Read the Traceback

读 traceback

≈ 15 min · python-debugging python-strings · Open in WolfCode

Parse a Python traceback as a string. Read it bottom-up: last line = error type, deepest frame = where the bug is.

Anatomy

Traceback (most recent call last):
  File "letter_grade.py", line 19, in <module>
    print(letter_grade(85))
  File "letter_grade.py", line 12, in letter_grade   ← deepest frame, usually the bug
    return grade
NameError: name 'grade' is not defined              ← error type + message

The two helpers

Python · runnable

def error_type(tb):
    last_line = tb.strip().splitlines()[-1]
    return last_line.split(":")[0].strip()

def last_frame(tb):
    filename = ""
    line_number = 0
    for raw in tb.splitlines():
        line = raw.strip()
        if line.startswith('File "'):
            rest = line[len('File "'):]
            close_q = rest.find('"')
            name = rest[:close_q]
            after = rest[close_q + 1:]
            i = after.find("line ")
            digits = ""
            for ch in after[i + 5:]:
                if ch.isdigit():
                    digits += ch
                else:
                    break
            filename = name
            line_number = int(digits)
    return (filename, line_number)

tb = '''Traceback (most recent call last):
  File "letter_grade.py", line 19, in <module>
    print(letter_grade(85))
  File "letter_grade.py", line 12, in letter_grade
    return grade
NameError: name 'grade' is not defined'''

print(error_type(tb))       # NameError
print(last_frame(tb))       # ('letter_grade.py', 12)

Loading Python runtime (~5 MB, one-time)…

The common runtime errors

Error	Most likely cause
`NameError`	Typo or used-before-defined
`TypeError`	Wrong type or arg count
`IndexError`	Index out of range
`KeyError`	Dict key not present
`AttributeError ... NoneType ...`	Function returned None; you called .x on it
`RecursionError`	Forgot the base case
`ZeroDivisionError`	Divided by 0