sqlachemy 查询方法如何工作? [英] How does sqlachemy query method work?

问题描述

我正在学习 sqlachemy，我对 Python 比较陌生.

I am learning sqlachemy, I'm relatively new to Python.

当我阅读它的文档时，我看到了这种用法，例如:

When I read its documentation, I saw this kind of usage, for example:

query.filter(User.name == 'ed')

query.filter(User.name == 'ed')

Python 不会评估表达式 User.name == 'ed' 然后将结果(布尔值)传递给 query.filter 方法吗?

Wouldn't Python evaluate the expression User.name == 'ed' and then pass the result, which is a boolean, to query.filter method?

这种语法是如何工作的?Python 是否支持某种运算符覆盖，例如 C++?

How does this kind of syntax work? Does Python support some kind of operator overriding like C++?

推荐答案

SQLAlchemy 使用各种 特殊方法钩子 用于重载运算符行为.

SQLAlchemy uses the various special method hooks to overload operator behaviour.

对于 ==，__eq__() 方法在编译时返回表示 SQL 表达式的特殊对象.引用有关丰富比较"挂钩的文档:

For ==, the __eq__() method returns special objects that signify a SQL expression when compiled. To quote the documentation on the 'rich comparison' hooks:

按照惯例，False 和 True 会返回以进行成功的比较.但是，这些方法可以返回任何值，因此如果在布尔上下文中使用比较运算符(例如，在 if 语句的条件中)，Python 将调用 bool() 上的值来确定结果是真还是假.

By convention, False and True are returned for a successful comparison. However, these methods can return any value, so if the comparison operator is used in a Boolean context (e.g., in the condition of an if statement), Python will call bool() on the value to determine if the result is true or false.

参见ColumnOperators class 在 SQLAlchemy 源代码中用于实现的特定钩子.

See the ColumnOperators class in the SQLAlchemy source for the specific hooks implemented.

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