将表达式传递给函数? [英] Passing expressions to functions?

问题描述

我不太确定我的意思，所以请忍受.

I'm not quite sure what I mean here, so please bear with me..

在SQLAlchemy中，似乎应该将表达式传递给

In SQLAlchemy, it appears I'm supposed to pass an expression to filter() in certain cases. When I try to implement something like this myself, I end up with:

>>> def someFunc(value):
...    print(value)

>>> someFunc(5 == 5)
True

如何从函数内部获取传递给==的值?

How do I get the values passed to == from inside the function?

我正在努力实现这样的目标

I'm trying to achieve something like this

 >>> def magic(left, op, right):
 ...    print(left+" "+op+" "+right)

 >>> magic(5 == 5)
 5 == 5

如果其中一个参数是对象怎么办?

What about if one of the parameters was an object?

推荐答案

如果将"op"设为函数，则可以实现示例:

You can achieve your example if you make "op" a function:

>>> def magic(left, op, right):
...     return op(left, right)
...
>>> magic(5, (lambda a, b: a == b), 5)
True
>>> magic(5, (lambda a, b: a == b), 4)
False

这比传递字符串更像Pythonic.像 sort() 之类的功能就是这样.

This is more Pythonic than passing a string. It's how functions like sort() work.

那些带有filter()的SQLAlchemy示例令人费解.我不了解有关SQLAlchemy的内部知识，但是我猜在像query.filter(User.name == 'ed')这样的示例中，正在发生的情况是User.name是特定于SQLAlchemy的类型，而对__eq()函数的奇怪实现会生成SQL filter()函数而不是进行比较.即:他们已经制作了特殊的类，使您可以键入发出SQL代码的Python表达式.这是一种不寻常的技术，除非构建一种将两种语言(例如ORM)架桥起来的方法，否则我将避免使用这种技术.

Those SQLAlchemy examples with filter() are puzzling. I don't know the internals about SQLAlchemy, but I'm guessing in an example like query.filter(User.name == 'ed') what's going on is that User.name is a SQLAlchemy-specific type, with an odd implementation of the __eq() function that generates SQL for the filter() function instead of doing a comparison. Ie: they've made special classes that let you type Python expressions that emit SQL code. It's an unusual technique, one I'd avoid unless building something that's bridging two languages like an ORM.

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