assign value to a variable using eval in a metaprogramming manner

Question

I want to assign value to a variable using eval in a metaprogramming manner. My attempt was shown below:

sample = None
var_name = "sample"
value = 0
eval("{0} = {1}".format(var_name, value))

However, I got the following error:

Traceback (most recent call last):
  File "tmp.py", line 4, in <module>
    eval("{0} = {1}".format(var_name, value))
  File "<string>", line 1
    sample = 0
           ^
SyntaxError: invalid syntax

Could you explain how can I do this? I think lower level function like assign(var, val) could exist and this enabled assignment using eval. But I couldn’t find such function.

eval metaprogramming python

GProst · Accepted Answer

Use exec instead:
sample = None
var_name = "sample"
value = 0
exec("{0} = {1}".format(var_name, value))

eval is for evaluating an expression, not an assignment statement

kouta-kun · Answer

If you're outside a function you can use

globals()[varname] = value

Example:

>>> a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined
>>> globals()['a'] = 5
>>> a
5

Inside a function there's locals() but it can only read from the symbol table and not update so you should use eval() like other answers have pointed out.

You can also cheat by using setattr/hasattr/getattr on an object:

>>> class Object:
...     pass
... 
>>> obj = Object()
>>> obj.a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Object' object has no attribute 'a'
>>> setattr(obj, 'a', 3)
>>> obj.a
3

Ironkey · Answer

does exec accomplish what you're trying to do?:
sample = None
var_name = "sample"
value = 5
exec(f"{var_name} = {value}")

print(sample)

output: 5

assign value to a variable using eval in a metaprogramming manner

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