python-动态加载-模块-类-对象

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有时希望从配置文件等地获取要被动态加载的 module，但是所读取的配置项通常为字符串类型，无法用 import 加载.

例如：

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>>> import 'os'
File "<stdin>", line 1
 import 'os'
  ^
SyntaxError: invalid syntax

使用内建函数import

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>>> __import__('os')
<module 'os' from '/usr/lib64/python2.7/os.pyc'>

但使用import的时候，如果module字符串有点号(.), 会有问题:

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>> __import__('os')
<module 'os' from '/usr/lib64/python2.7/os.pyc'>  #会发现与__import__('os')一样

方法1

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def import_class(import_str):
    """Returns a class from a string including module and class.
    .. versionadded:: 0.3
    """
    mod_str, _sep, class_str = import_str.rpartition('.')
    __import__(mod_str)
    try:
        return getattr(sys.modules[mod_str], class_str)
    except AttributeError:
        raise ImportError('Class %s cannot be found (%s)' %
                          (class_str,
                           traceback.format_exception(*sys.exc_info())))
def import_object(import_str, *args, **kwargs):
    """Import a class and return an instance of it.
    .. versionadded:: 0.3
    """
    return import_class(import_str)(*args, **kwargs)
def import_module(import_str):
    """Import a module.
    .. versionadded:: 0.3
    """
    __import__(import_str)
    return sys.modules[import_str]

参考：https://github.com/openstack/oslo.utils/blob/master/oslo_utils/importutils.py

方法2

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def import_object(name):
  # type: (_BaseString) -> Any
  """Imports an object by name.
  import_object('x') is equivalent to 'import x'.
  import_object('x.y.z') is equivalent to 'from x.y import z'.
  """
   if not isinstance(name, str):
        # on python 2 a byte string is required.
        name = name.encode('utf-8')
    if name.count('.') == 0:
        return __import__(name, None, None)
    parts = name.split('.')
    obj = __import__('.'.join(parts[:-1]), None, None, [parts[-1]], 0)
    try:
        return getattr(obj, parts[-1])
    except AttributeError:
        raise ImportError("No module named %s" % parts[-1])