Error checking if package with dotted name is installed (pip==20.2)

Hi,

I’ve got an issue while installing namespace distribution dependencies. While it worked fine with pip==20.1.1 resolving fails with pip==20.2.

• Python: 3.8.2
• Pip: 20.2
• Platform:

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 20.04 LTS
Release:	20.04
Codename:	focal

My namespace distributions follow PEP420. However, I use a slightly different naming scheme than proposed in https://github.com/pypa/sample-namespace-packages/tree/master/native.

To reproduce the issue the following steps need to be executed:

1. Clone https://github.com/pypa/sample-namespace-packages

$ git clone https://github.com/pypa/sample-namespace-packages.git && cd sample-namespace-packages

1. Apply the following diff:

$ git diff
diff --git a/native/pkg_a/setup.py b/native/pkg_a/setup.py
index 8021a7a..1238c0c 100644
--- a/native/pkg_a/setup.py
+++ b/native/pkg_a/setup.py
@@ -16,7 +16,7 @@ from setuptools import setup
 
 
 setup(
-    name='example_pkg_a',
+    name='example_pkg.a',
 
     version='1',
 
diff --git a/native/pkg_b/setup.py b/native/pkg_b/setup.py
index 872122a..1830dcc 100644
--- a/native/pkg_b/setup.py
+++ b/native/pkg_b/setup.py
@@ -16,7 +16,7 @@ from setuptools import setup
 
 
 setup(
-    name='example_pkg_b',
+    name='example_pkg.b',
 
     version='1',
 
@@ -27,6 +27,7 @@ setup(
     author_email='[e-mail]',
 
     license='Apache Software License',
+    install_requires=['example_pkg.a==1'],

1. Check pip version:

$ pip --version
pip 20.2 from /home/ubuntu/work/projects/sample-namespace-packages/venv/lib/python3.8/site-packages/pip (python 3.8)

1. Install pkg_a:

$ pip install native/pkg_a/
Processing ./native/pkg_a
Using legacy 'setup.py install' for example-pkg.a, since package 'wheel' is not installed.
Installing collected packages: example-pkg.a
    Running setup.py install for example-pkg.a ... done
Successfully installed example-pkg.a-1
$ pip freeze
example-pkg.a==1
pkg-resources==0.0.0

1. Try to install pkg_b which should fail:

$ pip install native/pkg_b/
Processing ./native/pkg_b
ERROR: Could not find a version that satisfies the requirement example_pkg.a==1 (from example-pkg.b==1) (from versions: none)
ERROR: No matching distribution found for example_pkg.a==1 (from example-pkg.b==1)

Though, when naming the namespace distributions as described in https://github.com/pypa/sample-namespace-packages/tree/master/native i.e. with a _ instead of a ., the distributions install correctly.

It took me a while until I noted that the issue is the naming convention, I didn’t follow thoroughly.

Question: Is the change from pip==20.1.1 to pip==20.2 on purpose?

If you need any further details, let me know.

[uranusjr]

I experiemented a bit, and it seems like switching from calling pkg_resources.get_distribution() to pip’s own would indeed solve this particular issue.

Since this is a quite significant issue (there are many popular packages preferring to be referred by a dot in the name, e.g. coverage.py), I’m going to do the one required replacement in a separate PR, so we can get a bugfix version out as soon as possible. #8114 and the other replacements can happen afterwards.

[uranusjr]

I opened #8659 to fix check_if_exists.