pip should be listed as a system requirement

[lochsh]

• I have searched the issues of this repo and believe that this is not a duplicate.

Issue

My understanding is that pip is a system requirement for poetry. See my comment here of poetry failing to work without pip: #612 (comment)

The current system requirements only list Python as a system requirement. Although the need for pip might seem obvious given poetry installs Python packages, I believe it should be explicitly listed as a requirement.

Although many standard Python installations come with Pip, installing Python 2 from source does not (I think this has changed with Python 3). I don't think it should be assumed that a Python installation being present on a machine means a pip installation will be too. At least not until Python 2 is officially unsupported (I know it's not long! Using Python 2 was not my choice.).

Do let me know if I've misunderstood anything.

[abn]

The only scenario where pip is externally managed is when Poetry is configured to use the system envrionment (poetry config virtualenvs.create false) and not create a project virtual environment.

In 1.0.z releases under Python 2.7 this pip is ensured by virtualenv and under Python 3 pip installation is ensured via ensurepip that is called by the venv built-in module. With 1.1.0 this becomes unified with the use of virutalenv and its embeded pip versions (#2666).

The custom installer can be used without issues in an environment that does not have pip. See below.

$ docker run --rm -it fedora:latest bash -c '\
>     type pip || :; \
>     type pip3 || :; \
>     python3 -m pip || :;\
>     curl -sLO https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py;\
>     python3 get-poetry.py --preview --yes; \
>     source $HOME/.poetry/env; \
>     poetry --version; \
>     poetry new foobar; \
>     pushd foobar; \
>     poetry install'
bash: line 1: type: pip: not found
bash: line 2: type: pip3: not found
/usr/bin/python3: No module named pip
Retrieving Poetry metadata

# Welcome to Poetry!

This will download and install the latest version of Poetry,
a dependency and package manager for Python.

It will add the `poetry` command to Poetry's bin directory, located at:

$HOME/.poetry/bin

This path will then be added to your `PATH` environment variable by
modifying the profile files located at:

$HOME/.profile
$HOME/.bash_profile

You can uninstall at any time by executing this script with the --uninstall option,
and these changes will be reverted.

Installing version: 1.1.0b2
  - Downloading poetry-1.1.0b2-linux.tar.gz (57.18MB)

Poetry (1.1.0b2) is installed now. Great!

To get started you need Poetry's bin directory ($HOME/.poetry/bin) in your `PATH`
environment variable. Next time you log in this will be done
automatically.

To configure your current shell run `source $HOME/.poetry/env`

Poetry version 1.1.0b2
Created package foobar in foobar
/foobar /
Creating virtualenv foobar-lWDpn5M1-py3.8 in /root/.cache/pypoetry/virtualenvs
Updating dependencies
Resolving dependencies... (1.8s)

Writing lock file

Package operations: 9 installs, 0 updates, 0 removals

  • Installing pyparsing (2.4.7)
  • Installing six (1.15.0)
  • Installing attrs (19.3.0)
  • Installing more-itertools (8.4.0)
  • Installing packaging (20.4)
  • Installing pluggy (0.13.1)
  • Installing py (1.9.0)
  • Installing wcwidth (0.2.5)
  • Installing pytest (5.4.3)

Installing the current project: foobar (0.1.0)

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