Python Function: UnboundLocalError: local variable 'tuple' referenced before assignment

Question

The first block of code works (lines 15 - 21).

The error is occurring in the second block (lines 24 - 30).

Here is my code:

# converting numerical input into list and tuple <-- line 15

data = input("provide numbers separated by ',': ")
list = data.split(",")
tuple = tuple(list)

print("list:", list, "tuple:", tuple)


def convert(): # <-- line 24
    data = input("provide numbers separated by ',': ")
    list = data.split(',')
    tuple = tuple(list)
    print("list:", list, "tuple:", tuple)

convert()

I have read solutions that suggest declaring the variable in the global namespace, but the solutions do not seem to be working for my function. And a solution still eludes me.

The function has the same code as the first block, its just wrapped in a function. I don't understand why the code works at the top level, but not inside a function.

Does anyone know what's happening here?

Answer 1

If you re-bind a non-global variable name (such as a = b) anywhere within a function⁽¹⁾, it is made a local variable for that entire function (even before the modification).

That means that tuple = tuple(list) is assigning to a local tuple, and your use of tuple(list) is using that variable, rather than the actual built-in tuple() function. Since that variable name is not bound at this point, you get the "use before set" error.

This is why it's a bad idea to use built-in function names as variable names, which is something you appear to have done with abandon, both tuple and list :-)

So I would suggest two things:

• rename your variables to prevent clashes (use my_list and my_tuple for example).
• make my_tuple global within the function (better would be to avoid globals altogether).

Avoiding globals can be as simple as:

def convert():
    data = input("provide numbers separated by ',': ")
    my_list = data.split(",")
    my_tuple = tuple(my_list)
    print("list:", my_list, "tuple:", my_tuple)
    return my_tuple

outer_tuple = convert()

---

⁽¹⁾ This does not include simply modifying the variable's value such as changing one item in a list:

x = [1, 2, 3]
x[1] = 42         # Mutation of x, not re-binding.
x = [7, 8, 9]     # Re-binding x to new object.

And note that for immutable types (like int for example), x = 7 is re-binding x to a different object 7, not changing the value of the object "behind" x. This particular aspect of Python (the fact that all things are objects and variable names are simply bound to those objects) was one of my greatest epiphanies when learning the language.