Syllabus Lesson 7 of 239 · Python Foundations
Python Foundations

Errors & Tracebacks

Errors are not failure, they are feedback. When Python cannot run a line it stops and prints a traceback. Read it from the bottom up: the last line names the error type and gives a short message, and the lines above point at where it happened.

The three you will meet first

  • SyntaxError means the code is shaped wrong, so Python cannot even start. Usually a missing :, a missing quote, or an unmatched bracket.
  • NameError means you used a name Python has never seen. Usually a typo, or using a variable before it is created.
  • TypeError means an operation got the wrong kind of value, like adding a number to a string: "age: " + 30.
print("hi)        # SyntaxError: missing closing quote
print(totl)       # NameError: 'totl' is not defined
print("n" + 5)    # TypeError: can only concatenate str (not "int") to str

How to fix a TypeError when mixing text and numbers

Convert the number to text first with str(...), or better, use an f-string which converts for you:

age = 30
print("age: " + str(age))   # age: 30
print(f"age: {age}")        # age: 30  (cleaner)

Tracebacks feel scary at first. After a week of reading them they become the fastest debugging tool you own.

Your turn

The snippet below is meant to define describe(age) so that describe(30) returns the string "age: 30", but it has a bug that raises a TypeError (you cannot add a number to a string). Fix the function so it returns the correct text. Use str(age) or an f-string. Then print describe(30).

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