Functions, Errors, and Files

Programming FoundationsLesson 4 of 875 minutesLab included

Learning objectives

By the end of this lesson

• Define a function with inputs and a return value.
• Read a traceback from the final exception backward.
• Catch an expected exception narrowly.
• Read a text file with deterministic cleanup.

Functions give behavior a boundary

def normalize_name(name):
    cleaned = name.strip().lower()
    return cleaned

result = normalize_name("  Ada  ")
print(result)

name is a parameter, the supplied string is an argument, and return sends one value back to the caller. The function’s boundary gives us a focused question: for this input, what output or failure results?

Read the last error first

Change the call to normalize_name(12). The traceback shows the chain of calls, followed by the exception type and message. Begin at the final line, then move upward to the first frame in your code. A traceback is a path to the failure, not a ceremonial wall of red text.

Catch only a failure you understand:

try:
    number = int("not-a-number")
except ValueError as error:
    print("invalid integer:", error)

Avoid a bare except: that turns unrelated bugs into the same vague message.

Files are external input

Create numbers.txt containing one integer per line, then create:

def load_numbers(path):
    numbers = []
    with open(path, "r", encoding="utf-8") as handle:
        for line_number, line in enumerate(handle, start=1):
            text = line.strip()
            if text:
                numbers.append(int(text))
    return numbers

print(load_numbers("numbers.txt"))

The with statement closes the file even when conversion fails. Add an invalid line and use the traceback to identify it. Then improve the function by catching ValueError, reporting line_number, and re-raising the error with raise.

Knowledge check

Question 1 of 2

Where should traceback investigation usually begin?

The final exception type and messageThe first line printed by the shell promptA random library frame

Check answer

What this gives us as researchers

You can isolate behavior, preserve the path to a failure, and treat file contents as untrusted input. The Python Behavior Lab combines those skills.