fix: raise chained errors with "from" #998

This makes exceptions report their causes correctly, as "The above exception was
the direct cause of the following exception" instead of "During handling of the
above exception, another exception occurred."

coverage/collector.py

@@ -137,12 +137,12 @@ def __init__(
                 self.threading = threading
             else:
                 raise CoverageException(f"Don't understand concurrency={concurrency}")
-        except ImportError:
+        except ImportError as ex:
             raise CoverageException(
                 "Couldn't trace with concurrency={}, the module isn't installed.".format(
                     self.concurrency,
                 )
-            )
+            ) from ex
 
         self.reset()
 
@@ -318,8 +318,8 @@ def start(self):
             (frame, event, arg), lineno = args
             try:
                 fn(frame, event, arg, lineno=lineno)
-            except TypeError:
-                raise Exception("fullcoverage must be run with the C trace function.")
+            except TypeError as ex:
+                raise Exception("fullcoverage must be run with the C trace function.") from ex
 
         # Install our installation tracer in threading, to jump-start other
         # threads.

coverage/config.py

@@ -125,7 +125,7 @@ def getregexlist(self, section, option):
             except re.error as e:
                 raise CoverageException(
                     f"Invalid [{section}].{option} value {value!r}: {e}"
-                )
+                ) from e
             if value:
                 value_list.append(value)
         return value_list
@@ -272,7 +272,7 @@ def from_file(self, filename, our_file):
         try:
             files_read = cp.read(filename)
         except (configparser.Error, TomlDecodeError) as err:
-            raise CoverageException(f"Couldn't read config file {filename}: {err}")
+            raise CoverageException(f"Couldn't read config file {filename}: {err}") from err
         if not files_read:
             return False
 
@@ -285,7 +285,7 @@ def from_file(self, filename, our_file):
                 if was_set:
                     any_set = True
         except ValueError as err:
-            raise CoverageException(f"Couldn't read config file {filename}: {err}")
+            raise CoverageException(f"Couldn't read config file {filename}: {err}") from err
 
         # Check that there are no unrecognized options.
         all_options = collections.defaultdict(set)

coverage/execfile.py

@@ -42,7 +42,7 @@ def find_module(modulename):
     try:
         spec = importlib.util.find_spec(modulename)
     except ImportError as err:
-        raise NoSource(str(err))
+        raise NoSource(str(err)) from err
     if not spec:
         raise NoSource(f"No module named {modulename!r}")
     pathname = spec.origin
@@ -193,7 +193,7 @@ def run(self):
             raise
         except Exception as exc:
             msg = "Couldn't run '{filename}' as Python code: {exc.__class__.__name__}: {exc}"
-            raise CoverageException(msg.format(filename=self.arg0, exc=exc))
+            raise CoverageException(msg.format(filename=self.arg0, exc=exc)) from exc
 
         # Execute the code object.
         # Return to the original directory in case the test code exits in
@@ -226,7 +226,7 @@ def run(self):
                 sys.excepthook(typ, err, tb.tb_next)
             except SystemExit:                      # pylint: disable=try-except-raise
                 raise
-            except Exception:
+            except Exception as exc:
                 # Getting the output right in the case of excepthook
                 # shenanigans is kind of involved.
                 sys.stderr.write("Error in sys.excepthook:\n")
@@ -236,7 +236,7 @@ def run(self):
                     err2.__traceback__ = err2.__traceback__.tb_next
                 sys.__excepthook__(typ2, err2, tb2.tb_next)
                 sys.stderr.write("\nOriginal exception was:\n")
-                raise ExceptionDuringRun(typ, err, tb.tb_next)
+                raise ExceptionDuringRun(typ, err, tb.tb_next) from exc
             else:
                 sys.exit(1)
         finally:
@@ -277,8 +277,8 @@ def make_code_from_py(filename):
     # Open the source file.
     try:
         source = get_python_source(filename)
-    except (OSError, NoSource):
-        raise NoSource("No file to run: '%s'" % filename)
+    except (OSError, NoSource) as exc:
+        raise NoSource("No file to run: '%s'" % filename) from exc
 
     code = compile_unicode(source, filename, "exec")
     return code
@@ -288,8 +288,8 @@ def make_code_from_pyc(filename):
     """Get a code object from a .pyc file."""
     try:
         fpyc = open(filename, "rb")
-    except OSError:
-        raise NoCode("No file to run: '%s'" % filename)
+    except OSError as exc:
+        raise NoCode("No file to run: '%s'" % filename) from exc
 
     with fpyc:
         # First four bytes are a version-specific magic number.  It has to

coverage/parser.py

@@ -41,9 +41,7 @@ def __init__(self, text=None, filename=None, exclude=None):
             try:
                 self.text = get_python_source(self.filename)
             except OSError as err:
-                raise NoSource(
-                    f"No source for code: '{self.filename}': {err}"
-                )
+                raise NoSource(f"No source for code: '{self.filename}': {err}") from err
 
         self.exclude = exclude
 
@@ -243,7 +241,7 @@ def parse_source(self):
                 "Couldn't parse '%s' as Python source: '%s' at line %d" % (
                     self.filename, err.args[0], lineno
                 )
-            )
+            ) from err
 
         self.excluded = self.first_lines(self.raw_excluded)
 
@@ -363,7 +361,7 @@ def __init__(self, text, code=None, filename=None):
                     "Couldn't parse '%s' as Python source: '%s' at line %d" % (
                         filename, synerr.msg, synerr.lineno
                     )
-                )
+                ) from synerr
 
         # Alternative Python implementations don't always provide all the
         # attributes on code objects that we need to do the analysis.

coverage/sqldata.py

@@ -291,7 +291,7 @@ def _read_db(self):
                     "Data file {!r} doesn't seem to be a coverage data file: {}".format(
                         self._filename, exc
                     )
-                )
+                ) from exc
             else:
                 if schema_version != SCHEMA_VERSION:
                     raise CoverageException(
@@ -1095,7 +1095,7 @@ def execute(self, sql, parameters=()):
                 pass
             if self.debug:
                 self.debug.write(f"EXCEPTION from execute: {msg}")
-            raise CoverageException(f"Couldn't use data file {self.filename!r}: {msg}")
+            raise CoverageException(f"Couldn't use data file {self.filename!r}: {msg}") from exc
 
     def execute_one(self, sql, parameters=()):
         """Execute a statement and return the one row that results.

coverage/templite.py

@@ -288,10 +288,10 @@ def _do_dots(self, value, *dots):
             except AttributeError:
                 try:
                     value = value[dot]
-                except (TypeError, KeyError):
+                except (TypeError, KeyError) as exc:
                     raise TempliteValueError(
                         f"Couldn't evaluate {value!r}.{dot}"
-                    )
+                    ) from exc
             if callable(value):
                 value = value()
         return value

coverage/tomlconfig.py

@@ -49,7 +49,7 @@ def read(self, filenames):
             try:
                 self.data = tomli.loads(toml_text)
             except tomli.TOMLDecodeError as err:
-                raise TomlDecodeError(str(err))
+                raise TomlDecodeError(str(err)) from err
             return [filename]
         else:
             has_toml = re.search(r"^\[tool\.coverage\.", toml_text, flags=re.MULTILINE)
@@ -95,8 +95,8 @@ def _get(self, section, option):
             raise configparser.NoSectionError(section)
         try:
             return name, data[option]
-        except KeyError:
-            raise configparser.NoOptionError(option, name)
+        except KeyError as exc:
+            raise configparser.NoOptionError(option, name) from exc
 
     def has_option(self, section, option):
         _, data = self._get_section(section)
@@ -149,7 +149,7 @@ def getregexlist(self, section, option):
             except re.error as e:
                 raise CoverageException(
                     f"Invalid [{name}].{option} value {value!r}: {e}"
-                )
+                ) from e
         return values
 
     def getint(self, section, option):

setup.py

@@ -161,21 +161,21 @@ def run(self):
         """Wrap `run` with `BuildFailed`."""
         try:
             build_ext.run(self)
-        except errors.DistutilsPlatformError:
-            raise BuildFailed()
+        except errors.DistutilsPlatformError as exc:
+            raise BuildFailed() from exc
 
     def build_extension(self, ext):
         """Wrap `build_extension` with `BuildFailed`."""
         try:
             # Uncomment to test compile failure handling:
             #   raise errors.CCompilerError("OOPS")
             build_ext.build_extension(self, ext)
-        except ext_errors:
-            raise BuildFailed()
+        except ext_errors as exc:
+            raise BuildFailed() from exc
         except ValueError as err:
             # this can happen on Windows 64 bit, see Python issue 7511
             if "'path'" in str(err):    # works with both py 2/3
-                raise BuildFailed()
+                raise BuildFailed() from err
             raise
 
 # There are a few reasons we might not be able to compile the C extension.