Regression since 2.5.0: empty commands section causes error

[asottile]

We often use this pattern such that tox is creating our virtual environments for us:

tox.ini

[tox]
skipsdist = true

[testenv:venv]
deps = pre-commit
commands =

Under 2.4.1

I get (what I expect):

$ tox -e venv
venv recreate: /tmp/fo/.tox/venv
venv installdeps: pre-commit
venv installed: appdirs==1.4.0,aspy.yaml==0.2.2,cached-property==1.3.0,functools32==3.2.3.post2,jsonschema==2.6.0,nodeenv==1.1.2,packaging==16.8,pre-commit==0.12.2,pyparsing==2.1.10,PyYAML==3.12,six==1.10.0,virtualenv==15.1.0
venv runtests: PYTHONHASHSEED='2591024316'
___________________________________ summary ____________________________________
  venv: commands succeeded
  congratulations :)

2.5.0

This is an error:

$ tox -e venv
venv create: /tmp/fo/.tox/venv
venv installdeps: pre-commit
venv installed: appdirs==1.4.0,aspy.yaml==0.2.2,cached-property==1.3.0,functools32==3.2.3.post2,jsonschema==2.6.0,nodeenv==1.1.2,packaging==16.8,pre-commit==0.12.2,pyparsing==2.1.10,PyYAML==3.12,six==1.10.0,virtualenv==15.1.0
ERROR: Commands not specified. Please update relevant section of /tmp/fo/tox.ini
___________________________________ summary ____________________________________
ERROR:   venv: nothing to do

workaround

A (kinda unfortunate) workaround is to do something like:

commands = python -c 'print("done!")'

[obestwalter]

Hi @asottile, that's correct. already identified here, but we should have a proper bug for this. Closed the other one in favour of this.

[asottile]

Since this doesn't seem simple to fix, should we start by reverting #407?

The parser doesn't seem to differentiate between "missing" and "present but empty"

[obestwalter]

I think that's a good idea, as it is better not to have a regression than a broken new feature.

[asottile]

This was reverted in #456 -- perhaps the original issue should be reopened and this one closed?

[obestwalter]

Done. Thanks @asottile!