No module named 'dataclasses' in Python 3.6.9

[l0b0]

Python 3.6 doesn't have dataclasses, so the following fails:

Traceback (most recent call last):
  File "…/.venv/bin/black", line 7, in <module>
    from black import patched_main
  File "…/.venv/lib/python3.6/site-packages/black/__init__.py", line 49, in <module>
    from dataclasses import dataclass, field, replace

[ichard26]

Not sure how that could happen as the requirement of dataclasses is listed in setup.py... 😕

black/setup.py

Lines 70 to 80 in 2b75f88

	install_requires=[
	"click>=7.1.2",
	"appdirs",
	"toml>=0.10.1",
	"typed-ast>=1.4.0",
	"regex>=2020.1.8",
	"pathspec>=0.6, <1",
	"dataclasses>=0.6; python_version < '3.7'",
	"typing_extensions>=3.7.4",
	"mypy_extensions>=0.4.3",
	],

[l0b0]

I tried changing pyproject.toml from black = {version = "^19.10b0", allow-prereleases = true} to black = {version = "*", allow-prereleases = true}. This results in an upgrade to 20.8b1 (poetry update prints "Updating black (19.10b0 -> 20.8b1)") and an entry in poetry.lock for the "dataclasses" package:

[[package]]
category = "main"
description = "A backport of the dataclasses module for Python 3.6"
marker = "python_version < \"3.7\""
name = "dataclasses"
optional = false
python-versions = ">=3.6, <3.7"
version = "0.7"

[package.source]
reference = "package proxy"
type = "legacy"
url = "http://…/root/pypi/+simple"

But for whatever reason it looks like it's not being installed - poetry update doesn't print anything about the "dataclasses" package, and the Virtualenv doesn't have it:

$ find .venv -name '*dataclasses*'
.venv/lib/python3.6/site-packages/isort/_future/__pycache__/_dataclasses.cpython-36.pyc
.venv/lib/python3.6/site-packages/isort/_future/_dataclasses.py
.venv/lib/python3.6/site-packages/astroid/brain/__pycache__/brain_dataclasses.cpython-36.pyc
.venv/lib/python3.6/site-packages/astroid/brain/brain_dataclasses.py
.venv/lib/python3.6/site-packages/mypy/plugins/dataclasses.py
.venv/lib/python3.6/site-packages/mypy/plugins/dataclasses.cpython-36m-x86_64-linux-gnu.so
.venv/lib/python3.6/site-packages/mypy/plugins/__pycache__/dataclasses.cpython-36.pyc
.venv/lib/python3.6/site-packages/mypy/typeshed/stdlib/3.7/dataclasses.pyi
.venv/lib/python3.6/site-packages/mypy/typeshed/third_party/3/dataclasses.pyi

Running poetry install afterwards doesn't make any difference.

[JelleZijlstra]

Yes, looks like a Poetry issue.

[Cheukting]

I did not use Poetry but came across the same issue. It's tox on a Travis build.

[Cheukting]

I think this maybe the issue: pypa/pip#8686

[erdnaxeli]

I think this maybe the issue: pypa/pip#8686

I am not sure this is it, this issue is about using extra != a. Here the check is about the python version.

I use pipenv and I can reproduce the bug.