Merge pull request ipython#1921 from bfroehle/_1890_magic_arguments_d…

…ocstring

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 ipythongh-1890.

IPython/core/magic_arguments.py

@@ -50,7 +50,13 @@ def magic_cool(self, arg):
 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.
@@ -62,7 +68,7 @@ def __init__(self,
                  epilog=None,
                  version=None,
                  parents=None,
-                 formatter_class=argparse.HelpFormatter,
+                 formatter_class=MagicHelpFormatter,
                  prefix_chars='-',
                  argument_default=None,
                  conflict_handler='error',