Queries Windows registry for Python installations, as specified in PEP514. The bitness is determined by invoking it, since only v3.5 and later include that in the registry.

pyinsts.py

# PEP514 defines these registry entries
# https://www.python.org/dev/peps/pep-0514/

from __future__ import print_function

# https://gist.github.com/sfaleron/6d31cfe2a7188b6bcea5ca67346254a1
import pybits

import sys

PY2 = sys.version_info[0] < 3

if PY2:
    import _winreg as wr
    FileNotFoundError = WindowsError
else:
    import  winreg as wr

hkcu = wr.HKEY_CURRENT_USER
hklm = wr.HKEY_LOCAL_MACHINE

import os.path as osp

class KeywordToAttr(object):
    __slots__ = ()
    def __init__(self, **kw):
        for k, v in kw.items():
            setattr(self, k, v)

class ByBits(KeywordToAttr):
    __slots__ = ('bits32', 'bits64')

class ByRole(KeywordToAttr):
    __slots__ = ('user', 'system')


rootKey = r'Software\Python\PythonCore'

def OpenKey6432(parent, subPath):
    if PY2:
        return wr.OpenKey(parent, subPath, 0,
            wr.KEY_READ | wr.KEY_WOW64_32KEY)
    else:
        return wr.OpenKey(parent, subPath,
            access=wr.KEY_READ | wr.KEY_WOW64_32KEY)

def OpenKey6464(parent, subPath):
    if PY2:
        return wr.OpenKey(parent, subPath, 0,
            wr.KEY_READ | wr.KEY_WOW64_64KEY)
    else:
        return wr.OpenKey(parent, subPath,
            access=wr.KEY_READ | wr.KEY_WOW64_64KEY)

def get_bits(path):
    return pybits.get_bits(osp.join(path, 'python.exe'))

def _inner_insts(OpenKey, hive, cont):
    try:
        k = OpenKey(hive, rootKey)
    except FileNotFoundError:
        return

    n = wr.QueryInfoKey(k)[0]
    for i in range(n):
        verStr = wr.EnumKey(k, i)
        kk = OpenKey(k, verStr)
        try:
            kPath = OpenKey(kk, 'InstallPath')
        except FileNotFoundError:
            continue

        m = wr.QueryInfoKey(kPath)[1]
        for j in range(m):
            name, val, typ = wr.EnumValue(kPath, j)
            if name == '':
                pypath = val
                if '-' in verStr:
                    verStr = verStr[:verStr.index('-')]
                if get_bits(pypath) == 64:
                    cont.bits64.add((verStr, pypath))
                else:
                    cont.bits32.add((verStr, pypath))

def find_insts():
    # defaults are okay: only one instance
    found = ByRole(
        user   = ByBits(bits32=set(), bits64=set()),
        system = ByBits(bits32=set(), bits64=set()))

    # one of the 6432/6464 calls is equivalent to not
    # using any extra flags, exactly which one depends
    # on the "bitness" of the interpreter.

    _inner_insts(OpenKey6432, hkcu, found.user)
    _inner_insts(OpenKey6464, hkcu, found.user)
    _inner_insts(OpenKey6432, hklm, found.system)
    _inner_insts(OpenKey6464, hklm, found.system)

    return found

def _toStr(l):
    return ''.join(['\n  {} {}'.format(v,p) for v,p in l])

if __name__ == '__main__':
    found = find_insts()

    print('User')
    print('32: ' + _toStr(found.user.bits32))
    print('64: ' + _toStr(found.user.bits64))

    print('System')
    print('32: ' + _toStr(found.system.bits32))
    print('64: ' + _toStr(found.system.bits64))