Support user-provided fixers

Avoid erroring out when a non-standard fixer is specified, and instead
hand it over to the refactoring tool so it can try to load it.

While at it, add a command-line option to add the current directory
to sys.path, to make it easier for users to let their fixers be found.

docs/fixers.rst

@@ -21,6 +21,15 @@ then use the ``--no-six`` flag.
 Fixers use the API defined by 2to3. For details of how this works, and how to
 implement your own fixers, see `Extending 2to3 with your own fixers, at
 python3porting.com <http://python3porting.com/fixers.html>`_.
+``python-modernize`` will try to load fixers whose full dotted-path is specified
+as a ``-f`` argument, but will fail if they are not found. By default, fixers
+will not be found in the current directory; use ``--fixers-here`` to make
+``python-modernize`` look for them there, or see the `Python tutorial on
+modules <https://docs.python.org/3/tutorial/modules.html>`_ (in particular,
+the parts on the `search path
+<https://docs.python.org/3/tutorial/modules.html#the-module-search-path>`_
+and `packages <https://docs.python.org/3/tutorial/modules.html#packages>`_)
+for more info on how Python finds modules.
 
 
 Default

libmodernize/main.py

@@ -10,6 +10,7 @@
 import sys
 import logging
 import optparse
+import os
 
 from lib2to3.main import warn, StdoutRefactoringTool
 from lib2to3 import refactor
@@ -23,10 +24,12 @@
 Usage: modernize [options] file|dir ...
 """ % __version__
 
+
 def format_usage(usage):
     """Method that doesn't output "Usage:" prefix"""
     return usage
 
+
 def main(args=None):
     """Main program.
 
@@ -41,11 +44,13 @@ def main(args=None):
     parser.add_option("--no-diffs", action="store_true",
                       help="Don't show diffs of the refactoring.")
     parser.add_option("-l", "--list-fixes", action="store_true",
-                      help="List available transformations.")
+                      help="List standard transformations.")
     parser.add_option("-d", "--doctests_only", action="store_true",
                       help="Fix up doctests only.")
     parser.add_option("-f", "--fix", action="append", default=[],
                       help="Each FIX specifies a transformation; '-f default' includes default fixers.")
+    parser.add_option("--fixers-here", action="store_true",
+                      help="Add current working directory to python path (so fixers can be found)")
     parser.add_option("-j", "--processes", action="store", default=1,
                       type="int", help="Run 2to3 concurrently.")
     parser.add_option("-x", "--nofix", action="append", default=[],
@@ -80,7 +85,7 @@ def main(args=None):
     if not options.write and options.nobackups:
         parser.error("Can't use '-n' without '-w'.")
     if options.list_fixes:
-        print("Available transformations for the -f/--fix and -x/--nofix options:")
+        print("Standard transformations available for the -f/--fix and -x/--nofix options:")
         for fixname in sorted(avail_fixes):
             print("    {}  ({})".format(fixname, fixname.split(".fix_", 1)[1]))
         print()
@@ -97,6 +102,8 @@ def main(args=None):
             return 2
     if options.print_function:
         flags["print_function"] = True
+    if options.fixers_here:
+        sys.path.append(os.getcwd())
 
     # Set up logging handler
     level = logging.DEBUG if options.verbose else logging.INFO
@@ -150,9 +157,8 @@ def main(args=None):
                         explicit.add(matched)
                         break
                 else:
-                    print("Error: fix '{}' was not found".format(fix),
-                          file=sys.stderr)
-                    return 2
+                    # A non-standard fix -- trust user to have supplied path
+                    explicit.add(fix)
         requested = default_fixes.union(explicit) if default_present else explicit
     else:
         requested = default_fixes