Question

我正在使用SQLalchemy编写我的第一个Python（3.4）应用程序。我有几种方法都有非常相似的模式。他们采用可选参数session，默认为None。如果传递session，则该函数使用该会话，否则它将打开并使用新会话。例如，请考虑以下方法：

def _stocks(self, session=None):
    """Return a list of all stocks in database."""
    newsession = False
    if not session:
        newsession = True
        session = self.db.Session()
    stocks = [stock.ticker for stock in session.query(Stock).all()]
    if newsession:
        session.close()
    return stocks

所以，作为Python的新手并渴望学习它的所有功能，我认为这就像是学习Python装饰器的最佳时机。所以经过大量的阅读，比如这个this series of blog posts和this非常好的答案，我写了下面的装饰：

from functools import wraps

def session_manager(func):
    """
    Manage creation of session for given function.

    If a session is passed to the decorated function, it is simply
    passed through, otherwise a new session is created.  Finally after
    execution of decorated function, the new session (if created) is
    closed/
    """
    @wraps(func)
    def inner(that, session=None, *args, **kwargs):
        newsession = False
        if not session:
            newsession = True
            session = that.db.Session()
        func(that, session, *args, **kwargs)
        if newsession:
            session.close()
        return func(that, session, *args, **kwargs)
    return inner

它看起来很棒。原始方法现在简化为：

@session_manager
def _stocks(self, session=None):
    """Return a list of all stocks in database."""
    return [stock.ticker for stock in session.query(Stock).all()]

但是，当我将装饰器应用于除了可选session之外还需要一些位置参数的函数时，我得到一个错误。所以试着写：

@session_manager
def stock_exists(self, ticker, session=None):
    """
    Check for existence of stock in database.

    Args:
        ticker (str): Ticker symbol for a given company's stock.
        session (obj, optional): Database session to use.  If not
            provided, opens, uses and closes a new session.

    Returns:
        bool: True if stock is in database, False otherwise.
    """
    return bool(session.query(Stock)
                .filter_by(ticker=ticker)
                .count()
                )

并且像print(client.manager.stock_exists('AAPL'))一样运行会为AttributeError提供以下追溯：

Traceback (most recent call last):
  File "C:\Code\development\Pynance\pynance.py", line 33, in <module>
    print(client.manager.stock_exists('GPX'))
  File "C:\Code\development\Pynance\pynance\decorators.py", line 24, in inner
    func(that, session, *args, **kwargs)
  File "C:\Code\development\Pynance\pynance\database\database.py", line 186, in stock_exists
    .count()
AttributeError: 'NoneType' object has no attribute 'query'
[Finished in 0.7s]

所以我在回溯中猜测，我正在弄乱参数的顺序，但我无法弄清楚如何正确地对它们进行排序。除了session之外，我还有一些我想要装饰的函数，可以使用0-3个参数。有人可以指出我方法中的错误吗？

Answer 1

更改

def inner(that, session=None, *args, **kwargs):

到

def inner(that, *args, session=None, **kwargs):

和

return func(that, session, *args, **kwargs)

到

return func(that, *args, session=session, **kwargs)

有效：

def session_manager(func):

    def inner(that, *args, session=None, **kwargs):
        if not session:
            session = object()
        return func(that, *args, session=session, **kwargs)

    return inner


class A():

    @session_manager
    def _stocks(self, session=None):
        print(session)
        return True

    @session_manager
    def stock_exists(self, ticker, session=None):
        print(ticker, session)
        return True

a = A()
a._stocks()
a.stock_exists('ticker')

输出：

$ python3 test.py
<object object at 0x7f4197810070>
ticker <object object at 0x7f4197810070>

使用def inner(that, session=None, *args, **kwargs)时，任何第二个位置参数（计数self）都被视为session参数。因此，当您致电manager.stock_exists('AAPL') session获取值AAPL时。

Answer 2

我注意到的第一件事是你正在调用两次装饰功能

@wraps(func)
    def inner(that, session=None, *args, **kwargs):
        newsession = False
        if not session:
            newsession = True
            session = that.db.Session()
        #calling first time
        func(that, session, *args, **kwargs)
        if newsession:
            session.close()
        #calling second time
        return func(that, session, *args, **kwargs)
    return inner

在第二次通话期间，会议已经关闭。此外，您无需在装饰器功能中明确接受that和session参数，它们已位于args和kwargs中。看看这个解决方案：

@wraps(func)
def inner(*args, **kwargs):
    session = None
    if not 'session' in kwargs:
        session = that.db.Session()
        kwargs['session'] = session
    result = func(*args, **kwargs)
    if session:
        session.close()
    return result
return inner

您可能还希望将会话结束代码放在finally块中，然后即使装饰函数抛出异常，您也会确定它已关闭

Python用可变数量的位置args和可选的arg来装饰方法

2 个答案: