add --why option to poetry show

[tgolsson]

Pull Request Check List

Resolves: #5396

• Added tests for changed code.
• Updated documentation for changed code. (none found for poetry show)

This adds a --why option which when combined with --tree and a single package starts the tree at the parent level to show why a specific dependency is included. Optimally this'd walk the tree to the root; but that requires a graph-like structure to easily do/filter, and feels out of scope for this PR.

Example output

Without --why:

$ poetry show requests --tree
requests 2.27.1 Python HTTP for Humans.
├── certifi >=2017.4.17
├── charset-normalizer >=2.0.0,<2.1.0
├── idna >=2.5,<4
└── urllib3 >=1.21.1,<1.27

With --why:

$ poetry show requests --tree --why
cachecontrol 0.12.10 httplib2 caching for requests
└── requests *
    ├── certifi >=2017.4.17
    ├── charset-normalizer >=2.0.0,<2.1.0
    ├── idna >=2.5,<4
    └── urllib3 >=1.21.1,<1.27
requests-toolbelt 0.9.1 A utility belt for advanced users of python-requests
└── requests >=2.0.1,<3.0.0
    ├── certifi >=2017.4.17
    ├── charset-normalizer >=2.0.0,<2.1.0
    ├── idna >=2.5,<4
    └── urllib3 >=1.21.1,<1.27

On a package with no parent:

$ poetry show --tree cachy --why
Package cachy is a direct dependency.
cachy 0.3.0 Cachy provides a simple yet effective caching library.

Forgetting to specify package:

$ poetry show --tree --why
Error: --why requires specifying a package name.

[abn]

@tgolsson can you also update doc?

In general, I like the idea. I'd use it. Would it be feasible to use --why without --tree as well?

[tgolsson]

@abn Found the docs in cli.md, is there any other spot? Also added an extra test, and added a column with --why for lists:

$ poetry show --why
attrs              21.4.0    from pytest                                                      Classes Without Boilerplate
cachecontrol       0.12.10                                                                    httplib2 caching for requests
cachy              0.3.0                                                                      Cachy provides a simple yet effective caching library.
certifi            2021.10.8 from requests                                                    Python package for providing Mozilla's CA Bundle.
importlib-metadata 4.11.2    from keyring,pluggy,poetry-core,pre-commit,pytest,tox,virtualenv Read metadata from Python packages
...

It won't accept --why for a single package still since it includes that info by default:

$ poetry show --why requests
Error: --why cannot be used when displaying a single package.

It will as before accept --why for a tree for a specific package:

$ poetry show --why --tree requests
cachecontrol 0.12.10 httplib2 caching for requests
└── requests *
    ├── certifi >=2017.4.17
    ├── charset-normalizer >=2.0.0,<2.1.0
    ├── idna >=2.5,<4
    └── urllib3 >=1.21.1,<1.27
requests-toolbelt 0.9.1 A utility belt for advanced users of python-requests
└── requests >=2.0.1,<3.0.0
    ├── certifi >=2017.4.17
    ├── charset-normalizer >=2.0.0,<2.1.0
    ├── idna >=2.5,<4
    └── urllib3 >=1.21.1,<1.27

But not for the global tree listing, since again it contains all info, and --why would add nothing.

Error: --why requires a package when combined with --tree.

[jacopok]

Thanks for this implementation! --why is very idiomatic 👍

[tgolsson]

Gentle poke @abn :-)

[abn]

@tgolsson consider me poked :)

[tgolsson]

@abn Good ;-) are there any other blockers or steps in order to get this merged? I can rebase it tomorrow.

[abn]

Minor comment. Otherwise looks good to rebase.

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