Passing a string to eval in method

Question

Struggling with passing a variable reference to a nested function. Using a dictionary is not an option in my use case. It's a much simplified MRE (real use passes an object with many nested objects).

def func(reference):
    eval('trueVal=' + reference)
    print(trueVal)                 #Expecting trueVal=15000

trueValue = 15000
reference = 'trueValue'
func(reference)

So what's the problem? If there's an error message, please include the whole thing. We shouldn't have to run the example just to see what the problem is. — wjandrea, Apr 20 '20 at 13:09
The whole thing? eval fails with SyntaxError: invalid syntax — misantroop, Apr 20 '20 at 13:13
Yes, edit the question to add the full error message with traceback. — wjandrea, Apr 20 '20 at 13:19
You cannot put '=' in an eval statement. You'd have to do it in an exec: `exec('trueVal=%s' % (reference),globals(),locals())` — User 12692182, Apr 20 '20 at 13:23
Does this answer your question? [eval SyntaxError: invalid syntax in python](https://stackoverflow.com/questions/22558548/eval-syntaxerror-invalid-syntax-in-python) — wjandrea, Apr 20 '20 at 13:24

score 3 · Accepted Answer · answered Apr 20 '20 at 13:17

3

eval evaluates expressions. The result of your expression in your example can then be assigned to trueVal explicitly:

trueVal = eval(reference)

answered Apr 20 '20 at 13:17

MatsLindh

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Good, thanks. I also arrived at the result with a much more complicated and roundabout way of exec('x=reference', globals()) and exec('trueVal=eval(x)', globals()) – misantroop Apr 20 '20 at 13:22

score 2 · Answer 2 · answered Apr 20 '20 at 13:23

I would not endorse using eval or exec, 99 times out of 100, there is a better way to do it, dictionary is not the only option but without posting your question its impossible to provide a better way to approach it. below is for reference as an example that works without hardcoding the variable name. But really there is always likely a better approach thatn eval or exec.

def func(reference, value):
    exec(reference + '="' + str(value) +'"')
    print(reference, ":", eval(reference))                 #Expecting trueVal=15000

trueValue = 15000
reference = 'trueVal'
func(reference, trueValue)

Passing a string to eval in method

2 Answers2