Enhance errors for exception/warnings matching (#8508)

Co-authored-by: Florian Bruhin <me@the-compiler.org>

changelog/8508.improvement.rst

@@ -0,0 +1,2 @@
+Introduce multiline display for warning matching  via :py:func:`pytest.warns` and
+enhance match comparison for :py:func:`_pytest._code.ExceptionInfo.match` as returned by :py:func:`pytest.raises`.

src/_pytest/_code/code.py

@@ -672,10 +672,11 @@ class ExceptionInfo(Generic[E]):
         If it matches `True` is returned, otherwise an `AssertionError` is raised.
         """
         __tracebackhide__ = True
-        msg = "Regex pattern {!r} does not match {!r}."
+        value = str(self.value)
-        if regexp == str(self.value):
+        msg = f"Regex pattern did not match.\n Regex: {regexp!r}\n Input: {value!r}"
-            msg += " Did you mean to `re.escape()` the regex?"
+        if regexp == value:
-        assert re.search(regexp, str(self.value)), msg.format(regexp, str(self.value))
+            msg += "\n Did you mean to `re.escape()` the regex?"
+        assert re.search(regexp, value), msg
         # Return True to allow for "assert excinfo.match()".
         return True
 

src/_pytest/recwarn.py

@@ -1,6 +1,7 @@
 """Record warnings during test function execution."""
 import re
 import warnings
+from pprint import pformat
 from types import TracebackType
 from typing import Any
 from typing import Callable
@@ -142,10 +143,11 @@ def warns(
     __tracebackhide__ = True
     if not args:
         if kwargs:
-            msg = "Unexpected keyword arguments passed to pytest.warns: "
+            argnames = ", ".join(sorted(kwargs))
-            msg += ", ".join(sorted(kwargs))
+            raise TypeError(
-            msg += "\nUse context-manager form instead?"
+                f"Unexpected keyword arguments passed to pytest.warns: {argnames}"
-            raise TypeError(msg)
+                "\nUse context-manager form instead?"
+            )
         return WarningsChecker(expected_warning, match_expr=match, _ispytest=True)
     else:
         func = args[0]
@@ -191,7 +193,7 @@ class WarningsRecorder(warnings.catch_warnings):
             if issubclass(w.category, cls):
                 return self._list.pop(i)
         __tracebackhide__ = True
-        raise AssertionError("%r not found in warning list" % cls)
+        raise AssertionError(f"{cls!r} not found in warning list")
 
     def clear(self) -> None:
         """Clear the list of recorded warnings."""
@@ -202,7 +204,7 @@ class WarningsRecorder(warnings.catch_warnings):
     def __enter__(self) -> "WarningsRecorder":  # type: ignore
         if self._entered:
             __tracebackhide__ = True
-            raise RuntimeError("Cannot enter %r twice" % self)
+            raise RuntimeError(f"Cannot enter {self!r} twice")
         _list = super().__enter__()
         # record=True means it's None.
         assert _list is not None
@@ -218,7 +220,7 @@ class WarningsRecorder(warnings.catch_warnings):
     ) -> None:
         if not self._entered:
             __tracebackhide__ = True
-            raise RuntimeError("Cannot exit %r without entering first" % self)
+            raise RuntimeError(f"Cannot exit {self!r} without entering first")
 
         super().__exit__(exc_type, exc_val, exc_tb)
 
@@ -268,16 +270,17 @@ class WarningsChecker(WarningsRecorder):
 
         __tracebackhide__ = True
 
+        def found_str():
+            return pformat([record.message for record in self], indent=2)
+
         # only check if we're not currently handling an exception
         if exc_type is None and exc_val is None and exc_tb is None:
             if self.expected_warning is not None:
                 if not any(issubclass(r.category, self.expected_warning) for r in self):
                     __tracebackhide__ = True
                     fail(
-                        "DID NOT WARN. No warnings of type {} were emitted. "
+                        f"DID NOT WARN. No warnings of type {self.expected_warning} were emitted.\n"
-                        "The list of emitted warnings is: {}.".format(
+                        f"The list of emitted warnings is: {found_str()}."
-                            self.expected_warning, [each.message for each in self]
-                        )
                     )
                 elif self.match_expr is not None:
                     for r in self:
@@ -286,11 +289,8 @@ class WarningsChecker(WarningsRecorder):
                                 break
                     else:
                         fail(
-                            "DID NOT WARN. No warnings of type {} matching"
+                            f"""\
-                            " ('{}') were emitted. The list of emitted warnings"
+DID NOT WARN. No warnings of type {self.expected_warning} matching the regex were emitted.
-                            " is: {}.".format(
+ Regex: {self.match_expr}
-                                self.expected_warning,
+ Emitted warnings: {found_str()}"""
-                                self.match_expr,
-                                [each.message for each in self],
-                            )
                         )

testing/code/test_excinfo.py

@@ -420,18 +420,20 @@ def test_match_raises_error(pytester: Pytester) -> None:
             excinfo.match(r'[123]+')
     """
     )
-    result = pytester.runpytest()
+    result = pytester.runpytest("--tb=short")
     assert result.ret != 0
 
-    exc_msg = "Regex pattern '[[]123[]]+' does not match 'division by zero'."
+    match = [
-    result.stdout.fnmatch_lines([f"E * AssertionError: {exc_msg}"])
+        r"E .* AssertionError: Regex pattern did not match.",
+        r"E .* Regex: '\[123\]\+'",
+        r"E .* Input: 'division by zero'",
+    ]
+    result.stdout.re_match_lines(match)
     result.stdout.no_fnmatch_line("*__tracebackhide__ = True*")
 
     result = pytester.runpytest("--fulltrace")
     assert result.ret != 0
-    result.stdout.fnmatch_lines(
+    result.stdout.re_match_lines([r".*__tracebackhide__ = True.*", *match])
-        ["*__tracebackhide__ = True*", f"E * AssertionError: {exc_msg}"]
-    )
 
 
 class TestFormattedExcinfo:

testing/python/raises.py

@@ -191,10 +191,12 @@ class TestRaises:
             int("asdf")
 
         msg = "with base 16"
-        expr = "Regex pattern {!r} does not match \"invalid literal for int() with base 10: 'asdf'\".".format(
+        expr = (
-            msg
+            "Regex pattern did not match.\n"
+            f" Regex: {msg!r}\n"
+            " Input: \"invalid literal for int() with base 10: 'asdf'\""
         )
-        with pytest.raises(AssertionError, match=re.escape(expr)):
+        with pytest.raises(AssertionError, match="(?m)" + re.escape(expr)):
             with pytest.raises(ValueError, match=msg):
                 int("asdf", base=10)
 
@@ -217,7 +219,7 @@ class TestRaises:
             with pytest.raises(AssertionError, match="'foo"):
                 raise AssertionError("'bar")
         (msg,) = excinfo.value.args
-        assert msg == 'Regex pattern "\'foo" does not match "\'bar".'
+        assert msg == '''Regex pattern did not match.\n Regex: "'foo"\n Input: "'bar"'''
 
     def test_match_failure_exact_string_message(self):
         message = "Oh here is a message with (42) numbers in parameters"
@@ -226,9 +228,10 @@ class TestRaises:
                 raise AssertionError(message)
         (msg,) = excinfo.value.args
         assert msg == (
-            "Regex pattern 'Oh here is a message with (42) numbers in "
+            "Regex pattern did not match.\n"
-            "parameters' does not match 'Oh here is a message with (42) "
+            " Regex: 'Oh here is a message with (42) numbers in parameters'\n"
-            "numbers in parameters'. Did you mean to `re.escape()` the regex?"
+            " Input: 'Oh here is a message with (42) numbers in parameters'\n"
+            " Did you mean to `re.escape()` the regex?"
         )
 
     def test_raises_match_wrong_type(self):

testing/test_recwarn.py

@@ -1,4 +1,3 @@
-import re
 import warnings
 from typing import Optional
 
@@ -263,7 +262,7 @@ class TestWarns:
             with pytest.warns(RuntimeWarning):
                 warnings.warn("user", UserWarning)
         excinfo.match(
-            r"DID NOT WARN. No warnings of type \(.+RuntimeWarning.+,\) were emitted. "
+            r"DID NOT WARN. No warnings of type \(.+RuntimeWarning.+,\) were emitted.\n"
             r"The list of emitted warnings is: \[UserWarning\('user',?\)\]."
         )
 
@@ -271,15 +270,15 @@ class TestWarns:
             with pytest.warns(UserWarning):
                 warnings.warn("runtime", RuntimeWarning)
         excinfo.match(
-            r"DID NOT WARN. No warnings of type \(.+UserWarning.+,\) were emitted. "
+            r"DID NOT WARN. No warnings of type \(.+UserWarning.+,\) were emitted.\n"
-            r"The list of emitted warnings is: \[RuntimeWarning\('runtime',?\)\]."
+            r"The list of emitted warnings is: \[RuntimeWarning\('runtime',?\)]."
         )
 
         with pytest.raises(pytest.fail.Exception) as excinfo:
             with pytest.warns(UserWarning):
                 pass
         excinfo.match(
-            r"DID NOT WARN. No warnings of type \(.+UserWarning.+,\) were emitted. "
+            r"DID NOT WARN. No warnings of type \(.+UserWarning.+,\) were emitted.\n"
             r"The list of emitted warnings is: \[\]."
         )
 
@@ -289,18 +288,14 @@ class TestWarns:
                 warnings.warn("runtime", RuntimeWarning)
                 warnings.warn("import", ImportWarning)
 
-        message_template = (
+        messages = [each.message for each in warninfo]
-            "DID NOT WARN. No warnings of type {0} were emitted. "
+        expected_str = (
-            "The list of emitted warnings is: {1}."
+            f"DID NOT WARN. No warnings of type {warning_classes} were emitted.\n"
-        )
+            f"The list of emitted warnings is: {messages}."
-        excinfo.match(
-            re.escape(
-                message_template.format(
-                    warning_classes, [each.message for each in warninfo]
-                )
-            )
         )
 
+        assert str(excinfo.value) == expected_str
+
     def test_record(self) -> None:
         with pytest.warns(UserWarning) as record:
             warnings.warn("user", UserWarning)