1 files changed, 27 insertions, 12 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index 9680a4aa9d5..b99e195f407 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -94,13 +94,13 @@
 ;;       python-shell-interpreter-args
 ;;       "-i C:\\Python27\\Scripts\\ipython-script.py")
 
-;; If you are experiencing missing or delayed output in your shells,
-;; that's likely caused by your Operating System's pipe buffering
-;; (e.g. this is known to happen running CPython 3.3.4 in Windows 7.
+;; Missing or delayed output used to happen due to differences between
+;; Operating Systems' pipe buffering (e.g. CPython 3.3.4 in Windows 7.
 ;; See URL `http://debbugs.gnu.org/cgi/bugreport.cgi?bug=17304').  To
-;; fix this, using CPython's "-u" commandline argument or setting the
-;; "PYTHONUNBUFFERED" environment variable should help: See URL
-;; `https://docs.python.org/3/using/cmdline.html#cmdoption-u'.
+;; avoid this, the `python-shell-unbuffered' defaults to non-nil and
+;; controls whether `python-shell-calculate-process-environment'
+;; should set the "PYTHONUNBUFFERED" environment variable on startup:
+;; See URL `https://docs.python.org/3/using/cmdline.html#cmdoption-u'.
 
 ;; The interaction relies upon having prompts for input (e.g. ">>> "
 ;; and "... " in standard Python shell) and output (e.g. "Out[1]: " in
@@ -1824,6 +1824,14 @@ Restart the Python shell after changing this variable for it to take effect."
   :group 'python
   :safe 'booleanp)
 
+(defcustom python-shell-unbuffered t
+  "Should shell output be unbuffered?.
+When non-nil, this may prevent delayed and missing output in the
+Python shell.  See commentary for details."
+  :type 'boolean
+  :group 'python
+  :safe 'booleanp)
+
 (defcustom python-shell-process-environment nil
   "List of environment variables for Python shell.
 This variable follows the same rules as `process-environment'
@@ -2116,6 +2124,8 @@ uniqueness for different types of configurations."
         (virtualenv (if python-shell-virtualenv-root
                         (directory-file-name python-shell-virtualenv-root)
                       nil)))
+    (when python-shell-unbuffered
+      (setenv "PYTHONUNBUFFERED" "1"))
     (when python-shell-extra-pythonpaths
       (setenv "PYTHONPATH" (python-shell-calculate-pythonpath)))
     (if (not virtualenv)
@@ -2849,25 +2859,30 @@ This function takes the list of setup code to send from the
 
 (defcustom python-shell-completion-setup-code
   "try:
-    import readline, rlcompleter
+    import __builtin__
 except ImportError:
+    # Python 3
+    import builtins as __builtin__
+try:
+    import readline, rlcompleter
+except:
     def __PYTHON_EL_get_completions(text):
         return []
 else:
     def __PYTHON_EL_get_completions(text):
+        builtins = dir(__builtin__)
         completions = []
         try:
             splits = text.split()
             is_module = splits and splits[0] in ('from', 'import')
-            is_ipython = getattr(
-                __builtins__, '__IPYTHON__',
-                getattr(__builtins__, '__IPYTHON__active', False))
+            is_ipython = ('__IPYTHON__' in builtins or
+                          '__IPYTHON__active' in builtins)
             if is_module:
                 from IPython.core.completerlib import module_completion
                 completions = module_completion(text.strip())
-            elif is_ipython and getattr(__builtins__, '__IP', None):
+            elif is_ipython and '__IP' in builtins:
                 completions = __IP.complete(text)
-            elif is_ipython and getattr(__builtins__, 'get_ipython', None):
+            elif is_ipython and 'get_ipython' in builtins:
                 completions = get_ipython().Completer.all_completions(text)
             else:
                 i = 0