Filter current file collections, create another file collections 
        contains all winfile with criterion=True.
        
        How to construct your own criterion function, see 
        :meth:`FileCollection.from_path_by_criterion`.

        :param criterion: customize filter function
        :type criterion: function
        :param keepboth: if True, returns two file collections, one is files
            with criterion=True, another is False.
        :type keepboth: boolean
        
        **中文文档**
        
        在当前的文件集合中, 根据criterion中的规则, 选择需要的生成
        FileCollection。当keepboth参数=True时, 返回两个FileCollection, 一个
        是符合条件的文件集合, 一个是不符合条件的。

def select(self, criterion, keepboth=False):
        """Filter current file collections, create another file collections 
        contains all winfile with criterion=True.
        
        How to construct your own criterion function, see 
        :meth:`FileCollection.from_path_by_criterion`.

        :param criterion: customize filter function
        :type criterion: function
        :param keepboth: if True, returns two file collections, one is files
            with criterion=True, another is False.
        :type keepboth: boolean
        
        **中文文档**
        
        在当前的文件集合中, 根据criterion中的规则, 选择需要的生成
        FileCollection。当keepboth参数=True时, 返回两个FileCollection, 一个
        是符合条件的文件集合, 一个是不符合条件的。
        """
        if keepboth:
            fcs_yes, fcs_no = FileCollection(), FileCollection()
            for winfile in self.files.values():
                if criterion(winfile):
                    fcs_yes.files[winfile.abspath] = winfile
                else:
                    fcs_no.files[winfile.abspath] = winfile
            return fcs_yes, fcs_no
        else:
            fcs = FileCollection()
            for winfile in self.files.values():
                if criterion(winfile):
                    fcs.files[winfile.abspath] = winfile
            
            return fcs