Improve missing cassette errors (#583)

* Improve missing cassette errors

When the freeze file or the cassette file is missing, we get pretty
werid errors. Let's try to make them more explicit.

* Add back fixture with a comment.

* Move check in freezer

Author: therve

tests/conftest.py

@@ -62,10 +62,7 @@ def pytest_bdd_before_step(request, feature, scenario, step, step_func):
 
     context = tracer.get_call_context()
     span = tracer.start_span(
-        step.type,
-        resource=step.name,
-        span_type=step.type,
-        child_of=context,
+        step.type, resource=step.name, span_type=step.type, child_of=context,
     )
     setattr(step_func, "__dd_span__", span)
 
@@ -211,11 +208,9 @@ def context(vcr, unique, unique_lower, freezer):
 @pytest.fixture(scope="session")
 def record_mode(request):
     """Manage compatibility with DD client libraries."""
-    return {
-        "false": "none",
-        "true": "rewrite",
-        "none": "new_episodes",
-    }[os.getenv("RECORD", "false").lower()]
+    return {"false": "none", "true": "rewrite", "none": "new_episodes",}[
+        os.getenv("RECORD", "false").lower()
+    ]
 
 
 def _disable_recording():
@@ -225,7 +220,7 @@ def _disable_recording():
 
 @pytest.fixture(scope="session")
 def disable_recording(request):
-    """Disable VCR.py integration."""
+    """Disable VCR.py integration. This overrides a pytest-recording fixture."""
     return _disable_recording()
 
 
@@ -257,9 +252,23 @@ def freezer(default_cassette_name, record_mode, vcr):
             with pathlib.Path(vcr._path).with_suffix(".frozen").open("w+") as f:
                 f.write(freeze_at)
     else:
-        with pathlib.Path(vcr._path).with_suffix(".frozen").open("r") as f:
+        freeze_file = pathlib.Path(vcr._path).with_suffix(".frozen")
+        if not freeze_file.exists():
+            msg = (
+                "Time file '{}' not found: create one setting `RECORD=true` or "
+                "ignore it using `RECORD=none`".format(freeze_file)
+            )
+            raise RuntimeError(msg)
+        with freeze_file.open("r") as f:
             freeze_at = f.readline().strip()
 
+        if not pathlib.Path(vcr._path).exists():
+            msg = (
+                "Cassette '{}' not found: create one setting `RECORD=true` or "
+                "ignore it using `RECORD=none`".format(vcr._path)
+            )
+            raise RuntimeError(msg)
+
     return freeze_time(parser.isoparse(freeze_at))