Creating a Python Object from a String in Python 3

Question

This is not a duplicate of creating a function object from a string because that is a Python 2 solution with exec not as a function()

Trying to implement that solution in Python 3 gets this:

d = {}
exec("def f(x): return x")in d
print(d)

returns:

{}

So the question is, how do I create a function in Python 3 from an arbitrary string? (In my case read in from a YAML file, but that's a side issue.)

I don't understand what the `in d` part is trying to do. If you just `exec` that code, you will be able to call `f(x)` in your code. Either way, I would never recommend running arbitrary strings, this seems like an XY problem — user3483203, Jul 18 '18 at 16:04
I need the function object so I can place it in an object to be used later. The use must be able to create an arbitrary function that uses data from the object. — Ray Salemi, Jul 18 '18 at 16:05
@user3483203 that's part of the syntax of the `exec` keyword in Python 2 (apparently). It seems like a honkin' bad idea to me. — Adam Smith, Jul 18 '18 at 16:05
Right. Understand that folks don't know my application. Moving beyond the question of whether others would do it, is it still possible to create a function object from an arbitrary string? — Ray Salemi, Jul 18 '18 at 16:06
@RaySalemi "The user must be able to create an arbitrary function" You realize how dangerous that is, right? — Adam Smith, Jul 18 '18 at 16:06
@RaySalemi hmm I guess I'm not imaginative enough to envision a use case where that danger is warranted ;) but I trust you've done your homework on that threat vector! — Adam Smith, Jul 18 '18 at 16:08
It returns `{}` as it is not being called anywhere. (wrong info, ignore.) — sP_, Jul 18 '18 at 16:09
@sP_ No, it returns `{}` because `d` is an empty dictionary. The middle line of code just checks if `None` is a member of `d`, which it is not. — Adam Smith, Jul 18 '18 at 16:11

score 3 · Accepted Answer · answered Jul 18 '18 at 16:08

3

X in d , returns True if X is found in element d.

Your exec call is defining function f in global scope.

This appears to do what you want it to:

>>> d = {}
>>> exec("def f(x): return x", d)
>>> d["f"]("Hello World")
'Hello World'

answered Jul 18 '18 at 16:08

Jake

Thanks! I see now how the `exec` has changed (and now that I understand it, improved.) – Ray Salemi Jul 18 '18 at 16:13

score 0 · Answer 2 · answered Jul 18 '18 at 16:10

0

>>> d={}
>>> exec("def f(x): return x",None,d)
>>> d['f'](2)
2
>>>

answered Jul 18 '18 at 16:10

blhsing

2 Answers2