Passing expressions to functions?

问题:

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

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

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

I'm trying to achieve something like this

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

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

What about if one of the parameters was an object?

回答1:

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

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

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.

回答2:

An even more pythonic variant of Nelson's solution is to use the operator functions from the operator module in the standard library; there is no need to create your own lambdas.

>>> from operator import eq
>>> def magic(left, op, right):
...   return op(left, right)
... 
>>> magic(5, eq, 5)
True

回答3:

You can't. The expression 5 == 5 is evaluated and only then is the result passed to someFunc. The function just gets True (the True object, to be precise), no matter what the expression was.

Edit: Concerning your edit, this question is kind of close.

Edit 2: You could just pass the expression as a string and use eval, like this:

>>> def someFunc(expression_string):
...    print(expression_string, "evaluates to", eval(expression_string))

>>> someFunc("5 == 5")
5 == 5 evaluates to True

Don't know whether that helps you. Keep in mind that eval is a powerful tool, so it's dangerous to pass arbitrary (and possibly even user-generated) input to it.

回答4:

It appears you can return tuples from eq:

class Foo:
    def __init__(self, value):
            self.value = value

    def __eq__(self, other):
            return (self.value, other.value)


f1 = Foo(5)
f2 = Foo(10)
print(f1 == f2)

回答5:

You have to implement __eq__() . For example ::

class A(object):
    def __eq__(self, other):
        return (self, '==', other)

Then, for the function, which you want to get the expression, like ::

def my_func(expr):
    # deal with the expression
    print(expr)

>>> a = A()
>>> my_func(a == 1)
(<__main__.A object at 0x1015eb978>, '==', 1)

回答6:

Short answer: You can't. The result of the expression evaluation is passed to the function rather than the expression itself.