout.re_match_lines((r".*: Member 'DoesNotExistExtension' not found.*",))
     assert not Path(
         testfile.tmpdir, "__snapshots__", "test_file", "test_default.raw"
     ).exists()

tests/integration/test_snapshot_option_extension_pythonpath.py

@@ -0,0 +1,101 @@
+import textwrap
+from pathlib import Path
+
+import pytest
+
+import syrupy
+
+SUBDIR = "subdir_not_on_default_path"
+
+
+@pytest.fixture(autouse=True)
+def cache_clear():
+    syrupy.__import_extension.cache_clear()
+
+
+@pytest.fixture
+def testfile(pytester):
+    subdir = pytester.mkpydir(SUBDIR)
+
+    Path(
+        subdir,
+        "extension_file.py",
+    ).write_text(
+        data=textwrap.dedent(
+            """
+            from syrupy.extensions.single_file import SingleFileSnapshotExtension
+            class MySingleFileExtension(SingleFileSnapshotExtension):
+                pass
+            """
+        ),
+        encoding="utf-8",
+    )
+
+    pytester.makepyfile(
+        test_file=(
+            """
+            def test_default(snapshot):
+                assert b"default extension serializer" == snapshot
+            """
+        )
+    )
+
+    return pytester
+
+
+def test_snapshot_default_extension_option_success(testfile):
+    testfile.makeini(
+        f"""
+        [pytest]
+        pythonpath =
+            {Path(testfile.path, SUBDIR).as_posix()}
+    """
+    )
+
+    result = testfile.runpytest(
+        "-v",
+        "--snapshot-update",
+        "--snapshot-default-extension",
+        "extension_file.MySingleFileExtension",
+    )
+    result.stdout.re_match_lines((r"1 snapshot generated\."))
+    assert Path(
+        testfile.path, "__snapshots__", "test_file", "test_default.raw"
+    ).exists()
+    assert not result.ret
+
+
+def test_snapshot_default_extension_option_module_not_found(testfile):
+    result = testfile.runpytest(
+        "-v",
+        "--snapshot-update",
+        "--snapshot-default-extension",
+        "extension_file.MySingleFileExtension",
+    )
+    result.stdout.re_match_lines((r".*: Module 'extension_file' does not exist.*",))
+    assert not Path(
+        testfile.path, "__snapshots__", "test_file", "test_default.raw"
+    ).exists()
+    assert result.ret
+
+
+def test_snapshot_default_extension_option_failure(testfile):
+    testfile.makeini(
+        f"""
+        [pytest]
+        pythonpath =
+            {Path(testfile.path, SUBDIR).as_posix()}
+    """
+    )
+
+    result = testfile.runpytest(
+        "-v",
+        "--snapshot-update",
+        "--snapshot-default-extension",
+        "extension_file.DoesNotExistExtension",
+    )
+    result.stdout.re_match_lines((r".*: Member 'DoesNotExistExtension' not found.*",))
+    assert not Path(
+        testfile.path, "__snapshots__", "test_file", "test_default.raw"
+    ).exists()
+    assert result.ret