Change this:

    print(calculate(splitter(expression)))

to this:

    print(calculate(* splitter(expression)))

The error is that calculate expects three float values (x, and y, and z), but splitter returns a tuple object ((x, y, z) as one object, similar to arrays in other languages):

>>> type(splitter("1 + 2"))
<class 'tuple'>

Prepending a tuple or a list with a * operator (unpacking or splatting) unpacks the object into its individual items that are then passed to the function as separate arguments.

In fact, str.split() already returns a tuple. By assigning multiple values at once in x, y, z = expression.split(), you actually unpack the returned tuple into individual values, then in return x, y, z, you pack those values into a tuple.

There's a !python@programming.dev community more appropriate to python specific questions

Functions in Python can only return a single value. While you are allowed to comma-separate additional values, as you have done, they are combined into a single tuple object that wraps the intended values.

As such, your splitter function is returning a single tuple containing three strings. Your calculate function is expecting three individual arguments (I'm guessing that the error trace mentions this).

To get around this, you can use the splat/asterisk operator to "unpack" the items from the tuple:

a, b, c = splitter(expression)
# or, inline
calculate(*splitter(expression))

Edit: removed bad asterisk

print(eval(input("Expression:")))

Unsafe coding is best coding ;)

Yeah others have pointer out the error, but I want to really recommend using VSCode with the Python extension and static types. It will make finding these errors super easy because it adds a red underline exactly where the problem is.

Static types means:

def splitter(expression: str) -> tuple[str, str, str]:
...
def calculate(x: str, y: str, z: str) -> str:

After the split x becomes (1 and z becomes 1). They can't be converted to float. I think that's why. Let me run the code. Edit - Also as you are returning a, b, c from splitter but Python functions return a single object. The a, b, c turns to a tuple. But you are using js syntax I think. So we have to unwrap inside the calculate function.

Now my original comment is not useful but you can try to introduce a conditional to check of the numbers x and z are actually numbers before converting to floats

# Ask user to enter an expression and display output

def main():
    expression = input("Expression: ")
    print(calculate (splitter (expression)))

# Split expression into components and assign to variables as float values

def splitter (expression):
    print(expression)
    x, y, z = expression.split()
    print(expression.split())
    return x, y, z

# Calculate expression result

# Changes Beginning
def calculate(numbers):
    x,y,z = numbers
# Changes End

    x, z = float(x), float(z)
    if y == "+":
        return str(round((x + z), 1))
    elif y == "-":
        return str(round((xz), 1))
    elif y == "*":
        return str(round((x*z), 1))
    else:
        return str(round((x / z), 1))

main()

you're missing the asterisk to expand the tuple returned by splitter:

print(calculate(*splitter(expression)))

if you're using VS Code, check out the error lens extension