How to substitute value of variables in Python expression, but not evaluate the expression?


I have a Python expression that looks like the following:

var1 = 'GOOGLE'
var2 = '5'
expr = 'df[df[var1]>=var2]'

In my workspace var1 and var2 are well defined so I can evaluate expr as follows:


However, I want to pass this expr (as string) to another function with values of var1 and var2 substituted in it. I do not want to pass the variables var1 and var2, as I can have any number of variables, not just two. How do I accomplish this?

Asked By: Zing



You can simply use Python f-string as demonstrated below

expr = f'df[df[{var1}] >= {var2}]'

Answered By: Maxwell D. Dorliea

You can parse the expression with ast.parse and use a subclass of ast.NodeTransformer to convert Name nodes to the corresponding values as Constant nodes, and then convert the AST back to code with ast.unparse:

import ast

var1 = 'GOOGLE'
var2 = '5'
expr = 'df[df[var1]>=var2]'

class NamesToConstants(ast.NodeTransformer):
    def visit_Name(self, node):
        if in globals(): # feel free to use your own dict instead of globals()
            value = globals()[]
            try: # convert value to integer if viable
                value = int(value)
            return ast.Constant(value=value)
        return node

tree = ast.parse(expr)

This outputs:

df[df['GOOGLE'] >= 5]

ast.unparse requires Python 3.10 or later. If you’re using an earlier version, you can use astunparse.unparse from the astunparse package instead.


Answered By: blhsing
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