Fix exception causes all over the codebase

jupyter_client/asynchronous/client.py

@@ -169,8 +169,8 @@ async def _recv_reply(self, msg_id, timeout=None, channel='shell'):
                     reply = await self.get_control_msg(timeout=timeout)
                 else:
                     reply = await self.get_shell_msg(timeout=timeout)
-            except Empty:
-                raise TimeoutError("Timeout waiting for reply")
+            except Empty as e:
+                raise TimeoutError("Timeout waiting for reply") from e
             if reply['parent_header'].get('msg_id') != msg_id:
                 # not my reply, someone may have forgotten to retrieve theirs
                 continue

jupyter_client/blocking/client.py

@@ -134,8 +134,8 @@ def _recv_reply(self, msg_id, timeout=None, channel='shell'):
                     reply = self.get_control_msg(timeout=timeout)
                 else:
                     reply = self.get_shell_msg(timeout=timeout)
-            except Empty:
-                raise TimeoutError("Timeout waiting for reply")
+            except Empty as e:
+                raise TimeoutError("Timeout waiting for reply") from e
             if reply['parent_header'].get('msg_id') != msg_id:
                 # not my reply, someone may have forgotten to retrieve theirs
                 continue

jupyter_client/session.py

@@ -379,8 +379,9 @@ def _signature_scheme_changed(self, change):
         hash_name = new.split('-', 1)[1]
         try:
             self.digest_mod = getattr(hashlib, hash_name)
-        except AttributeError:
-            raise TraitError("hashlib has no such attribute: %s" % hash_name)
+        except AttributeError as e:
+            raise TraitError("hashlib has no such attribute: %s" %
+                             hash_name) from e
         self._new_auth()
 
     digest_mod = Any()
@@ -537,7 +538,7 @@ def _check_packers(self):
                 jsonmsg = ""
             raise ValueError(
                 msg.format(packer=self.packer, e=e, jsonmsg=jsonmsg)
-            )
+            ) from e
 
         # ensure packed message is bytes
         if not isinstance(packed, bytes):
@@ -555,7 +556,7 @@ def _check_packers(self):
                 jsonmsg = ""
             raise ValueError(
                 msg.format(packer=self.packer, unpacker=self.unpacker, e=e, jsonmsg=jsonmsg)
-            )
+            ) from e
 
         # check datetime support
         msg = dict(t=utcnow())
@@ -733,8 +734,8 @@ def send(self, stream, msg_or_type, content=None, parent=None, ident=None,
                 try:
                     # check to see if buf supports the buffer protocol.
                     view = memoryview(buf)
-                except TypeError:
-                    raise TypeError("Buffer objects must support the buffer protocol.")
+                except TypeError as e:
+                    raise TypeError("Buffer objects must support the buffer protocol.") from e
             # memoryview.contiguous is new in 3.3,
             # just skip the check on Python 2
             if hasattr(view, 'contiguous') and not view.contiguous:

jupyter_client/ssh/tunnel.py

@@ -241,12 +241,12 @@ def openssh_tunnel(lport, rport, server, remoteip='127.0.0.1', keyfile=None, pas
                 raise SSHException('The authenticity of the host can\'t be established.')
         except pexpect.TIMEOUT:
             continue
-        except pexpect.EOF:
+        except pexpect.EOF as e:
             if tunnel.exitstatus:
                 print(tunnel.exitstatus)
                 print(tunnel.before)
                 print(tunnel.after)
-                raise RuntimeError("tunnel '%s' failed to start" % (cmd))
+                raise RuntimeError("tunnel '%s' failed to start" % (cmd)) from e
             else:
                 return tunnel.pid
         else: