---
tags: [Python Unit Tests at Hypothesis]
redirect_from:
  - /post/debugging-tests/
---

Debugging Failing Tests with pytest
===================================

This post briefly covers some tools for debugging failing tests when using pytest.

If looking at the traceback from a test failure isn't enough there are a few
tools to help debug a failing test:

1. The `--showlocals` argument to pytest will print out the values of all local
   variables after the traceback whenever a test fails. Sometimes this can be
   enough extra information to find the problem:

   ```bash
   $ tox -e py27-h tests/h -- --showlocals
   ```

2. Pytest will capture the output of any Python `print` statements and
   print it after the traceback for the failing test. The output of `print`
   statements in the test code or in the code under test is printed (but print
   statements from tests that passed are not printed).

   Add `print` statements to the tests and to the code under test to print
   out information that can help you to figure out what's going wrong - the
   values of arguments and local variables, which lines of code are called (and
   in which order), etc.

   Just remember to delete the `print`s again before committing the code!

   **Tip**: Try wrapping a `print` statement in an `if` clause, so that
   something is printed out only if some expression is true. This can be useful
   for only printing out the valuable information, and not a lot of noise, if
   the `print` statement needs to be on a line of code that's executed many
   times during the test.

3. The `--pdb` argument to pytest will drop into a
   [pdb](https://pymotw.com/2/pdb/) debugger shell whenever a test fails, at
   the line of code that fails:

   ```bash
   $ tox -e py27-h tests/h -- --pdb
   ```

   From this shell you can inspect the values of variables and arguments,
   evaluate expressions, step through code one line at a time, etc. For some
   tips see [pdb on pymotw](https://pymotw.com/2/pdb/).

4. `ipdb` is an enhanced debugger shell based on ipython, with syntax
   highlighting, tab completion, etc. To use it you first need to install
   ipdb into tox's virtual environment:

   ```bash
   $ .tox/py27-h/bin/pip install robpol86-pytest-ipdb
   ```

   We're using `robpol86-pytest-ipdb` because the `pytest-ipdb` package is
   broken.

   Then run the tests with the `--ipdb` argument instead of `--pdb`:

   ```
   $ tox -e py27-h tests/h -- --ipdb
   ```

   There are many more Python debuggers that you can try, for example
   [pdb++](https://pypi.python.org/pypi/pdbpp/) and
   [pudb](https://pypi.python.org/pypi/pudb).


5. `set_trace()` lets you drop into a debugger shell on any line of code,
   whether in a test or in the code under test, rather than just when a test
   fails. Put this line anywhere in the Python code:

   ```python
   import pdb; pdb.set_trace()
   ```

   and you'll drop into pdb whenever execution hits that line.

   For ipdb you would use `import ipdb; ipdb.set_trace()`.

   As with `print` statements, wrapping `set_trace()` lines in `if` statements
   can be a useful trick.

   As with `print` statements, remember to delete the `set_trace()` lines again
   before committing the code. Fortunately you can easily
   [grep](https://en.wikipedia.org/wiki/Grep) the entire codebase for
   `set_trace`, since this should never appear in production code.