SQLAlchemy中的链接比较 [英] Chained comparisons in SQLAlchemy

问题描述

Python 支持链接比较： 1 < 2< 3 翻译为（1 <2）和（2 <3）。

Python supports chained comparisons: 1 < 2 < 3 translates to (1 < 2) and (2 < 3).

我试图使用SQLAlchemy进行SQL查询，如下所示：

I am trying to make an SQL query using SQLAlchemy which looks like this:

results = session.query(Couple).filter(10 < Couple.NumOfResults < 20).all()

不如预期。我已经打开引擎的 echo = True 关键字，确实 - 生成的SQL查询只包括两个比较之一。

The results I got were not as expected. I've turned the engine's echo=True keyword, and indeed - the generated SQL query only included one of the two comparisons.

我找不到任何明确表示禁止的文档。我假设如果这种类型的表达式在Python中支持，它也应该在SQLAlchemy中支持。

I can't find any documentation that explicitly says this is forbidden. I assumed that if this type of expression is supported in Python, it should be supported in SQLAlchemy as well.

为什么这不工作？我有一个可能的解决方案（在答案中分享），但很高兴听到其他意见。

Why doesn't this work? I have one possible solution in mind (shared in answers), but will be glad to hear other opinions.

推荐答案

可能 Python实际评估此表达式：

The reason is probably that Python actually evaluates this expression:

（10< Couple.NumOfResults and Couple.NumOfResults< 20 ）

SQLAlchemy不支持和运算符$ c>和_ ）。因此，SQLAlchemy中不允许链接比较。

The and operator is unsupported in SQLAlchemy (one should use and_ instead). And thus - chained comparisons are not allowed in SQLAlchemy.

在原始示例中，您应该编写此代码：

In the original example, one should write this code instead:

results = session.query(Couple).filter(and_(10 < Couple.NumOfResults, 
                                            Couple.NumOfResults < 20).all()

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