Fix exception causes all over the codebase (#8998)

In some parts of the code, an exception is being caught and replaced with a more user-friendly error. In these cases the syntax `raise new_error from old_error` needs to be used.

Python 3's exception chaining means it shows not only the traceback of the current exception, but that of the original exception (and possibly more.) This is regardless of `raise from`. The usage of `raise from` tells Python to put a more accurate message between the tracebacks. Instead of this: 
```
    During handling of the above exception, another exception occurred:
```
You'll get this: 
```
    The above exception was the direct cause of the following exception:
```
The first is inaccurate, because it signifies a bug in the exception-handling code itself, which is a separate situation than wrapping an exception.

Author: cool-RR

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