envlist is only the list of environments that tox runs by default when called with no arguments. It's overridden (and ignored) if you pass the -e option on the command-line. (It's also what's returned by tox -a).

The [testenv] section is the one that is used for default settings. [testenv:foo] is used for settings that are only used for tox -e foo.

Using the plain [testenv] section allows us to run, for example, tox -e py38 to run the tests with Python 3.8, or tox -e pypy3, or tox -e py36-dev, and get a sensible behavior.

You can tweak the settings for different environments in the [testenv] by matching "factors". factors are the bits of the environment name separated by dashes:

• environment py3 has factors py3
• environment pypy3-dev has factors pypy3 and dev

All tox settings can be controlled by factors, by prefixing the line with <factor-match>:. This is what happens at line 11: the ipdb dependency is only enabled when using an environment with the dev factor.

Fair enough, although we don't really _need_ a separate environment for that, you can just pass --no-cov to pytest by calling tox -e py3 -- --no-cov.