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From: Thomas Passin <list1@tompassin.net>
Newsgroups: comp.lang.python
Subject: Re: Pip installs to unexpected place
Date: Wed, 16 Apr 2025 12:50:27 -0400
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On 4/15/2025 5:38 PM, rbowman via Python-list wrote:
> On Tue, 15 Apr 2025 14:12:19 -0400, Thomas Passin wrote:
> 
>> On Linux, at least, it's standard for pip to install into the user's
>> site-packages location if it's not invoked with admin privileges - even
>> without --user. Pip will emit a message saying so. Well, that used to be
>> true but nowadays Pip wants you to use the --break-system-packages flag
>> if you want to insist on installing into the system's Python install,
>> even if it's going to go into --user. I'm not sure if the restriction
>> will be in place given that the OP built his own Python version.
> 
> Is that pip or a distro's version of pip? On Fedora I get the message
> about defaulting to user. On Ubuntu I get a message to use a venv or if I
> really want a global install to use something like 'pip install python3-
> black'. Ubuntu's is pip 24.2, Fedor's is 24.3.1 but neither of them show
> '--break-system-packages' in --help.

The behavior is specifed in

https://packaging.python.org/en/latest/specifications/externally-managed-environments/#externally-managed-environments

Exactly how pip works and what messages it emits are specified by this 
document, and the details depend on how the distro's packagers configure 
it.  For example, here is a bit of the spec:

"Before a Python-specific package installer (that is, a tool such as pip 
- not an external tool such as apt) installs a package into a certain 
Python context, it should make the following checks by default:

     Is it running outside of a virtual environment? It can determine 
this by whether sys.prefix == sys.base_prefix.

     Is there an EXTERNALLY-MANAGED file in the directory identified by 
sysconfig.get_path("stdlib", sysconfig.get_default_scheme())?

If both of these conditions are true, the installer should exit with an 
error message indicating that package installation into this Python 
interpreter’s directory are disabled outside of a virtual environment.

The installer should have a way for the user to override these rules, 
such as a command-line flag --break-system-packages. This option should 
not be enabled by default and should carry some connotation that its use 
is risky."