Why can't I use a library I just installed with Pip?

Pip is itself implemented in Python, and even when the command you use is pip, a Python interpreter needs to run. (On Mac and Linux, pip is a Python script that has a shebang line and executable bit set; on Windows, it's a very small program that spawns a Python process to run the Pip code - sort of like a bundled-interpreter application, but without actually bundling the interpreter.)

Pip generally installs packages according to the path where that Python interpreter was found. This means you don't have to tell it every time, and is convenient for some systems that are carefully configured to make it work properly. (Activating a virtual environment could be considered as this kind of configuration.)

There can only be one file (executable or not) with the same name in the same folder; and when you run a command at the command line, it can only find one of those programs. So pip means whichever program (if any) is found first by looking at entries in the PATH environment variable, one by one - once Pip is found, the search stops. Similarly for python (or python3 etc.)

What happens next depends on the exact system configuration.

• When you install Python on Windows, each Python minor version gets a separate sub-folder within Program Files or a similarly named folder. (Long ago, Python would put its install folders directly under C:\.) By default, these folders are not added to PATH, in part because over time that could make the PATH really long (long enough to bump into system-imposed limits). Instead, you're expected to use the Python launcher for Windows, called py (which is common to all versions, and sites directly in C:\Windows). But there is no equivalent "launcher" for Pip (of course, py -m pip solves the problem anyway).

  If pip is runnable as a command, it's because some Python folder was added to PATH - and it might not be the one you want. Normally, if pip and python both works as commands, then they would find the same installation folder. But this wouldn't work if, for example, Pip were not installed for first Python listed: then python would find that Python, but pip wouldn't (and could find a different Python).

• When you install Python on Linux, typically it puts versioned pip and python executables (for example, they might be named pip3.12 and python3.12) side by side right in /usr/bin, and sets up symlinks for python, python3 etc. But those symlinks can only mean one version at a time, and it's possible for them to get out of sync. So pip and python can use different Python installations even though they're in the same folder.

  Also, keep in mind that it's common for a Python installation that comes with Linux, to have several features disabled - especially Pip - typically for security reasons. (It would be bad if, for example, you could easily use Pip to upgrade a third-party library that came pre-installed in your system Python, which a critical system script depends on, and the update broke that script. Even worse, PyPI can host malware - anyone can offer a package, and not everyone is reading the source code for every version of every upload. Never use sudo to run Pip.)