Python知识点： __import__

用法：libvirt = __import__('libvirt')

help(__import__)

__import__(...)
    __import__(name, globals={}, locals={}, fromlist=[], level=-1) -> module

　　导入一个模块。

　　globals：决定上下文，不修改。

　　locals：当前未使用。

　　fromlist：有值表示``from name import ...''，为空表示``import name''。

　　当从一个包中导入模块时，注意__import__('A.B', ...)当fromlist为空时返回包A，

　　不为空时返回子模块B。

　　level：用于决定是否执行绝对导入。-1是绝对相对都会尝试，默认。0是绝对。正数

　　表示相对当前模块的父目录的层数。

Import a module.  The globals are only used to determine the context;
    they are not modified.  The locals are currently unused.  The fromlist
    should be a list of names to emulate ``from name import ...'', or an
    empty list to emulate ``import name''.
    When importing a module from a package, note that __import__('A.B', ...)
    returns package A when fromlist is empty, but its submodule B when
    fromlist is not empty.  Level is used to determine whether to perform
    absolute or relative imports.  -1 is the original strategy of attempting
    both absolute and relative imports, 0 is absolute, a positive number
    is the number of parent directories to search relative to the current module.