macOS: Fix yt-dlp launch through mpv - #791

why is this necessary?

I was trying to address the macOS yt-dlp/mpv problem discussed in #585 and #785, while making the fix work more universally rather than depending on a particular yt-dlp/Python installation.

For context, I'm doing my best to get something workable while not being a macOS user let alone developer.

I know what you're trying to accomplish, I authored that PR :)
my question is about the addition in this PR relative to mine (extending PATH with 'common' paths) - what makes you think PATH is not sufficiently populated therefore needs to be extended?

Fair question. The PATH change isn't needed for the "PYTHONHOME"/"PYTHONPATH" fix itself. I added it because removing the old workaround also removes its explicit "/opt/homebrew/bin:/usr/local/bin:/usr/bin" PATH handling.

My concern is the normal DMG/Finder-launched use case: py2app documents that applications launched normally get a minimal environment and don't pick up changes from the user's shell profile. This was also discussed in #276, where it was noted that macOS GUI apps may not have "/usr/local/bin" in PATH and the suggestion was to append it rather than replace the existing PATH.

So the intention was to preserve the old code's ability to find a Homebrew-installed downloader, while avoiding overwriting any PATH that is already present. "/opt/homebrew/bin" covers current Apple Silicon Homebrew and "/usr/local/bin" covers Intel Homebrew.

On reflection, I think the patch can be narrower here: rather than adding all of the standard system paths, it could just preserve the inherited PATH and append "/opt/homebrew/bin" and "/usr/local/bin" if missing.

What's your thoughts on it being tweaked to:
if isMacOS():
env.pop('PYTHONHOME', None)
env.pop('PYTHONPATH', None)

# Add common Homebrew locations while preserving inherited PATH entries.
pathEntries = [path for path in env.get('PATH', '').split(os.pathsep) if path]
for path in ['/opt/homebrew/bin', '/usr/local/bin']:
    if path not in pathEntries:
        pathEntries.append(path)
env['PATH'] = os.pathsep.join(pathEntries)

macos applications, launched from dock, finder, or as login items generally don't pick up the PATH from the user's shell profile as you have noted.
this is intended by apple and not specific to py2app. the proper mechanism to configure this is via /etc/paths and /etc/paths.d (see man path_helper).
if homebrew users have not added homebrew to the system-wide path, then that is indicative of a misconfigured system, in my opinion. now, whether this is common or not, I wouldn't be able to tell you - I do not use homebrew (and am most knowledgeable about Linux too). and I'm not installing homebrew just to try this, but it's worth noting that some package managers (and proprietary software packages) install their paths into /etc/paths.d automatically.
should syncplay accommodate for this misconfiguration of unknown commonness? probably not. but it's not a hill I'm willing to die on. so, your call :)