How to handle AssertionError in Python and find out which line or statement it occurred on?

Question

I want to handle AssertionErrors both to hide unnecessary parts of the stack trace from the user and to print a message as to why the error occurred and what the user should do about it.

Is there any way to find out on which line or statement the assert failed within the except block?

try:
    assert True
    assert 7 == 7
    assert 1 == 2
    # many more statements like this
except AssertionError:
    print 'Houston, we have a problem.'
    print
    print 'An error occurred on line ???? in statement ???'
    exit(1)

I don’t want to have to add this to every assert statement:

assert 7 == 7, "7 == 7"

because it repeats information.

Asked By: devtk

||

Source

Answer 1

Use the traceback module:

import sys
import traceback

try:
    assert True
    assert 7 == 7
    assert 1 == 2
    # many more statements like this
except AssertionError:
    _, _, tb = sys.exc_info()
    traceback.print_tb(tb) # Fixed format
    tb_info = traceback.extract_tb(tb)
    filename, line, func, text = tb_info[-1]

    print('An error occurred on line {} in statement {}'.format(line, text))
    exit(1)

Answered By: phihag

Answer 2

The traceback module and sys.exc_info are overkill for tracking down the source of an exception. That’s all in the default traceback. So instead of calling exit(1) just re-raise:

try:
    assert "birthday cake" == "ice cream cake", "Should've asked for pie"
except AssertionError:
    print 'Houston, we have a problem.'
    raise

Which gives the following output that includes the offending statement and line number:

Houston, we have a problem.
Traceback (most recent call last):
  File "/tmp/poop.py", line 2, in <module>
    assert "birthday cake" == "ice cream cake", "Should've asked for pie"
AssertionError: Should've asked for pie

Similarly the logging module makes it easy to log a traceback for any exception (including those which are caught and never re-raised):

import logging

try:
    assert False == True 
except AssertionError:
    logging.error("Nothing is real but I can't quit...", exc_info=True)

Answered By: notpeter

How to handle AssertionError in Python and find out which line or statement it occurred on?

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