magic_arguments: dedent but otherwise preserve indentation.

Previously magic_arguments stripped all whitespace at the beginning of
each line, interfering with formatting syntax which relies on indentation
to give context (e.g., code blocks). Now the docstring text is passed
through dedent to strip the global indentation before being handed off
to RawDescriptionHelpFormatter which preserves any remaining indentation.

Thanks to @rkern for suggesting the solution approach.

Closes gh-1890.

IPython/core/magic_arguments.py

@@ -1,217 +1,223 b''
 ''' A decorator-based method of constructing IPython magics with `argparse`
 option handling.
 
 New magic functions can be defined like so::
 
     from IPython.core.magic_arguments import (argument, magic_arguments,
         parse_argstring)
 
     @magic_arguments()
     @argument('-o', '--option', help='An optional argument.')
     @argument('arg', type=int, help='An integer positional argument.')
     def magic_cool(self, arg):
         """ A really cool magic command.
 
     """
         args = parse_argstring(magic_cool, arg)
         ...
 
 The `@magic_arguments` decorator marks the function as having argparse arguments.
 The `@argument` decorator adds an argument using the same syntax as argparse's
 `add_argument()` method. More sophisticated uses may also require the
 `@argument_group` or `@kwds` decorator to customize the formatting and the
 parsing.
 
 Help text for the magic is automatically generated from the docstring and the
 arguments::
 
     In[1]: %cool?
         %cool [-o OPTION] arg
         
         A really cool magic command.
         
         positional arguments:
           arg                   An integer positional argument.
         
         optional arguments:
           -o OPTION, --option OPTION
                                 An optional argument.
 
 '''
 #-----------------------------------------------------------------------------
 # Copyright (C) 2010-2011, IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 # Our own imports
 from IPython.external import argparse
 from IPython.core.error import UsageError
 from IPython.utils.process import arg_split
+from IPython.utils.text import dedent
 
+class MagicHelpFormatter(argparse.RawDescriptionHelpFormatter):
+    """ A HelpFormatter which dedents but otherwise preserves indentation.
+    """
+    def _fill_text(self, text, width, indent):
+        return argparse.RawDescriptionHelpFormatter._fill_text(self, dedent(text), width, indent)
 
 class MagicArgumentParser(argparse.ArgumentParser):
     """ An ArgumentParser tweaked for use by IPython magics.
     """
     def __init__(self,
                  prog=None,
                  usage=None,
                  description=None,
                  epilog=None,
                  version=None,
                  parents=None,
-                 formatter_class=argparse.HelpFormatter,
+                 formatter_class=MagicHelpFormatter,
                  prefix_chars='-',
                  argument_default=None,
                  conflict_handler='error',
                  add_help=False):
         if parents is None:
             parents = []
         super(MagicArgumentParser, self).__init__(prog=prog, usage=usage,
             description=description, epilog=epilog, version=version,
             parents=parents, formatter_class=formatter_class,
             prefix_chars=prefix_chars, argument_default=argument_default,
             conflict_handler=conflict_handler, add_help=add_help)
 
     def error(self, message):
         """ Raise a catchable error instead of exiting.
         """
         raise UsageError(message)
 
     def parse_argstring(self, argstring):
         """ Split a string into an argument list and parse that argument list.
         """
         argv = arg_split(argstring)
         return self.parse_args(argv)
 
 
 def construct_parser(magic_func):
     """ Construct an argument parser using the function decorations.
     """
     kwds = getattr(magic_func, 'argcmd_kwds', {})
     if 'description' not in kwds:
         kwds['description'] = getattr(magic_func, '__doc__', None)
     arg_name = real_name(magic_func)
     parser = MagicArgumentParser(arg_name, **kwds)
     # Reverse the list of decorators in order to apply them in the
     # order in which they appear in the source.
     group = None
     for deco in magic_func.decorators[::-1]:
         result = deco.add_to_parser(parser, group)
         if result is not None:
             group = result
 
     # Replace the starting 'usage: ' with IPython's %.
     help_text = parser.format_help()
     if help_text.startswith('usage: '):
         help_text = help_text.replace('usage: ', '%', 1)
     else:
         help_text = '%' + help_text
 
     # Replace the magic function's docstring with the full help text.
     magic_func.__doc__ = help_text
 
     return parser
 
 
 def parse_argstring(magic_func, argstring):
     """ Parse the string of arguments for the given magic function.
     """
     return magic_func.parser.parse_argstring(argstring)
 
 
 def real_name(magic_func):
     """ Find the real name of the magic.
     """
     magic_name = magic_func.__name__
     if magic_name.startswith('magic_'):
         magic_name = magic_name[len('magic_'):]
     return getattr(magic_func, 'argcmd_name', magic_name)
 
 
 class ArgDecorator(object):
     """ Base class for decorators to add ArgumentParser information to a method.
     """
 
     def __call__(self, func):
         if not getattr(func, 'has_arguments', False):
             func.has_arguments = True
             func.decorators = []
         func.decorators.append(self)
         return func
 
     def add_to_parser(self, parser, group):
         """ Add this object's information to the parser, if necessary.
         """
         pass
 
 
 class magic_arguments(ArgDecorator):
     """ Mark the magic as having argparse arguments and possibly adjust the
     name.
     """
 
     def __init__(self, name=None):
         self.name = name
 
     def __call__(self, func):
         if not getattr(func, 'has_arguments', False):
             func.has_arguments = True
             func.decorators = []
         if self.name is not None:
             func.argcmd_name = self.name
         # This should be the first decorator in the list of decorators, thus the
         # last to execute. Build the parser.
         func.parser = construct_parser(func)
         return func
 
 
 class argument(ArgDecorator):
     """ Store arguments and keywords to pass to add_argument().
 
     Instances also serve to decorate command methods.
     """
     def __init__(self, *args, **kwds):
         self.args = args
         self.kwds = kwds
 
     def add_to_parser(self, parser, group):
         """ Add this object's information to the parser.
         """
         if group is not None:
             parser = group
         parser.add_argument(*self.args, **self.kwds)
         return None
 
 
 class argument_group(ArgDecorator):
     """ Store arguments and keywords to pass to add_argument_group().
 
     Instances also serve to decorate command methods.
     """
     def __init__(self, *args, **kwds):
         self.args = args
         self.kwds = kwds
 
     def add_to_parser(self, parser, group):
         """ Add this object's information to the parser.
         """
         return parser.add_argument_group(*self.args, **self.kwds)
 
 
 class kwds(ArgDecorator):
     """ Provide other keywords to the sub-parser constructor.
     """
     def __init__(self, **kwds):
         self.kwds = kwds
 
     def __call__(self, func):
         func = super(kwds, self).__call__(func)
         func.argcmd_kwds = self.kwds
         return func
 
 
 __all__ = ['magic_arguments', 'argument', 'argument_group', 'kwds',
     'parse_argstring']