SQLAlchemy:使用`and`和`or`时出现意外结果 [英] SQLAlchemy: unexpected results when using `and` and `or`

问题描述

我有一个声明性的基类News:

I have a declarative base class News:

class News(Base):
    __tablename__ = "news"
    id = Column(Integer, primary_key = True)
    title = Column(String)
    author = Column(String)
    url = Column(String)
    comments = Column(Integer)
    points = Column(Integer)
    label = Column(String)

我还有一个函数f(title)，该函数获取一个字符串并返回3个字符串变体之一:"good"，"maybe"或"never". 我尝试获取过滤的行:

I also have a function f(title), that gets a string and returns one of 3 variants of strings: 'good', 'maybe' or 'never'. I try to get filtered rows:

rows = s.query(News).filter(News.label == None and f(News.title) == 'good').all()

但是程序失败，引发此错误:

But the program fails, raising this error:

raise TypeError("Boolean value of this clause is not defined")

我该如何解决?

推荐答案

问题是这样的:

News.label == None and f(News.title) == 'good'
#                  ^^^ here

Python不允许覆盖布尔操作 and和or的行为.您可以在Python中使用 __bool__ 在一定程度上影响它们3和 __nonzero__ 在Python 2中是它定义对象的真值

Python does not allow overriding the behaviour of boolean operations and and or. You can influence them to some extent with __bool__ in Python 3 and __nonzero__ in Python 2, but all that does is that it defines the truth value of your object.

如果所讨论的对象未实现__bool__并引发了错误，或者未实现，则由于

If the objects in question had not implemented __bool__ and thrown the error, or the implementation had not thrown, you would've gotten possibly rather cryptic errors due to the short-circuiting nature of and and or:

In [19]: (News.label == 'asdf') and True
Out[19]: <sqlalchemy.sql.elements.BinaryExpression object at 0x7f62c416fa58>

In [24]: (News.label == 'asdf') or True
Out[24]: True

因为

In [26]: bool(News.label == 'asdf')
Out[26]: False

这可能并且将导致以不正确的SQL表达式的形式拉头发:

This could and would lead to hair pulling in the form of incorrect SQL expressions:

In [28]: print(News.label == 'asdf' or News.author == 'NOT WHAT YOU EXPECTED')
news.author = :author_1

要生成布尔型SQL表达式，请使用 and_() ， or_() 和 not_() a> sql表达式函数或二进制 & ， | 和 ~ 运算符重载:

To produce boolean SQL expressions either use the and_(), or_(), and not_() sql expression functions, or the binary &, |, and ~ operator overloads:

# Parentheses required due to operator precedence
filter((News.label == None) & (f(News.title) == 'good'))

或

filter(and_(News.label == None, f(News.title) == 'good'))

或将多个条件传递给对

or pass multiple criterion to a call to Query.filter():

filter(News.label == None, f(News.title) == 'good')

或组合多个对filter()的调用:

filter(News.label == None).filter(f(News.title) == 'good')

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