Windows Store version of pip is incompatible with Briefcase

[freakboy3742]

Reported via Gitter, using Windows 10 x64, pip 20.1.1, and Python 3.7 x64 installed via the Window Store.

Running briefcase create generates the error:

ERROR: Can not combine '--user' and '--target'
Unable to install dependencies. This may be because one of your dependencies is invalid, or because pip was unable to connect to the PyPI server.

This is caused by the request to install app dependencies. When Briefcase installs dependencies (CreateCommand.install_app_dependencies()), it internally makes a call to:

python -m pip install --upgrade --target <app location> <package list>

With a "vanilla" pip, this works fine. However, in order to accomodate per-user site-packages, the Windows Store version of pip injects a --user argument. This argument is incompatible with --target, raising the error.

This doesn't occur with the official Python installer, as pip doesn't use the implied --user argument.

[zooba]

This is pypa/pip#4390 again, so not specific to Windows or the Store install (I only skimmed, but it looks like all the other complaints on the pip thread are to do with Linux distros adding default pip configuration).

The --system workaround doesn't apply here. That's part of a Debian(?) patch.

[zooba]

FYI, there's an (undocumented) --no-user option that you should be able to safely use whenever you use --target.