Understanding error messages

Tip

The visible error and its root cause may be located in very different parts of your code.

If there's an error in your code that causes the program to terminate, read the error message and see what it can tell you.

Most of the time, the error message should allow you to identify:

• What went wrong? For example, did it try to read data from a file that does not exist?

• Where did this happen? On which line of which file did the error occur?

Stack traces

When an error occurs, one useful piece of information is knowing which functions were called in order to make the error occur.

Below we have example Python and R scripts that produce an error.

Question

Can you identify where the error occurred, just by looking at the error message?

Tip

R may not give you a useful stack trace unless you instruct it do so. In the example, we use:

options(error = rlang::entrace)

OverviewPythonR

You can download each script and run them on your own computer:

• stacktrace.py
• stacktrace.R

The error message is:

Traceback (most recent call last):
  File "stacktrace.py", line 46, in <module>
    status = main()
  File "stacktrace.py", line 7, in main
    do_big_tasks()
  File "stacktrace.py", line 17, in do_big_tasks
    do_third_step(i, quiet=quiet)
  File "stacktrace.py", line 38, in do_third_step
    try_something()
  File "stacktrace.py", line 42, in try_something
    raise ValueError("Whoops, this failed")
ValueError: Whoops, this failed

Source code

#!/usr/bin/env python3

import sys


def main():
    do_big_tasks()
    return 0


def do_big_tasks(num_tasks=20, quiet=True):
    for i in range(num_tasks):
        prepare_things(i, quiet=quiet)
        do_first_step(i, quiet=quiet)
        do_second_step(i, quiet=quiet)
        if i > 15:
            do_third_step(i, quiet=quiet)


def prepare_things(task_num, quiet=True):
    if not quiet:
        print(f'Preparing for task #{task_num}')


def do_first_step(task_num, quiet=True):
    if not quiet:
        print(f'Task #{task_num}: doing step #1')


def do_second_step(task_num, quiet=True):
    if not quiet:
        print(f'Task #{task_num}: doing step #2')


def do_third_step(task_num, quiet=True):
    if not quiet:
        print(f'Task #{task_num}: doing step #3')
    try_something()


def try_something():
    raise ValueError('Whoops, this failed')


if __name__ == '__main__':
    status = main()
    sys.exit(status)

The error message is:

Error in try_something() : Whoops, this failed
Calls: main -> do_big_tasks -> do_third_step -> try_something
Backtrace:
    ▆
 1. └─global main()
 2.   └─global do_big_tasks()
 3.     └─global do_third_step(i, quiet = quiet)
 4.       └─global try_something()
Execution halted

Source code

#!/usr/bin/env -S Rscript --vanilla

options(error = rlang::entrace)


main <- function() {
  do_big_tasks()
  invisible(0)
}

do_big_tasks <- function(num_tasks = 20, quiet = TRUE) {
  for (i in seq_len(num_tasks)) {
    prepare_things(i, quiet = quiet)
    do_first_step(i, quiet = quiet)
    do_second_step(i, quiet = quiet)
    if (i > 15) {
      do_third_step(i, quiet = quiet)
    }
  }
}

prepare_things <- function(task_num, quiet = TRUE) {
  if (!quiet) {
    cat("Preparing for task #", task_num, "\n", sep = "")
  }
}

do_first_step <- function(task_num, quiet = TRUE) {
  if (!quiet) {
    cat("Task #", task_num, ": doing step #1\n", sep = "")
  }
}

do_second_step <- function(task_num, quiet = TRUE) {
  if (!quiet) {
    cat("Task #", task_num, ": doing step #2\n", sep = "")
  }
}

do_third_step <- function(task_num, quiet = TRUE) {
  if (!quiet) {
    cat("Task #", task_num, ": doing step #3\n", sep = "")
  }
  try_something()
}

try_something <- function() {
  stop("Whoops, this failed")
}

if (! interactive()) {
  status <- main()
  quit(status = status)
}

Very long stack traces

You may get a very long stack trace when a error occurs deep within a software package or library. This means that the cause of the error, and location where the error is reported can be very far apart.

For example, the following Python script has an incorrect parameter name on line 7 (cbap should instead be cmap):

#!/usr/bin/env python3

import matplotlib.pyplot as plt
import matplotlib.cm as cm

example_counts = [1, 2, 3, 4, 5]
plt.plot(example_counts, cbap=cm.Blues)

And here is the stacktrace you should see when running this Python script:

Traceback (most recent call last):
  File "long_stacktrace.py", line 7, in <module>
    plt.plot(example_counts, cbap=cm.Blues)
  File "venv/lib/python3.10/site-packages/matplotlib/pyplot.py", line 3794, in plot
    return gca().plot(
  File "venv/lib/python3.10/site-packages/matplotlib/axes/_axes.py", line 1779, in plot
    lines = [*self._get_lines(self, *args, data=data, **kwargs)]
  File "venv/lib/python3.10/site-packages/matplotlib/axes/_base.py", line 296, in __call__
    yield from self._plot_args(
  File "venv/lib/python3.10/site-packages/matplotlib/axes/_base.py", line 534, in _plot_args
    return [l[0] for l in result]
  File "venv/lib/python3.10/site-packages/matplotlib/axes/_base.py", line 534, in <listcomp>
    return [l[0] for l in result]
  File "venv/lib/python3.10/site-packages/matplotlib/axes/_base.py", line 527, in <genexpr>
    result = (make_artist(axes, x[:, j % ncx], y[:, j % ncy], kw,
  File "venv/lib/python3.10/site-packages/matplotlib/axes/_base.py", line 335, in _makeline
    seg = mlines.Line2D(x, y, **kw)
  File "venv/lib/python3.10/site-packages/matplotlib/lines.py", line 407, in __init__
    self._internal_update(kwargs)
  File "venv/lib/python3.10/site-packages/matplotlib/artist.py", line 1216, in _internal_update
    return self._update_props(
  File "venv/lib/python3.10/site-packages/matplotlib/artist.py", line 1190, in _update_props
    raise AttributeError(
AttributeError: Line2D.set() got an unexpected keyword argument 'cbap'

While it does tell you the cause of the error, there are ten function calls between the root cause (line 3 of the stack trace) and the error (line 24 of the stack trace).