The full path to executables on the system Path are discovered by Python shutil.which. On Windows, this also auto-adds the .exe extension.

A caveat on Unix-like systems (Linux, MacOS) is that shell aliases are not necessarily found by shutil.which. You should instead append the directory of the desired executable to environment variable PATH.

import shutil

# None if executable not found
exe = shutil.which('ls')

cmd = [exe, '-l']
print(cmd)

Since shutil.which() returns None for non-found executable it is convenient for pytest.mark.skipif

For non-system utilities or other programs not on PATH, where the executable path is known, the path can be specified:

shutil.which('myexe', path=mypath)

Fixed in Python 3.8, but present in earlier Python versions is a bug that requires a string for shutil.which(..., path=str(mypath)).