Tox "ERROR: file not found: <project_name>"

Question

I'm running a super simple tox.ini file based on the Tox basic example and a walkthrough video entitled Automating Build, Test and Release Workflows with tox. The problem is everytime I run the tox command, I get a bunch of invocation errors and "ERROR: file not found:

tox.ini:

# tox (https://tox.readthedocs.io/) is a tool for running tests
# in multiple virtualenvs. This configuration file will run the
# test suite on all supported python versions. To use it, "pip install tox"
# and then run "tox" from this directory.

[tox]
skipsdist = true
envlist = py36, py37, build
skip_missing_interpreters = true

[testenv]
whitelist_externals = *
passenv = *
deps = -rrequirements-test.txt
commands = {[testenv:unit]commands}

[travis]
python =
    3.6: py36
    3.7: py37, build

[testenv:unit]
deps = {[testenv]deps}
commands = pytest {posargs:--cov={envsitepackagesdir}/project_name {envsitepackagesdir}/project_name src/tests}

[testenv:test]
deps =
    {[testenv]deps}
    {[testenv:build]deps}
commands =
    {[testenv:unit]commands}
    {[testenv:build]commands}

[testenv:build]
skip_install = true
deps =
    wheel
commands =
    rm -rf dist build
    python -W ignore setup.py -q sdist bdist_wheel

;[testenv:release]
;deps =
;    {[testenv:build]deps}
;    twine
;commands =
;    {[testenv:build]commands}
;    twine upload dist/*
;

# From https://pytest-cov.readthedocs.io/en/latest/tox.html
[testenv:clean]
deps = coverage
skip_install = true
commands = coverage erase

[pytest]
addopts =
    -v -s
    --color=yes
    --cov
    --cov-append
    --cov-report=term-missing
    --cov-config=setup.cfg

[coverage:run]
omit =
    */tests/*
    */test_*
    */distutils/*
    */site-packages/*

[coverage:report]
show_missing = True

[flake8]
exclude = .tox

[report]
omit = */usr/*

CLI:

$ tox
py36 create: /path/to/my/project/python/project_namepy/.tox/py36
py36 installdeps: -rrequirements-test.txt
py36 installed: atomicwrites==1.3.0,attrs==19.1.0,bleach==3.1.0,certifi==2019.6.16,chardet==3.0.4,colorama==0.4.1,coverage==4.5.4,docutils==0.15.2,Faker==2.0.0,filelock==3.0.12,idna==2.8,importlib-metadata==0.19,more-itertools==7.2.0,packaging==19.1,pkginfo==1.5.0.1,pluggy==0.12.0,py==1.8.0,Pygments==2.4.2,pyparsing==2.4.2,pytest==5.0.1,pytest-cov==2.7.1,python-dateutil==2.8.0,readme-renderer==24.0,requests==2.22.0,requests-toolbelt==0.9.1,six==1.12.0,text-unidecode==1.2,toml==0.10.0,tox==3.13.2,tqdm==4.33.0,twine==1.13.0,urllib3==1.25.3,virtualenv==16.7.2,wcwidth==0.1.7,webencodings==0.5.1,zipp==0.5.2
py36 run-test-pre: PYTHONHASHSEED='2972258678'
py36 run-test: commands[0] | pytest --cov=/path/to/my/project/python/project_namepy/.tox/py36/lib/python3.6/site-packages/project_name /path/to/my/project/python/project_namepy/.tox/py36/lib/python3.6/site-packages/project_name src/tests
==================================================================================== test session starts ====================================================================================
platform darwin -- Python 3.6.5, pytest-5.0.1, py-1.8.0, pluggy-0.12.0 -- /usr/local/Cellar/python/3.6.5/bin/python3.6
cachedir: .tox/py36/.pytest_cache
rootdir: /path/to/my/project/python/project_namepy, inifile: tox.ini
plugins: cov-2.7.1
collected 0 items                                                                                                                                                                           

=============================================================================== no tests ran in 0.02 seconds ================================================================================
ERROR: file not found: /path/to/my/project/python/project_namepy/.tox/py36/lib/python3.6/site-packages/project_name

ERROR: InvocationError for command /path/to/my/project/python/project_namepy/.tox/py36/bin/pytest --cov=/path/to/my/project/python/project_namepy/.tox/py36/lib/python3.6/site-packages/project_name /path/to/my/project/python/project_namepy/.tox/py36/lib/python3.6/site-packages/project_name src/tests (exited with code 4)
py37 create: /path/to/my/project/python/project_namepy/.tox/py37
py37 installdeps: -rrequirements-test.txt
py37 installed: atomicwrites==1.3.0,attrs==19.1.0,bleach==3.1.0,certifi==2019.6.16,chardet==3.0.4,colorama==0.4.1,coverage==4.5.4,docutils==0.15.2,Faker==2.0.0,filelock==3.0.12,idna==2.8,importlib-metadata==0.19,more-itertools==7.2.0,packaging==19.1,pkginfo==1.5.0.1,pluggy==0.12.0,py==1.8.0,Pygments==2.4.2,pyparsing==2.4.2,pytest==5.0.1,pytest-cov==2.7.1,python-dateutil==2.8.0,readme-renderer==24.0,requests==2.22.0,requests-toolbelt==0.9.1,six==1.12.0,text-unidecode==1.2,toml==0.10.0,tox==3.13.2,tqdm==4.33.0,twine==1.13.0,urllib3==1.25.3,virtualenv==16.7.2,wcwidth==0.1.7,webencodings==0.5.1,zipp==0.5.2
py37 run-test-pre: PYTHONHASHSEED='2972258678'
py37 run-test: commands[0] | pytest --cov=/path/to/my/project/python/project_namepy/.tox/py37/lib/python3.7/site-packages/project_name /path/to/my/project/python/project_namepy/.tox/py37/lib/python3.7/site-packages/project_name src/tests
==================================================================================== test session starts ====================================================================================
platform darwin -- Python 3.7.4, pytest-5.0.1, py-1.8.0, pluggy-0.12.0 -- /usr/local/Cellar/python/3.6.5/bin/python3.6
cachedir: .tox/py37/.pytest_cache
rootdir: /path/to/my/project/python/project_namepy, inifile: tox.ini
plugins: cov-2.7.1
collected 0 items                                                                                                                                                                           

=============================================================================== no tests ran in 0.01 seconds ================================================================================
ERROR: file not found: /path/to/my/project/python/project_namepy/.tox/py37/lib/python3.7/site-packages/project_name

ERROR: InvocationError for command /path/to/my/project/python/project_namepy/.tox/py37/bin/pytest --cov=/path/to/my/project/python/project_namepy/.tox/py37/lib/python3.7/site-packages/project_name /path/to/my/project/python/project_namepy/.tox/py37/lib/python3.7/site-packages/project_name src/tests (exited with code 4)
build create: /path/to/my/project/python/project_namepy/.tox/build
build installdeps: wheel
build run-test-pre: PYTHONHASHSEED='2972258678'
build run-test: commands[0] | rm -rf dist build
build run-test: commands[1] | python -W ignore setup.py -q sdist bdist_wheel
__________________________________________________________________________________________ summary __________________________________________________________________________________________
ERROR:   py36: commands failed
ERROR:   py37: commands failed
  build: commands succeeded

Answer 1

During the aforementioned video, one of the first line items in the tox.ini setup is writing the line having to do with skipping the creation of the sdist, which is considered "expensive":

[tox]
skipsdist = true

It turns out this is a very important step if your tests rely on importing your package (hint: they pretty much have to. Otherwise, what are they testing?). The narrator of the video (and also a key contributor to the project) does mention installing the package in "development" mode (pip install -e .), but neglects to mention (or perhaps I misheard) that the package does actually need to be installed at some point.

Changing this value to skipsdist = false or simply removing the line completely fixes the issue.