CodeBlanks Logo Icon CodeBlanks
Login Register

traceback

Learn stack traces with the traceback module.

The traceback module

To see the full call stack of an error you use the traceback module. You import it with import traceback and get the stack trace at the error moment as a string with traceback.format_exc().

This string shows which file and line the error started on; it lists each call frame in order. It's useful for error logging in production applications; not shown to the user but tells the developer a lot.

Practice Print the first line of traceback.format_exc

Start with import traceback. In a try block call x = 1 / 0. In except ZeroDivisionError: get the stack trace with tb = traceback.format_exc(). Print the first line with print(tb.splitlines()[0]). The first line is "Traceback (most recent call last):".

Starter code

main.py
# Import the traceback module
# Try a division by zero in a try block
# Catch ZeroDivisionError, get the trace
# Print the first line

Sample solution

main.py
import traceback

# Call 1 / 0 in a try block
try:
    x = 1 / 0
# In except ZeroDivisionError: get the trace with format_exc
except ZeroDivisionError:
    tb = traceback.format_exc()
    # Print the first line
    print(tb.splitlines()[0])

Expected output

Traceback (most recent call last):

Reading a stack trace

A stack trace has three parts: the header (Traceback (most recent call last):), the call frames (file name, line, code) and the final line with the error type and message. The bottom line tells the actual error.

Frames show calls from top to bottom in aging order; the bottom frame is where the error happened. When debugging you start at the bottom and go up; this makes finding the source easier.

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does traceback.format_exc() return?

  • Only the line number that raised
  • Only the error type
  • The error stack trace as a string Correct answer
  • A file object

Explanation: The string contains file, line and type info.

2 Complete the format_exc call

Question 
import traceback
tb = traceback.____()
Answer 
import traceback
tb = traceback.format_exc()

Explanation: format_exc returns the stack trace as a string.

3 Match the traceback concepts

Term Meaning
format_exc Returns the trace as a string
import traceback Imports the module
splitlines Splits into lines
ZeroDivisionError Division by zero error

All lessons in Intermediate

1 Classes

  1. class
  2. Attributes
  3. Methods
  4. __init__
  5. __str__ and __repr__
  6. Summary - Classes

2 Inheritance

  1. Inheritance basics
  2. super()
  3. Method overriding
  4. Multiple inheritance
  5. isinstance and issubclass
  6. Summary - Inheritance

3 Project: Helper library

  1. Class design
  2. String helpers
  3. Number helpers
  4. Date helpers
  5. Putting it together

4 Encapsulation

  1. Private attributes
  2. @property
  3. @x.setter
  4. @dataclass
  5. __slots__
  6. Summary - Encapsulation

5 Static and class methods

  1. @staticmethod
  2. @classmethod
  3. Factory pattern
  4. Class variables
  5. Constants
  6. Summary - Static and class methods

6 Magic methods

  1. __eq__ and __lt__
  2. __add__
  3. __len__ and __getitem__
  4. __iter__ and __next__
  5. __contains__
  6. Summary - Magic methods

7 Property and decorator

  1. Decorator basics
  2. Function decorator
  3. @property details
  4. @cached_property
  5. functools.wraps
  6. Summary - Property and decorator

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Abstract classes

  1. ABC
  2. @abstractmethod
  3. Abstract property
  4. Protocol
  5. Interface vs abstract
  6. Summary - Abstract classes

10 Dataclasses

  1. @dataclass basics
  2. field()
  3. frozen
  4. __post_init__
  5. asdict and astuple
  6. Summary - Dataclasses

11 Project: Blog app

  1. Model design
  2. Blog class
  3. Adding comments
  4. Search and filter
  5. Putting it together

12 File I/O

  1. open
  2. read and readline
  3. write and writelines
  4. with statement
  5. File modes
  6. Summary - File I/O

13 CSV and JSON

  1. csv.reader
  2. csv.writer
  3. DictReader and DictWriter
  4. json.load and dump
  5. json.loads and dumps
  6. Summary - CSV and JSON

14 Pathlib

  1. Path
  2. / operator
  3. exists and is_file
  4. mkdir and rmdir
  5. glob and rglob
  6. Summary - Pathlib

15 Advanced exceptions

  1. Multiple except
  2. except tuple
  3. Exception chaining
  4. except as
  5. traceback
  6. Summary - Advanced exceptions

16 Collections module

  1. Counter
  2. defaultdict
  3. OrderedDict
  4. deque
  5. namedtuple
  6. Summary - Collections

17 Itertools

  1. chain
  2. count and cycle
  3. islice
  4. combinations and permutations
  5. groupby
  6. Summary - Itertools

18 Generators

  1. yield
  2. Generator object
  3. yield from
  4. send and throw
  5. close
  6. Summary - Generators

19 Context managers

  1. with statement
  2. __enter__ and __exit__
  3. @contextmanager
  4. Catching exceptions
  5. ExitStack
  6. Summary - Context managers

20 Intermediate final

  1. Project design
  2. OOP model
  3. Generator layer
  4. File I/O
  5. Putting it together