fix: don't route specced Mocks through str/dict tracing

changelog/796.bugfix.rst

@@ -0,0 +1,5 @@
+Fixed a crash in hook tracing when a specced ``unittest.mock.Mock``
+(``Mock(spec=str)`` or ``Mock(spec=dict)``) was passed as a hook argument
+or returned as a first-result value: the tracer's ``isinstance`` checks
+routed the mock through the string/dict special-casing and raised
+``TypeError`` instead of rendering it.

src/pluggy/_tracing.py

@@ -63,7 +63,7 @@ def _render_value(obj: object) -> str:
     # Before the str check: a StrEnum member is a str whose str() drops the name.
     if isinstance(obj, (enum.Enum, os.PathLike)):
         return _safe_repr(obj)
-    if isinstance(obj, str):
+    if type(obj) is str:
         if len(obj.splitlines()) > 1:
             return _safe_str(obj)
         if _is_plain_token(obj):
@@ -94,7 +94,7 @@ def get(self, name: str) -> TagTracerSub:
         return TagTracerSub(self, (name,))
 
     def _format_message(self, tags: Sequence[str], args: Sequence[object]) -> str:
-        if isinstance(args[-1], dict):
+        if type(args[-1]) is dict:
             extra = args[-1]
             args = args[:-1]
         else:

testing/test_tracer.py

@@ -1,6 +1,7 @@
 import enum
 import os
 import pathlib
+from unittest.mock import Mock
 
 import pytest
 
@@ -164,6 +165,41 @@ def hello_again(self, arg):
     ]
 
 
+@pytest.mark.parametrize("spec", [str, dict])
+def test_tracing_specced_mock_does_not_raise(pm: PluginManager, spec: type) -> None:
+    """A specced ``Mock`` must not be routed through the str/dict special-casing.
+
+    ``Mock(spec=str)`` / ``Mock(spec=dict)`` report their spec through
+    ``__class__``, so ``isinstance`` would send them down the string
+    formatting or dict metadata path and raise ``TypeError``. See #792.
+    """
+
+    class Api:
+        @hookspec(firstresult=True)
+        def echo(self, value: object) -> None:
+            "return the value"
+
+    pm.add_hookspecs(Api)
+    test_hc = pm.hook.echo
+
+    class Plugin:
+        @hookimpl
+        def echo(self, value):
+            return value
+
+    value = Mock(spec=spec)
+    pm.register(Plugin())
+
+    trace_out: list[str] = []
+    pm.trace.root.setwriter(trace_out.append)
+    pm.enable_tracing()
+
+    out = test_hc(value=value)
+    assert out is value
+    assert "echo [hook]" in "".join(trace_out)
+    assert "finish echo" in "".join(trace_out)
+
+
 class BrokenRepr:
     def __repr__(self) -> str:
         raise RuntimeError("repr is broken")