ValueError: not enough values to unpack (expected 3, got 1) when using split()-CodePudding

I keep getting this error:

x, y, z = expression.split(" ") 
ValueError: not enough values to unpack (expected 3, got 1)

This is my code:

expression = input("Expression: ")
# Parse variables.
x, y, z = expression.split(" ")
new_x=float(x)
new_z=float(z)
#calculate
if y == " ":
    result = new_x   new_z
if y == "-":
    result = new_x - new_z
if y == "*":
    result = new_x * new_z
if y == "/":
    result= new_x / new_z
print(round(result, 1))

CodePudding user response：

The issue is that the input does not have enough delimited fragments to split into three pieces. You can play with this in the REPL:

>>> x, y, z = "a b c".split(" ")
>>> x
'a'
>>> y
'b'
>>> z
'c'
>>> x, y, z = "a b".split(" ")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: not enough values to unpack (expected 3, got 2)
>>> x, y, z = "a".split(" ")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: not enough values to unpack (expected 3, got 1)

For your use case where you are accepting input, the best bet is to handle this specific exception.

valid_input = false
x, y, z = None, None, None
while not valid_input:
  try:
    expression = input("Expression: ")
    x, y, z = expression.split(" ")
    valid_input = true
  except ValueError:
    print("Invalid input provided. Be sure to provide input with exactly 2 spaces in it!")

...  # Continue with the rest of your code

Another way to do this is:

def input_expression(attempt: int = 0) -> [str, str, str]:
  if attempt > 3:
    raise Exception("Too many invalid attempts. Quitting.")
  try:
    expression = input("Expression: ")
    x, y, z = expression.split(" ")
    return x, y, z
  except ValueError:
    print("Invalid input provided. Be sure to provide input with exactly 2 spaces in it!")
    return input_expression(attempt   1)

x, y, z = input_expression()
... # The rest of your code

CodePudding user response：

it means that expression.split(" ") returns only one value, but since you unpacked its value to three variables x,y,z, it should return three values. So the problem is actually with your input.