按字符串名称以编程方式查找运算符

Question

Suppose I have the following code:

def process(x, y, op):
    if op == 'eq':
        return x == y
    elif op == 'gt':
        return x > y
    elif op == 'lt':
        return x < y
    ... for many more operators, e.g. >=, <=, !=

How can I distill this down more programmatically? I'm thinking of creating a lookup like this:

op_lookup = {'eq': '==', 'gt': '>', 'lt': '<', ...}

Then doing something like this:

def process(x, y, op):
    return x op_lookup[op] y

Which obviously isn't valid Python...

Something like eval might work:

def process(x, y, op):
    return eval('{} {} {}'.format(x, op_lookup[op], y))

Which produces this, in an example:

>>> process(1, 1, 'eq')
True
>>> process(1, 1, 'lt')
False

Is there a better (safer?) way of accomplishing this?

Answer 1

Operators as callables are hidden in the operator module. You can dynamically access a specific operator by its name with the getattr builtin.

Demo:

>>> import operator         
>>> getattr(operator, 'eq')(1, 1)
True
>>> getattr(operator, 'eq')(1, 2)
False
>>> getattr(operator, 'lt')(1, 2)
True
>>> getattr(operator, 'lt')(2, 1)
False

We can use this to rewrite your function as follows.

import operator

def process(x, y, op):
    return getattr(operator, op)(x, y)

(Add sanity checks as required.)

Answer 2

The operator module is what you want:

>>> import operator
>>> op_lookup = {'eq': operator.eq, 'gt': operator.gt, 'lt': operator.lt}
>>> 
>>> def process(x, y, op):
...     return op_lookup[op](x, y)
... 
>>> process(1, 1, 'eq')
True
>>> process(1, 1, 'lt')
False