間接訪問Python實例屬性而不使用點符號

Question

給定了sqlalchemy中的幾個簡單表，它們具有簡單的一對多關係，我試圖編寫一個通用函數來將子項添加到關係集合中。這些表是這樣的：

class StockItem(Base): 

    __tablename__ = 'stock_items' 

    stock_id = Column(Integer, primary_key=True) 
    description = Column(String, nullable=False, unique=True) 
    department = Column(String) 
    images = relationship('ImageKey', backref='stock_item', lazy='dynamic') 

    def __repr__(self): 
     return '<StockItem(Stock ID:{}, Description: {}, Department: {})>'.\ 
      format(self.stock_id, self.description, self.department) 


class ImageKey(Base): 

    __tablename__ = 'image_keys' 

    s3_key = Column(String, primary_key=True) 
    stock_id = Column(Integer, ForeignKey('stock_items.stock_id')) 

    def __repr__(self): 
     return '<ImageKey(AWS S3 Key: {}, Stock Item: {})>'.\ 
      format(self.s3_key, self.stock_id) 

因此，與給定的設置，我可以將項目添加到收藏images對於給定StockItem：

item = StockItem(stock_id=42, description='Frobnistication for Foozlebars', 
       department='Books') 
image = ImageKey(s3_key='listings/images/Frob1.jpg', stock_id=42) 
item.images.append(image) 

確定。到現在爲止還挺好。實際上，我的應用將會有幾張關係表。當我試圖將其推廣到處理任意關係的函數時，我的問題就出現了。 Here'e我寫的（注意，add_item()只是它封裝了對象構造一個try/except處理IntegrityError的一種方法）：

@session_manager 
def _add_collection_item(self, Parent, Child, key, collection, 
         session=None, **kwargs): 
    """Add a Child object to the collection object of Parent.""" 
    child = self.add_item(Child, session=session, **kwargs) 
    parent = session.query(Parent).get(key) 
    parent.collection.append(child) # This line obviously throws error. 
    session.add(parent) 

我所說的功能，如：

db._add_collection_item(StockItem, ImageKey, 42, 'images', 
         s3_key='listings/images/Frob1.jpg', 
         stock_id=42) 

這顯然錯誤，因爲父表沒有collection屬性。回溯：

Traceback (most recent call last): 
    File "C:\Code\development\pyBay\demo1.py", line 25, in <module> 
    stock_id=1) 
    File "C:\Code\development\pyBay\pybay\database\client.py", line 67, in add_context_manager 
    result = func(self, *args, session=session, **kwargs) 
    File "C:\Code\development\pyBay\pybay\database\client.py", line 113, in _add_collection_item 
    parent.collection.append(child) 
AttributeError: 'StockItem' object has no attribute 'collection' 

我認爲它會拋出一個錯誤，因爲集合名稱是作爲字符串傳遞的。

所以我的問題是，如何通過關鍵詞而不是使用'點'表示法將項目添加到集合？

Answer 1

做：

getattr(parent, collection).append(child) 

更多信息

• https://docs.python.org/2/library/functions.html#getattr

• https://docs.python.org/3.4/library/functions.html#getattr