Fix exception causes all over the codebase

mypy/build.py

@@ -1988,13 +1988,13 @@ def parse_file(self) -> None:
                     raise CompileError([
                         "mypy: can't read file '{}': {}".format(
                             self.path, os.strerror(ioerr.errno))],
-                        module_with_blocker=self.id)
+                        module_with_blocker=self.id) from ioerr
                 except (UnicodeDecodeError, DecodeError) as decodeerr:
                     if self.path.endswith('.pyd'):
                         err = "mypy: stubgen does not support .pyd files: '{}'".format(self.path)
                     else:
                         err = "mypy: can't decode file '{}': {}".format(self.path, str(decodeerr))
-                    raise CompileError([err], module_with_blocker=self.id)
+                    raise CompileError([err], module_with_blocker=self.id) from decodeerr
             else:
                 assert source is not None
                 self.source_hash = compute_hash(source)

mypy/dmypy/client.py

@@ -534,8 +534,8 @@ def read_status(status_file: str) -> Dict[str, object]:
     with open(status_file) as f:
         try:
             data = json.load(f)
-        except Exception:
-            raise BadStatus("Malformed status file (not JSON)")
+        except Exception as e:
+            raise BadStatus("Malformed status file (not JSON)") from e
     if not isinstance(data, dict):
         raise BadStatus("Invalid status file (not a dict)")
     return data

mypy/dmypy_util.py

@@ -24,8 +24,8 @@ def receive(connection: IPCBase) -> Any:
         raise OSError("No data received")
     try:
         data = json.loads(bdata.decode('utf8'))
-    except Exception:
-        raise OSError("Data received is not valid JSON")
+    except Exception as e:
+        raise OSError("Data received is not valid JSON") from e
     if not isinstance(data, dict):
         raise OSError("Data received is not a dict (%s)" % str(type(data)))
     return data

mypy/ipc.py

@@ -109,7 +109,7 @@ def write(self, data: bytes) -> None:
                 assert err == 0, err
                 assert bytes_written == len(data)
             except WindowsError as e:
-                raise IPCException("Failed to write with error: {}".format(e.winerror))
+                raise IPCException("Failed to write with error: {}".format(e.winerror)) from e
         else:
             self.connection.sendall(data)
             self.connection.shutdown(socket.SHUT_WR)
@@ -131,11 +131,11 @@ def __init__(self, name: str, timeout: Optional[float]) -> None:
             timeout = int(self.timeout * 1000) if self.timeout else _winapi.NMPWAIT_WAIT_FOREVER
             try:
                 _winapi.WaitNamedPipe(self.name, timeout)
-            except FileNotFoundError:
-                raise IPCException("The NamedPipe at {} was not found.".format(self.name))
+            except FileNotFoundError as e:
+                raise IPCException("The NamedPipe at {} was not found.".format(self.name)) from e
             except WindowsError as e:
                 if e.winerror == _winapi.ERROR_SEM_TIMEOUT:
-                    raise IPCException("Timed out waiting for connection.")
+                    raise IPCException("Timed out waiting for connection.") from e
                 else:
                     raise
             try:
@@ -150,7 +150,7 @@ def __init__(self, name: str, timeout: Optional[float]) -> None:
                 )
             except WindowsError as e:
                 if e.winerror == _winapi.ERROR_PIPE_BUSY:
-                    raise IPCException("The connection is busy.")
+                    raise IPCException("The connection is busy.") from e
                 else:
                     raise
             _winapi.SetNamedPipeHandleState(self.connection,
@@ -237,8 +237,8 @@ def __enter__(self) -> 'IPCServer':
         else:
             try:
                 self.connection, _ = self.sock.accept()
-            except socket.timeout:
-                raise IPCException('The socket timed out')
+            except socket.timeout as e:
+                raise IPCException('The socket timed out') from e
         return self
 
     def __exit__(self,

mypy/main.py

@@ -192,10 +192,11 @@ def _python_executable_from_version(python_version: Tuple[int, int]) -> str:
                                           ['-c', 'import sys; print(sys.executable)'],
                                           stderr=subprocess.STDOUT).decode().strip()
         return sys_exe
-    except (subprocess.CalledProcessError, FileNotFoundError):
+    except (subprocess.CalledProcessError, FileNotFoundError) as e:
         raise PythonExecutableInferenceError(
             'failed to find a Python executable matching version {},'
-            ' perhaps try --python-executable, or --no-site-packages?'.format(python_version))
+            ' perhaps try --python-executable, or --no-site-packages?'.format(python_version)
+        ) from e
 
 
 def infer_python_executable(options: Options,

mypy/moduleinspect.py

@@ -44,7 +44,7 @@ def get_package_properties(package_id: str) -> ModuleProperties:
     try:
         package = importlib.import_module(package_id)
     except BaseException as e:
-        raise InspectError(str(e))
+        raise InspectError(str(e)) from e
     name = getattr(package, '__name__', None)
     file = getattr(package, '__file__', None)
     path = getattr(package, '__path__', None)  # type: Optional[List[str]]

mypy/stubgen.py

@@ -1191,7 +1191,7 @@ def collect_build_targets(options: Options, mypy_opts: MypyOptions) -> Tuple[Lis
         try:
             source_list = create_source_list(options.files, mypy_opts)
         except InvalidSourceList as e:
-            raise SystemExit(str(e))
+            raise SystemExit(str(e)) from e
         py_modules = [StubSource(m.module, m.path) for m in source_list]
         c_modules = []
 
@@ -1362,7 +1362,7 @@ def generate_asts_for_modules(py_modules: List[StubSource],
     try:
         res = build(list(py_modules), mypy_options)
     except CompileError as e:
-        raise SystemExit("Critical error during semantic analysis: {}".format(e))
+        raise SystemExit("Critical error during semantic analysis: {}".format(e)) from e
 
     for mod in py_modules:
         mod.ast = res.graph[mod.module].tree

mypy/stubtest.py

@@ -957,7 +957,7 @@ def build_stubs(modules: List[str], options: Options, find_submodules: bool = Fa
     except mypy.errors.CompileError as e:
         output = [_style("error: ", color="red", bold=True), "failed mypy compile.\n", str(e)]
         print("".join(output))
-        raise RuntimeError
+        raise RuntimeError from e
     if res.errors:
         output = [_style("error: ", color="red", bold=True), "failed mypy build.\n"]
         print("".join(output) + "\n".join(res.errors))

mypy/stubutil.py

@@ -91,7 +91,7 @@ def find_module_path_and_all_py2(module: str,
     except subprocess.CalledProcessError as e:
         path = find_module_path_using_py2_sys_path(module, interpreter)
         if path is None:
-            raise CantImport(module, str(e))
+            raise CantImport(module, str(e)) from e
         return path, None
     output = output_bytes.decode('ascii').strip().splitlines()
     module_path = output[0]
@@ -153,7 +153,7 @@ def find_module_path_and_all_py3(inspect: ModuleInspect,
         # Fall back to finding the module using sys.path.
         path = find_module_path_using_sys_path(module, sys.path)
         if path is None:
-            raise CantImport(module, str(e))
+            raise CantImport(module, str(e)) from e
         return path, None
     if mod.is_c_module:
         return None

mypy/suggestions.py

@@ -568,8 +568,8 @@ def find_node_by_file_and_line(self, file: str, line: int) -> Tuple[str, SymbolN
             raise SuggestionFailure('Source file is not a Python file')
         try:
             modname, _ = self.finder.crawl_up(os.path.normpath(file))
-        except InvalidSourceList:
-            raise SuggestionFailure('Invalid source file name: ' + file)
+        except InvalidSourceList as e:
+            raise SuggestionFailure('Invalid source file name: ' + file) from e
         if modname not in self.graph:
             raise SuggestionFailure('Unknown module: ' + modname)
         # We must be sure about any edits in this file as this might affect the line numbers.

mypy/util.py

@@ -132,7 +132,7 @@ def decode_python_encoding(source: bytes, pyversion: Tuple[int, int]) -> str:
     try:
         source_text = source.decode(encoding)
     except LookupError as lookuperr:
-        raise DecodeError(str(lookuperr))
+        raise DecodeError(str(lookuperr)) from lookuperr
     return source_text
 
 

mypyc/test/test_run.py

@@ -82,7 +82,7 @@ def run_setup(script_name: str, script_args: List[str]) -> bool:
         # "interrupted" as the argument. Convert it back so that
         # pytest will exit instead of just failing the test.
         if code == "interrupted":
-            raise KeyboardInterrupt
+            raise KeyboardInterrupt from e
 
         return code == 0 or code is None