Hello!

I'm on Windows, using Python3 from Anaconda distrib. I've set option
 '(python-shell-interpreter "py")

When doing
M-x run-python
I get
Warning (python): Your ‘python-shell-interpreter’ doesn’t seem to support
readline, yet ‘python-shell-completion-native-enable’ was t and "py" is not
part of the ‘python-shell-completion-native-disabled-interpreters’ list.
Native completions have been disabled locally.

Python 3.6.1 |Anaconda 4.4.0 (64-bit)| (default, May 11 2017, 13:25:24)
[MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> python.el: native completion setup failed
>>>

It is expected that Python completion works out-of-the-box, without
warnings.

Ready to provide any additional info,
Andrey Paramonov

In GNU Emacs 25.3.1 (x86_64-w64-mingw32)
 of 2017-09-17 built on LAPHROAIG
Windowing system distributor 'Microsoft Corp.', version 6.3.9600
Configured using:
 'configure --without-dbus --without-compress-install 'CFLAGS=-O2
 -static -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS

Important settings:
  value of $LANG: RUS
  locale-coding-system: cp1252

Major mode: Special

Minor modes in effect:
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...
You can run the command    run-python    with C-c C-p
Shell native completion is disabled, using fallback

Load-path shadows:
None found.

Features:
(shadow sort mail-extr thingatpt emacsbug message dired rfc822 mml
mml-sec epg mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums mail-utils
warnings compile tramp-cache python tramp-sh tramp tramp-compat
auth-source cl-seq eieio eieio-core cl-macs gnus-util mm-util help-fns
mail-prsvr password-cache tramp-loaddefs trampver ucs-normalize shell
pcomplete format-spec advice json map comint ring ansi-color finder-inf
info tex-site package epg-config seq byte-opt gv bytecomp byte-compile
cl-extra help-mode easymenu cconv cl-loaddefs pcase cl-lib time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel dos-w32 ls-lisp disp-table w32-win w32-vars
term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core frame cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese charscript case-table epa-hook jka-cmpr-hook help
simple abbrev minibuffer cl-preloaded nadvice loaddefs button faces
cus-face macroexp files text-properties overlay sha1 md5 base64 format
env code-pages mule custom widget hashtable-print-readable backquote
w32notify w32 multi-tty make-network-process emacs)

Memory information:
((conses 16 238322 10475)
 (symbols 56 24227 0)
 (miscs 48 129 150)
 (strings 32 31786 7274)
 (string-bytes 1 989742)
 (vectors 16 40361)
 (vector-slots 8 734537 6096)
 (floats 8 256 140)
 (intervals 56 426 0)
 (buffers 976 24))