Fix exception causes in 7 modules

IPython/core/formatters.py

@@ -643,17 +643,17 b' class PlainTextFormatter(BaseFormatter):'
             fmt = new
             try:
                 fmt%3.14159
-            except Exception:
-                raise ValueError("Precision must be int or format string, not %r"%new)
+            except Exception as e:
+                raise ValueError("Precision must be int or format string, not %r"%new) from e
         elif new:
             # otherwise, should be an int
             try:
                 i = int(new)
                 assert i >= 0
-            except ValueError:
-                raise ValueError("Precision must be int or format string, not %r"%new)
-            except AssertionError:
-                raise ValueError("int precision must be non-negative, not %r"%i)
+            except ValueError as e:
+                raise ValueError("Precision must be int or format string, not %r"%new) from e
+            except AssertionError as e:
+                raise ValueError("int precision must be non-negative, not %r"%i) from e
 
             fmt = '%%.%if'%i
             if 'numpy' in sys.modules:

IPython/core/interactiveshell.py

@@ -1490,8 +1490,8 b' class InteractiveShell(SingletonConfigurable):'
         else:                         # Delete by object
             try:
                 obj = self.user_ns[varname]
-            except KeyError:
-                raise NameError("name '%s' is not defined" % varname)
+            except KeyError as e:
+                raise NameError("name '%s' is not defined" % varname) from e
             # Also check in output history
             ns_refs.append(self.history_manager.output_hist)
             for ns in ns_refs:
@@ -1521,8 +1521,8 b' class InteractiveShell(SingletonConfigurable):'
         if regex is not None:
             try:
                 m = re.compile(regex)
-            except TypeError:
-                raise TypeError('regex must be a string or compiled pattern')
+            except TypeError as e:
+                raise TypeError('regex must be a string or compiled pattern') from e
             # Search for keys in each namespace that match the given regex
             # If a match is found, delete the key/value pair.
             for ns in self.all_ns_refs:
@@ -3614,13 +3614,13 b' class InteractiveShell(SingletonConfigurable):'
         try:
             if target.startswith(('http://', 'https://')):
                 return openpy.read_py_url(target, skip_encoding_cookie=skip_encoding_cookie)
-        except UnicodeDecodeError:
+        except UnicodeDecodeError as e:
             if not py_only :
                 # Deferred import
                 from urllib.request import urlopen
                 response = urlopen(target)
                 return response.read().decode('latin1')
-            raise ValueError(("'%s' seem to be unreadable.") % target)
+            raise ValueError(("'%s' seem to be unreadable.") % target) from e
 
         potential_target = [target]
         try :
@@ -3632,11 +3632,11 b' class InteractiveShell(SingletonConfigurable):'
             if os.path.isfile(tgt):                        # Read file
                 try :
                     return openpy.read_py_file(tgt, skip_encoding_cookie=skip_encoding_cookie)
-                except UnicodeDecodeError :
+                except UnicodeDecodeError as e:
                     if not py_only :
                         with io_open(tgt,'r', encoding='latin1') as f :
                             return f.read()
-                    raise ValueError(("'%s' seem to be unreadable.") % target)
+                    raise ValueError(("'%s' seem to be unreadable.") % target) from e
             elif os.path.isdir(os.path.expanduser(tgt)):
                 raise ValueError("'%s' is a directory, not a regular file." % target)
 
@@ -3648,9 +3648,9 b' class InteractiveShell(SingletonConfigurable):'
 
         try:                                              # User namespace
             codeobj = eval(target, self.user_ns)
-        except Exception:
+        except Exception as e:
             raise ValueError(("'%s' was not found in history, as a file, url, "
-                                "nor in the user namespace.") % target)
+                                "nor in the user namespace.") % target) from e
 
         if isinstance(codeobj, str):
             return codeobj

IPython/core/magic.py

@@ -630,7 +630,7 b' class Magics(Configurable):'
                 opts,args = getopt(argv, opt_str, long_opts)
             except GetoptError as e:
                 raise UsageError('%s ( allowed: "%s" %s)' % (e.msg,opt_str,
-                                        " ".join(long_opts)))
+                                        " ".join(long_opts))) from e
             for o,a in opts:
                 if o.startswith('--'):
                     o = o[2:]

IPython/core/magics/code.py

@@ -440,10 +440,10 b' class CodeMagics(Magics):'
                         return (None, None, None)
                     use_temp = False
 
-                except DataIsObject:
+                except DataIsObject as e:
                     # macros have a special edit function
                     if isinstance(data, Macro):
-                        raise MacroToEdit(data)
+                        raise MacroToEdit(data) from e
 
                     # For objects, try to edit the file where they are defined
                     filename = find_file(data)
@@ -467,8 +467,8 b' class CodeMagics(Magics):'
                         
                         m = ipython_input_pat.match(os.path.basename(filename))
                         if m:
-                            raise InteractivelyDefined(int(m.groups()[0]))
-                        
+                            raise InteractivelyDefined(int(m.groups()[0])) from e
+
                         datafile = 1
                     if filename is None:
                         filename = make_filename(args)

IPython/core/magics/execution.py

@@ -679,9 +679,9 b' class ExecutionMagics(Magics):'
             fpath = None # initialize to make sure fpath is in scope later
             fpath = arg_lst[0]
             filename = file_finder(fpath)
-        except IndexError:
+        except IndexError as e:
             msg = 'you must provide at least a filename.'
-            raise Exception(msg)
+            raise Exception(msg) from e
         except IOError as e:
             try:
                 msg = str(e)
@@ -689,7 +689,7 b' class ExecutionMagics(Magics):'
                 msg = e.message
             if os.name == 'nt' and re.match(r"^'.*'$",fpath):
                 warn('For Windows, use double quotes to wrap a filename: %run "mypath\\myfile.py"')
-            raise Exception(msg)
+            raise Exception(msg) from e
         except TypeError:
             if fpath in sys.meta_path:
                 filename = ""

IPython/core/magics/namespace.py

@@ -683,8 +683,8 b' class NamespaceMagics(Magics):'
         else:
             try:
                 m = re.compile(regex)
-            except TypeError:
-                raise TypeError('regex must be a string or compiled pattern')
+            except TypeError as e:
+                raise TypeError('regex must be a string or compiled pattern') from e
             for i in self.who_ls():
                 if m.search(i):
                     del(user_ns[i])

IPython/core/magics/osm.py

@@ -293,8 +293,8 b' class OSMagics(Magics):'
         """
         try:
             return os.getcwd()
-        except FileNotFoundError:
-            raise UsageError("CWD no longer exists - please use %cd to change directory.")
+        except FileNotFoundError as e:
+            raise UsageError("CWD no longer exists - please use %cd to change directory.") from e
 
     @skip_doctest
     @line_magic
@@ -386,8 +386,8 b' class OSMagics(Magics):'
         if ps == '-':
             try:
                 ps = self.shell.user_ns['_dh'][-2]
-            except IndexError:
-                raise UsageError('%cd -: No previous directory to change to.')
+            except IndexError as e:
+                raise UsageError('%cd -: No previous directory to change to.') from e
         # jump to bookmark if needed
         else:
             if not os.path.isdir(ps) or 'b' in opts:
@@ -764,15 +764,15 b' class OSMagics(Magics):'
         if 'd' in opts:
             try:
                 todel = args[0]
-            except IndexError:
+            except IndexError as e:
                 raise UsageError(
-                    "%bookmark -d: must provide a bookmark to delete")
+                    "%bookmark -d: must provide a bookmark to delete") from e
             else:
                 try:
                     del bkms[todel]
-                except KeyError:
+                except KeyError as e:
                     raise UsageError(
-                        "%%bookmark -d: Can't delete bookmark '%s'" % todel)
+                        "%%bookmark -d: Can't delete bookmark '%s'" % todel) from e
 
         elif 'r' in opts:
             bkms = {}