"pip show" incorrect result for "Required-by"

[Svetlitski]

Environment

• pip version: 19.2.3
• Python version: 3.7.4
• OS: macOS Mojave 10.14.6

Description

Running pip show <package> does not display all dependent packages under "Required-by:", even when <package> shows up in the "Requires:" field of a dependent package.

Expected behavior
Running pip show <package> should show all packages dependent on <package> under the "Required-by:" field of the output.

How to Reproduce

virtualenv pip_show_bug
source pip_show_bug/bin/activate
pip install black
pip show click
pip show black

click is clearly a dependency of black, but black does not show up under "Required-by:" when you run pip show click. All of black's other dependencies (i.e. appdirs, attrs, and toml) behave as expected with pip show, and running pip show black does show click under "Required:"

Output

bash-5.0$ virtualenv pip_show_broken
Using base prefix '/usr/local/Cellar/python/3.7.4/Frameworks/Python.framework/Versions/3.7'
New python executable in /Users/kevin/pip_show_broken/bin/python3.7
Also creating executable in /Users/kevin/pip_show_broken/bin/python
Installing setuptools, pip, wheel...
done.
bash-5.0$ source pip_show_broken/bin/activate
(pip_show_broken) bash-5.0$ pip install black
Collecting black
  Using cached https://files.pythonhosted.org/packages/30/62/cf549544a5fe990bbaeca21e9c419501b2de7a701ab0afb377bc81676600/black-19.3b0-py36-none-any.whl
Collecting appdirs (from black)
  Using cached https://files.pythonhosted.org/packages/56/eb/810e700ed1349edde4cbdc1b2a21e28cdf115f9faf263f6bbf8447c1abf3/appdirs-1.4.3-py2.py3-none-any.whl
Collecting attrs>=18.1.0 (from black)
  Using cached https://files.pythonhosted.org/packages/23/96/d828354fa2dbdf216eaa7b7de0db692f12c234f7ef888cc14980ef40d1d2/attrs-19.1.0-py2.py3-none-any.whl
Collecting toml>=0.9.4 (from black)
  Using cached https://files.pythonhosted.org/packages/a2/12/ced7105d2de62fa7c8fb5fce92cc4ce66b57c95fb875e9318dba7f8c5db0/toml-0.10.0-py2.py3-none-any.whl
Collecting click>=6.5 (from black)
  Using cached https://files.pythonhosted.org/packages/fa/37/45185cb5abbc30d7257104c434fe0b07e5a195a6847506c074527aa599ec/Click-7.0-py2.py3-none-any.whl
Installing collected packages: appdirs, attrs, toml, click, black
Successfully installed appdirs-1.4.3 attrs-19.1.0 black-19.3b0 click-7.0 toml-0.10.0
(pip_show_broken) bash-5.0$ pip show click
Name: Click
Version: 7.0
Summary: Composable command line interface toolkit
Home-page: https://palletsprojects.com/p/click/
Author: Armin Ronacher
Author-email: armin.ronacher@active-4.com
License: BSD
Location: /Users/kevin/pip_show_broken/lib/python3.7/site-packages
Requires: 
Required-by: 
(pip_show_broken) bash-5.0$ pip show black
Name: black
Version: 19.3b0
Summary: The uncompromising code formatter.
Home-page: https://github.com/ambv/black
Author: Łukasz Langa
Author-email: lukasz@langa.pl
License: MIT
Location: /Users/kevin/pip_show_broken/lib/python3.7/site-packages
Requires: appdirs, click, toml, attrs
Required-by: 

[xavfernandez]

Thanks for the report, I can reproduce the issue.

The fix should be quite simple:

pip/src/pip/_internal/commands/show.py

Lines 140 to 144 in 3b2d138

	name = dist.get('name', '')
	required_by = [
	pkg.project_name for pkg in pkg_resources.working_set
	if name in [required.name for required in pkg.requires()]
	]

We are comparing the project names without normalisation and Click would require it since it uses a capital C.

>>> [pkg for pkg in pkg_resources.working_set if pkg.project_name == 'Click'][0].project_name
'Click'
>>> [pkg for pkg in pkg_resources.working_set if pkg.project_name == 'black'][0].requires()
[Requirement.parse('click>=6.5'), Requirement.parse('attrs>=18.1.0'), Requirement.parse('appdirs'), Requirement.parse('toml>=0.9.4')]

For the contributors interested in fixing it, canonicalize_name would be the function to use.