python convert a string to an operator [duplicate]

Question

Is it possible to convert a string to an operator in python? I would like to pass a condition to a function

Ideally it would look like this:

def foo(self, attribute, operator_string, right_value):
    left_value = getattr(self, attribute)
    if left_value get_operator(operator_string) right_value:
         return True
    else:
         return False

bar.x = 10
bar.foo('x', '>', 10)
[out] False
bar.foo('x', '>=', 10)
[out] True

I could make a dictionary where keys are strings and values are functions of the operator module. I would have to change foo definition slightly:

operator_dict = {'>', operator.lt,
                 '>=', operator.le}
def foo(self, attribute, operator_string, right_value):
    left_value = getattr(self, attribute)
    operator_func = operator_dict[operator_string]
    if operator_func(left_value, right_value):
         return True
    else:
         return False

This means I have to make this dictionary, but is it really necessary?

As long as you have a manageable set of operators, a dictionary is just fine. — Oben Sonne
– Oben Sonne, Commented Feb 25, 2011 at 12:29

eumiro · Accepted Answer · 2011-02-25 14:27:30Z

4

You can use eval to dynamically build a piece of Python code and execute it, but apart from that there are no real alternatives. The dictionary-based solution is much more elegant and safe, however.

Apart from that, is it really that bad? Why not shorten it a bit …

return operator_dict[operator_string](left_value, right_value)

edited Feb 25, 2011 at 14:27

eumiro

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answered Feb 25, 2011 at 12:24

Alexander Gessler

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1 Comment

Ben2209 Over a year ago

Thanks for making me aware of eval. but yes, you are right, it is not that bad to create this dictionary.

stderr · Accepted Answer · 2011-02-25 19:48:32Z

3

The way the problem is specified I don't see why you can't pass operator.le to the function instead of ">=".

If this operator_string coming from a database or file or something or are you passing it around in your code?

bar.foo('x', operator.le , 10)

Are you just looking to have a convenient shorthand? Then you might do something like:

from operator import le
bar.foo('x', le, 10)

If the real problem here is that you have code or business rules coming in from a database or datafile then maybe you actually need to look at writing a little parser that will map your input into these objects and then you could take a look at using a library like pyparsing, ply, codetalker, etc.

edited Feb 25, 2011 at 19:48

answered Feb 25, 2011 at 13:26

stderr

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2 Comments

Ben2209 Over a year ago

I was just after a convenient shortcut. So your solution sounds good as well, thanks for that.

kindall Over a year ago

Or you could just do from operator import eq, ne, gt, lt, le, ge rather than having to write assignments for all of them.

Ani bairy · Accepted Answer · 2021-05-31 05:43:22Z

0

#This is very simple to do with eval()

score=1
trigger_conditon=">="
trigger_value=4

eval(f"{score}{trigger_conditon}{trigger_value}")

#luckily fstring also takes care of int/float or relavaent datatype

operator_str="ge"
import operator
eval(f"operator.{operator_str}({score},{trigger_value})")

answered May 31, 2021 at 5:43

Ani bairy

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