Convert a string to a callable function in Python

Question

I want to take written function (string) from a user input and convert it to a callable function in Python. How to achieve it despite any security concerns?

Example (string):

""" def test(): \n print('success') \n return 'success' """

I want to call / evaluate the said string by calling test() and make it print to the console.

use value or func = eval(some code string), you might need value = func(params) afterwards. — Lei Yang
– Lei Yang, Commented Feb 24, 2022 at 1:11
@JohnnyMopp literal eval is for literal data only (like 53.136, or (1, 2, 3), or {"x": 3}). You cannot parse function definitions with it. — ddejohn
– ddejohn, Commented Feb 24, 2022 at 1:23
eval evaluates expressions; you need exec to execute a def statement. — chepner
– chepner, Commented Feb 24, 2022 at 1:24
@JohnnyMopp I tried using a node generated by ast.parse before the ast.literal_eval, but got a ValueError: malformed node or string. That was using indentation. With no indentation I got indentation error. — Julio S.
– Julio S., Commented Feb 24, 2022 at 1:25

chepner · Accepted Answer · 2022-02-24 01:26:17Z

2

When appropriate, this is a job for the exec function.

>>> exec("""def test(): \n print('success') \n return 'success' """, globals())
>>> test()
success
'success'

answered Feb 24, 2022 at 1:26

chepner

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2 Comments

Julio S. Over a year ago

Why should one use exec instead of compile? Thanks!

Lei Yang Over a year ago

in short, exec is shorter. by compile you need another invoke statement.

maya · Accepted Answer · 2022-02-24 01:26:18Z

2

Try this, using the compile method combined with eval or exec：

a = compile("""def test(): \n print('success') \n return 'success' """, "", "exec")
eval(a)

test()

answered Feb 24, 2022 at 1:26

maya

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2 Comments

Julio S. Over a year ago

Why compile should be picked instead of exec? Thanks!

maya Over a year ago

Both are possible, but I think this way is more concise and less error-prone

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