One way to set up MacVim for Python and Python 3 on MacOS (assuming Python and Python 3 were installed through MacPorts)

vimrc

if toupper(substitute(system('uname'), '\n', '', '')) =~ 'DARWIN'
    \ && exists('+pythonhome')
    \ && exists('+pythondll')
    \ && exists('+pythonthreehome')
    \ && exists('+pythonthreedll')
    let python_path = '/opt/local/Library/Frameworks/Python.framework/Versions'
    let &pythonhome = ''
    let &pythondll  = ''
    let &pythonthreehome = ''
    let &pythonthreedll  = ''
    for i in range(0, 11)
        if filereadable(python_path . '/2.' . i . '/lib/libpython2.' . i . '.dylib')
                    \ && &pythonhome == ''
                    \ && &pythondll  == ''
            let &pythonhome = python_path . '/2.' . i
            let &pythondll  = python_path . '/2.' . i . '/lib/libpython2.' . i . '.dylib'
        endif
        if filereadable(python_path . '/3.' . i . '/lib/libpython3.' . i . '.dylib')
                    \ && &pythonthreehome == ''
                    \ && &pythonthreedll  == ''
            let &pythonthreehome = python_path . '/3.' . i
            let &pythonthreedll  = python_path . '/3.' . i . '/lib/libpython3.' . i . '.dylib'
        endif
    endfor
endif