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Shadowing a Built-in Function (Part 1, 4: Arithmetic operations)

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Assessment

Difficulty
1/5
Estimated time
Under an hour
Newbie friendliness
62/100
Issue type
Documentation
Clarity
Mostly clear
Activity status
Quiet
Tech stack
python
Domain
documentation

Research direction

Locate the lesson containing the shown Python arithmetic example and run the snippet, including the commented-out initialization, to observe the built-in shadowing behavior. Update the example and explanation so the accumulator uses a non-shadowing name such as total_sum, while preserving the explanation of the resulting error.

Written by the indexing model from the issue text.

Description

sum = 0

number = int(input("First number: "))
sum = sum + number

number = int(input("Second number: "))
sum = sum + number

number = int(input("Third number: "))
sum = sum + number

print(f"The sum of the numbers: {sum}")

As you may well be aware, sum is a built-in function in Python (used for adding up items in lists, etc.). While Python allows us to use sum as a variable name, it "shadows" the built-in function, which can lead to confusing bugs later in our code. It is safer to use a name like total_sum instead.

For example, comment out the first line and try to run the code. It should result in a NameError, but it raises a TypeError instead. Why? A NameError only occurs when Python cannot find a name anywhere (not in our local variables, global variables, or built-in functions). Because # sum = 0 was commented out, Python didn't find sum as a variable, but it did find the pre-existing built-in function sum() that comes built into Python. Therefore, it didn't raise a NameError.

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