Python if-statement with variable mathematical operator

Question

I'm trying to insert a variable mathematical operator into a if statement, an example of what I'm trying to achieve in parsing user-supplied mathematical expressions:

maths_operator = "=="

if "test" maths_operator "test":
       print "match found"

maths_operator = "!="

if "test" maths_operator "test":
       print "match found"
else:
       print "match not found"

obviously the above fails with SyntaxError: invalid syntax. I've tried using exec and eval but neither work in an if statement, what options do I have to get around this?

Nathan · Accepted Answer · 2012-08-07 13:50:20Z

21

Use the operator package together with a dictionary to look up the operators according to their text equivalents. All of these must be either unary or binary operators to work consistently.

import operator
ops = {'==' : operator.eq,
       '!=' : operator.ne,
       '<=' : operator.le,
       '>=' : operator.ge,
       '>'  : operator.gt,
       '<'  : operator.lt}

maths_operator = "=="

if ops[maths_operator]("test", "test"):
    print "match found"

maths_operator = "!="

if ops[maths_operator]("test", "test"):
    print "match found"
else:
    print "match not found"

answered Aug 7, 2012 at 13:50

Nathan

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Comments

Mark Byers · Accepted Answer · 2012-08-07 14:03:14Z

15

Use the operator module:

import operator
op = operator.eq

if op("test", "test"):
   print "match found"

edited Aug 7, 2012 at 14:03

answered Aug 7, 2012 at 13:45

Mark Byers

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1 Comment

binhex Over a year ago

thanks for your answer Mark, the operator module def is the way to get around this.

w-m · Accepted Answer · 2012-08-14 13:21:49Z

1

I've tried using exec and eval but neither work in an if statement

For the sake of completeness it should be mentioned that they do work, even if the posted answers provide a better solution. You'll have to eval() the whole comparison, not just the operator:

maths_operator = "=="

if eval('"test"' + maths_operator '"test"'):
       print "match found"

or exec the line:

exec 'if "test"' + maths_operator + '"test": print "match found"'

answered Aug 14, 2012 at 13:21

w-m

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