Python新手求助：求和代码报TypeError错误，求排查解决

Hey there! Let's work through the issues in your code and get it working properly.

First, let's understand why you're getting that error #

The error TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str' comes from two key problems:

• You're initializing total = None inside the loop, so every time the loop runs, total gets reset to None—a value that represents "nothing." You can't use the += operator on None because it doesn't know how to add anything to it.
• The input() function returns a string by default. Even if you fixed the None issue, you'd be concatenating strings instead of adding numbers (e.g., inputting 1, 2, 3 would give you "012" instead of 6).

Here's the fixed code #

total = 0  # Initialize total to 0 (the starting point for sums) outside the loop
for i in range(1, 4):  # Use 1-4 so your prompts show "number 1", "number 2", "number 3"
    # Convert input to a float (supports decimals) or int (only whole numbers)
    num = float(input(f'Please enter number {i}:'))
    total += num
print(f'The sum of your three numbers is: {total}')

Let's break down the changes: #

• Move total = 0 outside the loop: This way, total keeps its value between loop iterations and you're actually accumulating the sum instead of resetting it every time. We use 0 because it's the identity element for addition—adding 0 to any number doesn't change it.
• Convert input to a numeric type: Using float() (or int() if you only need whole numbers) turns the user's string input into a number that Python can add mathematically.
• Adjust the range(): range(1,4) gives you the numbers 1, 2, 3, which makes your prompt more user-friendly (people usually count starting at 1, not 0!).

Quick note about total = None #

In Python, you don't need to "declare" variables before using them—you can just assign a value when you need it. None is meant for representing an empty or missing value, not for initializing variables that will hold numbers. For numeric sums, starting at 0 is always the right call.

内容的提问来源于stack exchange，提问作者Stamp