如何修复“ValueError：无法将字符串转换为浮点数”？

Question

I'm writing a python code where the use can input as many numbers until they click enter without entering a number. The it does calculations to find the max, min, average, and total.

But I keep running into that error.

numberList = []

while True:
  enterNumber = input("Please enter a number (RETURN/ENTER when done): ")
  if enterNumber == " ":
    break
  convertFloat = float(enterNumber)
  numberList.append(convertFloat)

theMax = numberList[0]
theMin = numberList[0]
theTotal = 0.0

for value in numberList:
  if value > theMax:
    theMax = value

  if value < theMin:
    themin = value

  theTotal = theTotal + value

average = theTotal / len(numberList)

print("The numbers entered were: ")
for numbers in numberList:
    print(" ", numbers)
print("Max value was: " + str(theMax))
print("Min value was: " + str(theMin))
print("The total is: " + str(theTotal))
print("Average value was: ") + str(avg)

EDIT: I fixed it thanks to the person in the comments. However, there is a new error

Traceback (most recent call last):
  File "main.py", line 31, in <module>
    print("Average value was: ") + str(average)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Answer 1

This error occurs because the program fails to convert the str value to float value.

Why? your str value isn't in a float format, maybe it contains only characters and words but you need to input float string value and then the program will cast it into float successfully.

If you want to handle this situation learn about try, except .

Answer 2

The error comes from these lines:

enterNumber = input("Please enter a number (RETURN/ENTER when done): ")
if enterNumber == " ":
    break
convertFloat = float(enterNumber)

When done, the user inputs blank which corresponds to "" . However, your code checks only for " " (a blank space) and so the input "" gets passed to float and raises an error. A quick fix is to change if enterNumber == " ": to if enterNumber == "": .

Also there's a further error down at line 31 where you call variable avg which is not referenced. I'd change it to average .

Answer 3

As it was noted in the other answers, you compare an entered string with a space instead of an empty string. But even with the mistake fixed, the user can enter a wrong formatted number. The cases are named as Exceptions, and you should handle it using try/except construction. Also there are max , min and sum functions. By the way, you can use str.join method to join a list into a string.

I would suggest such a code:

numberList = []

while True:
    enterNumber = input("Please enter a number (RETURN/ENTER when done): ")
    print(enterNumber)
    if not enterNumber:
        break
    try:
        convertFloat = float(enterNumber)
        numberList.append(convertFloat)
    except ValueError:
        print("Your entered a wrong formatted number, try one more time.")

theMax = max(numberList)
theMin = min(numberList)
theTotal = sum(numberList)
average = theTotal / len(numberList)

print("The numbers entered were: ")
print(" ".join(map(str, numberList)))
print("Max value was: " + str(theMax))
print("Min value was: " + str(theMin))
print("The total is: " + str(theTotal))
print("Average value was: " + str(average))