Homebrew show reversed dependencies

brewrdeps.py

"""
Reverse dependencies for homebrew

Homebrew gives the package dependencies with `brew deps --installed` which each line of output
is `package: dependencies`. It is easy to see which package depends on nothing but difficult to
tell if nothing depends on it. Output of this script reversed it. For example, if we want to tell
which package can be uninstalled without breaking anything else in homebrew (i.e., output of
`brew leaves`), we can get the list with

    python brewrdeps.py | grep ': $'
"""
import subprocess

rdeps = {}
deps = subprocess.check_output(["brew", "deps", "--installed"], text=True)
for pkg, deps in [ln.split(":") for ln in deps.splitlines()]:
    pkg = pkg.strip()
    if pkg not in rdeps:
        rdeps[pkg] = []
    for dep_pkg in deps.strip().split():
        if dep_pkg not in rdeps:
            rdeps[dep_pkg] = []
        rdeps[dep_pkg].append(pkg)

for pkg in sorted(rdeps.keys()):
    print(f"{pkg}: {' '.join(rdeps[pkg])}")