Reading Error Messages

Traceback (most recent call last):
  File "analysis.py", line 12, in <module>
    result = average(scores)          <- the line that called the failing code
  File "analysis.py", line 7, in average
    return total / len(values)        <- the line that actually failed
ZeroDivisionError: division by zero    <- what went wrong

print("unclosed          # SyntaxError

def f():
return 1                  # IndentationError

print(undefined_name)     # NameError

print("age: " + 30)       # TypeError: can only concatenate str to str

print([1, 2, 3][5])       # IndexError: list index out of range

def average(values):
    print("got:", values, "type:", type(values))   # what did I actually receive?
    total = sum(values)
    print("total:", total, "count:", len(values))  # are both what I expect?
    return total / len(values)

Hold on to

• An error is information, not judgment — read the bottom line first
• Five error types cover almost everything you will hit early
• When there is no error, print the values and compare against your assumption

Try each one before opening the solution. Getting it wrong first is most of where the learning happens.

1. Deliberately produce all five error types from the lesson. For each, write down the bottom line of the traceback and the fix in your own words.
   Hint

   Copy each snippet from the lesson into its own file and run it one at a time.

   Solution

   SyntaxError: unterminated string — add the closing quote. IndentationError: expected an indented block — indent the body four spaces. NameError: name is not defined — assign it first, or fix the typo. TypeError: can only concatenate str — convert with `str(30)` or use an f-string. IndexError: list index out of range — the list has 3 items, so valid indices are 0, 1, 2.

   print("closed now")             # SyntaxError fixed
   
   def f():
       return 1                    # IndentationError fixed
   
   defined_name = 1
   print(defined_name)             # NameError fixed
   
   print("age: " + str(30))        # TypeError fixed
   print(f"age: {30}")             # or better
   
   print([1, 2, 3][2])             # IndexError fixed

2. Write a function that divides two numbers, call it with a zero denominator, and read the traceback. Identify which line failed and which line called it.
   Hint

   The traceback lists frames oldest first — the failing line is the last file/line pair.

   Solution

   The bottom frame points at `return a / b`, which is where the division happened. The frame above it points at your call site. The bottom line names the type: ZeroDivisionError. Fixing it means deciding what division by zero should mean for your program — raise a clearer error, or return a defined value — which is a design choice, not a syntax fix.

   def divide(a, b):
       if b == 0:
           raise ValueError("denominator must be non-zero")
       return a / b
   
   divide(1, 0)   # now fails with a message that says what you did wrong

   Check your work

   Paste this after your own code. If it runs without raising, you have it.

   try:
       divide(1, 0)
   except ValueError as exc:
       assert "non-zero" in str(exc)
   else:
       raise AssertionError("expected a ValueError")
   assert divide(6, 3) == 2
   print("ok")

3. Take a function that returns a wrong answer (write one on purpose — say, a sum that forgets the last element), and find the bug using only print statements. Note how many prints it took.
   Hint

   Print the inputs at the top of the function and the result just before returning.

   Solution

   The classic off-by-one: `range(len(values) - 1)` stops one element early. Two prints find it — one showing the input has 5 items, one showing the loop ran 4 times. Most bugs need two or three prints, not a debugger. The number of prints it took you is a useful thing to notice, because it drops fast with practice.

   def broken_sum(values):
       total = 0
       for i in range(len(values) - 1):   # bug: misses the last element
           total += values[i]
       return total
   
   def fixed_sum(values):
       total = 0
       for value in values:               # iterate the items, not the indices
           total += value
       return total

   Check your work

   Paste this after your own code. If it runs without raising, you have it.

   assert fixed_sum([1, 2, 3, 4, 5]) == 15
   assert fixed_sum([]) == 0
   print("ok")

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