My How and Why: pyproject.toml & the 'src' Project Structure

[ how-why packaging python testing ]

UPDATE 4 (5 May 2020): In addition to the links below, Brett Cannon wrote a new post in March 2020 providing further background on PEP517/518 and pyproject.toml, and showing a couple of examples of minimal configurations for using setuptools with pyproject.toml.

UPDATE 3 (4 Feb 2020): It’s been brought to my attention that Google is returning this post near the top of the results list for a variety of Python packaging-related keywords (ummm…yikes?!). As the post is written assuming a fair bit of Python packaging knowledge, here are a few links to some introductory material for the unfamiliar:

UPDATE 2 (16 May 2019): With the release of pip v19.1.1 , editable installs are again allowed in the presence of pyproject.toml. Both the pip install -e . and python develop approaches should now work. Discussion is ongoing on a more robust, setuptools-independent specification of editable installs.

UPDATE (26 Apr 2019): Just like with many others in the Python ecosystem, I was caught by the switch to rigorous observance of PEP517 in pip 19.1, which prohibits editable installs in projects declaring PEP517 compliance via the presence of pyproject.toml. However, I discovered that switching from a pip-mediated editable install to a setuptools-mediated ‘develop’ install (via python develop) seems to have bypassed the problem. Edits/updates have been made below where relevant.

At various points over the last year or so, I’d heard or seen various things about the new pyproject.toml file and how it interacts with Python packaging—in particular, I’d listened to the Python Bytes and Test and Code episodes that covered it. I’d also paid passing attention to the various debates about the merits of the src and non-src approaches to structuring projects. The most compelling argument I’d seen for src was made by Hynek Schlawack :

Inigo has opinions about testing.

I’d tried moving my code to src a couple of times, but it promptly broke things every time and I never cared enough to bother with figuring it out. When I recently came across Bernat Gabor’s three-part series on the state of Python packaging (covered not long ago on Python Bytes), though, it got me curious enough to seriously attempt a conversion both to pyproject.toml and a src project layout. I needed to tweak one of my Python packages and put out a new patch version anyways (stdio-mgr ), so it seemed like a natural time to put in the work to get these in place. The main resources I used in this process were Bernat’s series, the above article by Hynek, and Brett Cannon’s article on PEP 517/518.

In the end, as far as I can tell my problems boiled down to two things:

  1. I hadn’t paid close enough attention to the instructions in Hynek’s article for the src layout conversion, and had failed to set package_dir in addition to packages in the setuptools.setup() call.
  2. The exact chicken-and-egg problem that motivated PEP 517/518, where the dependencies needed to run are defined dynamically within

Ultimately, what makes me really glad I looked into this was realizing that I had been shipping broken sdists to PyPI. (Time for some fun with pip install --no-binary, *sigh*…) Turns out it was a highly useful thing that tox builds and installs the package to test from sdist, as opposed to just installing from a wheel, because it made it very clear to me that I had been doing The Wrong Thing™.

An unanticipated knock-on benefit from making both of these changes has been that I can now just include -e . explicitly in my requirements-dev.txt to install the project in developer mode as part of my dev virtualenv. Thus, I only need to invoke one command, pip install -r requirements-dev.txt, to set up the virtualenv, instead of having to then follow with a pip install -e . (I’m sticking with the requirements.txt paradigm mainly because I don’t know how otherwise to specify custom dependencies for Read the Docs builds.)

(While dealing with the pip 19.1 behavior change, I removed -e . from my requirements files, and instead ran python develop to install the package working tree into the relevant environment after calling pip install. For my local working directory, I just ran the command manually; for CI, I inserted it as a separate command in the pre-test phase of the build (install for Travis).)

Below is some commentary on the various files relevant to the changes I made. The files below are in the state of this commit, except where revised due to the pip 19.1 strict enforcement of PEP517.


Per the PEP 517 and PEP 518 specs, the following two lines in the [build-system] option block are the primary reason for introduction of pyproject.toml:

requires = ["wheel", "setuptools", "attrs>=17.1"]
build-backend = "setuptools.build_meta"

I’m still just using the setuptools build workflow, so wheel and setuptools naturally are build requirements. I also needed attrs at the time I attempted this conversion so that from stdio_mgr import __version__ didn’t break when is executed. (This import-via-the-package approach to single-sourcing the version number is #6 in the list put out in the Python Packaging Guide… I’m not really a fan of the path manipulation hijinks required to make this method work in the src configuration, though (see, below), so I’ve actually already switched to option #3 in the dev branch of the project.)

If I weren’t enabling isolated_build in tox.ini (see below), I wouldn’t have to specify build-backend here, as PEP 517 specifies to fall back to the setuptools/ backend configuration if build-backend is missing. However, tox doesn’t like it if build-backend isn’t explicitly specified when isolated_build is enabled:

$ tox -e sdist_install
ERROR: missing build-backend key at build-system section inside /.../pyproject.toml

I found it distinctly handy to be able to include my black configuration in here as well; specifying line length and include/exclude patterns:

line-length = 79
include = '''
  | ^/src/stdio_mgr/
  | ^/setup[.]py
  | ^/conftest[.]py
exclude = '''

black uses a regex filter, so that __pycache__ in exclude matches any cache folder throughout the project. Note also that the names of items as generated from black’s search algoritm always have a leading backslash, which has to be accounted for when constructing regexes anchored at the project root. (As an aside, I really like this formatting for multiline regexes with alternative patterns ((...|...|...)), and will be stealing it wholesale.)

I briefly considered also moving my tox config into pyproject.toml, but there’s only a string-key legacy method implemented at the moment, and pytest isn’t able to find its config info within that legacy_tox_ini string.

It looks like recent versions of setuptools automatically add pyproject.toml to the sdist build manifest, along with LICENSE, README, and CHANGELOG-like files. But, for my peace of mind I like to include them explicitly:

include LICENSE.txt README.rst pyproject.toml


As noted above, since my build requirements are now successfully specified in pyproject.toml, and pyproject.toml is bundled with the sdist by being included in (or by default), I can include -e . in my requirements-xyz.txts and (barring some weird compatibility issues with old versions of Python/pip/setuptools I’ve encountered on CI) a pip install -r requirements-dev.txt just works:

(fresh-env) $ pip install -r requirements-dev.txt
Obtaining file:///... (from -r requirements-dev.txt (line 12))
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Collecting attrs>=17 (from -r requirements-dev.txt (line 1))
  Using cached
Collecting black (from -r requirements-dev.txt (line 2))
  Using cached
... {many more packages} ...
Installing collected packages: {many packages}
  Running develop for stdio-mgr
Successfully installed {packages} stdio-mgr {more packages}

OTOH, if I delete pyproject.toml and try to install, it does NOT work:

(fresh-env) $ pip install -r requirements-dev.txt
Obtaining file:///... (from -r requirements-dev.txt (line 12))
    Complete output from command python egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/.../", line 6, in <module>
        from stdio_mgr import __version__
      File "/.../src/stdio_mgr/", line 34, in <module>
        from .stdio_mgr import stdio_mgr
      File "/.../src/stdio_mgr/", line 32, in <module>
        import attr
    ModuleNotFoundError: No module named 'attr'

Command "python egg_info" failed with error code 1 in /.../

As noted above, the strict enforcement of PEP517 in pip 19.1 disallowed editable installs:

>pip install -e .
Obtaining file:///.../stdiomgr
ERROR: Error installing 'file:///.../stdiomgr': editable mode is not supported for pyproject.toml
-style projects. This project is being processed as pyproject.toml-style because it has a pyproject.
toml file with a "build-backend" key in the "build_system" value. See PEP 517 for the relevant speci

In some (many? most?) cases, a separate invocation of python develop appeared to work as an alternative to the pip-mediated -e . editable install:

>python develop
running develop
running egg_info
writing src\stdio_mgr.egg-info\PKG-INFO
writing dependency_links to src\stdio_mgr.egg-info\dependency_links.txt
writing requirements to src\stdio_mgr.egg-info\requires.txt
writing top-level names to src\stdio_mgr.egg-info\top_level.txt
reading manifest file 'src\stdio_mgr.egg-info\SOURCES.txt'
reading manifest template ''
writing manifest file 'src\stdio_mgr.egg-info\SOURCES.txt'
running build_ext
Creating c:\...\stdiomgr\env\lib\site-packages\stdio-mgr.egg-link (link to src)
Adding stdio-mgr 1.0.2.dev1 to easy-install.pth file

Installed c:\...\stdiomgr\src
Processing dependencies for stdio-mgr==1.0.2.dev1
Searching for attrs==19.1.0
Best match: attrs 19.1.0
Adding attrs 19.1.0 to easy-install.pth file

Using c:\...\stdiomgr\env\lib\site-packages
Finished processing dependencies for stdio-mgr==1.0.2.dev1

This approach worked smoothly for both of my projects that I’d already converted to the src layout and PEP517 build system. As noted above, I would just run the develop command manually for my local development trees, and had added it as an extra install step in .travis.yml for CI.

For pip versions 19.1.1 and above, the pip install -e . approach again works without issue.


In order to have tox build the package under test in isolation from the development code, I added the standard:


Also, I realized that all of my test environments specifically included an attrs version:


    attrs_17_1:     attrs==17.1
    attrs_17_2:     attrs==17.2
    attrs_17_3:     attrs==17.3
    attrs_17_4:     attrs==17.4
    attrs_18_1:     attrs==18.1
    attrs_18_2:     attrs==18.2
    attrs_latest:   attrs

    attrs_17_1:     pytest==3.2.5
    attrs_17_{2,3}: pytest==3.4.2
    attrs_17_4:     pytest
    attrs_18_{1,2}: pytest
    attrs_latest:   pytest

In the absence of pyproject.toml and isolated_build=True, this has the potential to mask any problems arising from an incorrectly-specified build environment, because with isolated_build=False, tox installs everything in the deps BEFORE it installs the built package to be tested.

Thus, for example, if I delete pyproject.toml and remove isolated_build from tox.ini, the following tox invocation completes successfully:

$ tox -qr -e py36-attrs_latest
========================================================================== test session starts ===========================================================================
platform linux -- Python 3.6.6, pytest-4.2.0, py-1.7.0, pluggy-0.8.1
cachedir: .tox/py36-attrs_latest/.pytest_cache
rootdir: /..., inifile: tox.ini
collected 5 items

README.rst .                                                                                                                                                       [ 20%]
tests/ ....                                                                                                                                   [100%]

======================================================================== 5 passed in 0.06 seconds ========================================================================
________________________________________________________________________________ summary _________________________________________________________________________________
  py36-attrs_latest: commands succeeded
  congratulations :)

BUT, say I define a new tox environment with no deps, for the sole purpose of ensuring that the built package installs into and imports from within a clean enviroment without error:

    python -c "import stdio_mgr"

With no pyproject.toml and isolated_build=False, this tox environment FAILS:

$ tox -qr -e sdist_install
ERROR: invocation failed (exit code 1), logfile: /.../.tox/sdist_install/log/sdist_install-1.log
ERROR: actionid: sdist_install
msg: installpkg
cmdargs: '/.../.tox/sdist_install/bin/python -m pip install --exists-action w /.../.tox/.tmp/package/1/'

Processing ./.tox/.tmp/package/1/
    Complete output from command python egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-req-build-k0deu53y/", line 6, in <module>
        from stdio_mgr import __version__
      File "/tmp/pip-req-build-k0deu53y/src/stdio_mgr/", line 34, in <module>
        from .stdio_mgr import stdio_mgr
      File "/tmp/pip-req-build-k0deu53y/src/stdio_mgr/", line 32, in <module>
        import attr
    ModuleNotFoundError: No module named 'attr'

Command "python egg_info" failed with error code 1 in /tmp/pip-req-build-k0deu53y/

________________________________________________________________________________ summary _________________________________________________________________________________
ERROR:   sdist_install: InvocationError for command /.../.tox/sdist_install/bin/python -m pip install --exists-action w /.../.tox/.tmp/package/1/ (see /.../.tox/sdist_install/log/sdist_install-1.log) (exited with code 1)

With pyproject.toml restored and isolated_build=True, though, it works fine:

$ tox -qr -e sdist_install
________________________________________________________________________________ summary _________________________________________________________________________________
  sdist_install: commands succeeded
  congratulations :)

FWIW, as a final point I’ll note that I also just folded my pytest configuration into tox.ini, rather than creating a new pytest.ini or setup.cfg:

addopts = -p no:warnings --doctest-glob="README.rst"

I figure I’ll probably keep pytest config in here in general, even if it grows, to avoid creating the extra file.

Only minor things to point out here, save for noting again that it was critical to define package_dir in order for setuptools to grok the project source correctly.

    package_dir={"": "src"},

Since I was still using the import-through-the-project approach to single-source versioning at the particular commit I’m talking about here, I did have to do some path hijinks in order to access the __version__ defined within src/stdio_mgr/

from stdio_mgr import __version__

As noted above, I have since switched to single-source-version option #3 in the dev branch of the project:

with open(osp.join(*["src", "stdio_mgr", ""])) as f:

(NOTE (2019-06-12): I realized recently that the above construction is unnecessarily complicated—the unpacking of the list literal is superfluous. This would be much more cleanly written as .join("src", "stdio_mgr", ""). I think the unpacking must have been left over from something else I was fiddling with…?)

I could have moved a bunch of the configuration options to setup into setup.cfg, but I’d rather not create yet another new file for only this single purpose. If setuptools gets an update such that arguments to setup could be sited in pyproject.toml, I would strongly consider shifting them over there.

Written on April 1, 2019