Better error messages for common magic commands.

Print a helpful message when the user calls a magic command with a missing
argument, rather than a long traceback.

IPython/core/magics/code.py

@@ -21,7 +21,7 b' import sys'
 from urllib2 import urlopen
 
 # Our own packages
-from IPython.core.error import TryNext, StdinNotImplementedError
+from IPython.core.error import TryNext, StdinNotImplementedError, UsageError
 from IPython.core.macro import Macro
 from IPython.core.magic import Magics, magics_class, line_magic
 from IPython.core.oinspect import find_file, find_source_lines
@@ -73,6 +73,8 b' class CodeMagics(Magics):'
         """
 
         opts,args = self.parse_options(parameter_s,'fra',mode='list')
+        if not args:
+            raise UsageError('Missing filename.')
         raw = 'r' in opts
         force = 'f' in opts
         append = 'a' in opts
@@ -178,6 +180,9 b' class CodeMagics(Magics):'
         %load http://www.example.com/myscript.py
         """
         opts,args = self.parse_options(arg_s,'y')
+        if not args:
+            raise UsageError('Missing filename, URL, input history range, '
+                             'or macro.')
 
         contents = self.shell.find_user_code(args)
         l = len(contents)

IPython/core/magics/extension.py

@@ -16,6 +16,7 b''
 import os
 
 # Our own packages
+from IPython.core.error import UsageError
 from IPython.core.magic import Magics, magics_class, line_magic
 
 #-----------------------------------------------------------------------------
@@ -56,14 +57,20 b' class ExtensionMagics(Magics):'
     @line_magic
     def load_ext(self, module_str):
         """Load an IPython extension by its module name."""
+        if not module_str:
+            raise UsageError('Missing module name.')
         return self.shell.extension_manager.load_extension(module_str)
 
     @line_magic
     def unload_ext(self, module_str):
         """Unload an IPython extension by its module name."""
+        if not module_str:
+            raise UsageError('Missing module name.')
         self.shell.extension_manager.unload_extension(module_str)
 
     @line_magic
     def reload_ext(self, module_str):
         """Reload an IPython extension by its module name."""
+        if not module_str:
+            raise UsageError('Missing module name.')
         self.shell.extension_manager.reload_extension(module_str)

IPython/core/magics/namespace.py

@@ -19,7 +19,7 b' import sys'
 
 # Our own packages
 from IPython.core import page
-from IPython.core.error import StdinNotImplementedError
+from IPython.core.error import StdinNotImplementedError, UsageError
 from IPython.core.magic import Magics, magics_class, line_magic
 from IPython.testing.skipdoctest import skip_doctest
 from IPython.utils.encoding import DEFAULT_ENCODING
@@ -92,6 +92,8 b' class NamespaceMagics(Magics):'
     @line_magic
     def psource(self, parameter_s='', namespaces=None):
         """Print (or run through pager) the source code for an object."""
+        if not parameter_s:
+            raise UsageError('Missing object name.')
         self.shell._inspect('psource',parameter_s, namespaces)
 
     @line_magic

IPython/core/magics/osm.py

@@ -681,6 +681,9 b' class OSMagics(Magics):'
         %pycat myMacro
         %pycat http://www.example.com/myscript.py
         """
+        if not parameter_s:
+            raise UsageError('Missing filename, URL, input history range, '
+                             'or macro.')
 
         try :
             cont = self.shell.find_user_code(parameter_s)

IPython/zmq/zmqshell.py

@@ -30,6 +30,7 b' from IPython.core.interactiveshell import ('
 from IPython.core import page
 from IPython.core.autocall import ZMQExitAutocall
 from IPython.core.displaypub import DisplayPublisher
+from IPython.core.error import UsageError
 from IPython.core.magics import MacroToEdit, CodeMagics
 from IPython.core.magic import magics_class, line_magic, Magics
 from IPython.core.payloadpage import install_payload_page
@@ -349,6 +350,9 b' class KernelMagics(Magics):'
         """Show a file through the pager.
 
         Files ending in .py are syntax-highlighted."""
+        if not arg_s:
+            raise UsageError('Missing filename.')
+
         cont = open(arg_s).read()
         if arg_s.endswith('.py'):
             cont = self.shell.pycolorize(cont)