app/mapreduce/lib/pipeline/util.py

#!/usr/bin/env python
#
# Copyright 2010 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Utility functions for use with the Google App Engine Pipeline API."""

__all__ = ["for_name", "is_generator_function"]


import inspect
import logging


def for_name(fq_name, recursive=False):
  """Find class/function/method specified by its fully qualified name.

  Fully qualified can be specified as:
    * <module_name>.<class_name>
    * <module_name>.<function_name>
    * <module_name>.<class_name>.<method_name> (an unbound method will be
      returned in this case).

  for_name works by doing __import__ for <module_name>, and looks for
  <class_name>/<function_name> in module's __dict__/attrs. If fully qualified
  name doesn't contain '.', the current module will be used.

  Args:
    fq_name: fully qualified name of something to find

  Returns:
    class object.

  Raises:
    ImportError: when specified module could not be loaded or the class
    was not found in the module.
  """
  fq_name = str(fq_name)
  module_name = __name__
  short_name = fq_name

  if fq_name.rfind(".") >= 0:
    (module_name, short_name) = (fq_name[:fq_name.rfind(".")],
                                 fq_name[fq_name.rfind(".") + 1:])

  try:
    result = __import__(module_name, None, None, [short_name])
    return result.__dict__[short_name]
  except KeyError:
    # If we're recursively inside a for_name() chain, then we want to raise
    # this error as a key error so we can report the actual source of the
    # problem. If we're *not* recursively being called, that means the
    # module was found and the specific item could not be loaded, and thus
    # we want to raise an ImportError directly.
    if recursive:
      raise
    else:
      raise ImportError("Could not find '%s' on path '%s'" % (
                        short_name, module_name))
  except ImportError, e:
    # module_name is not actually a module. Try for_name for it to figure
    # out what's this.
    try:
      module = for_name(module_name, recursive=True)
      if hasattr(module, short_name):
        return getattr(module, short_name)
      else:
        # The module was found, but the function component is missing.
        raise KeyError()
    except KeyError:
      raise ImportError("Could not find '%s' on path '%s'" % (
                        short_name, module_name))
    except ImportError:
      # This means recursive import attempts failed, thus we will raise the
      # first ImportError we encountered, since it's likely the most accurate.
      pass
    # Raise the original import error that caused all of this, since it is
    # likely the real cause of the overall problem.
    raise


def is_generator_function(obj):
  """Return true if the object is a user-defined generator function.

  Generator function objects provides same attributes as functions.
  See isfunction.__doc__ for attributes listing.

  Adapted from Python 2.6.

  Args:
    obj: an object to test.

  Returns:
    true if the object is generator function.
  """
  CO_GENERATOR = 0x20
  return bool(((inspect.isfunction(obj) or inspect.ismethod(obj)) and
               obj.func_code.co_flags & CO_GENERATOR))