[Python-checkins] peps: pep-0492: Add operator precedence table for await expression.

[yury.selivanov]

https://hg.python.org/peps/rev/d355918bc0d7
changeset:   5807:d355918bc0d7
user:        Yury Selivanov <yselivanov at sprymix.com>
date:        Wed Apr 29 18:07:18 2015 -0400
summary:
  pep-0492: Add operator precedence table for await expression.

files:
  pep-0492.txt |  74 +++++++++++++++++++++++++++++++++++++--
  1 files changed, 70 insertions(+), 4 deletions(-)


diff --git a/pep-0492.txt b/pep-0492.txt
--- a/pep-0492.txt
+++ b/pep-0492.txt
@@ -164,13 +164,80 @@
 to an ``await`` expression.
 
 
-Syntax of "await" expression
-''''''''''''''''''''''''''''
+Updated operator precedence table
+'''''''''''''''''''''''''''''''''
 
 ``await`` keyword is defined differently from ``yield`` and ``yield
-from``.  The main difference is that *await expressions* do not require
+from`` in the Grammar.
+
+The key difference is that *await expressions* do not require
 parentheses around them most of the times.
 
+Also, ``yield from`` allows any expression as its argument, including
+expressions like ``yield from a() + b()``, that would be parsed as
+``yield from (a() + b())``, which is almost always a bug.  In general,
+the result of any arithmetic operation is not an *awaitable* object.
+To avoid this kind of mistakes, it was decided to make ``await``
+precedence lower than ``[]``, ``()``, and ``.``, but higher than ``**``
+operators.
+
++------------------------------+-----------------------------------+
+| Operator                     | Description                       |
++==============================+===================================+
+| ``yield``, ``yield from``    | Yield expression                  |
++------------------------------+-----------------------------------+
+| ``lambda``                   | Lambda expression                 |
++------------------------------+-----------------------------------+
+| ``if`` -- ``else``           | Conditional expression            |
++------------------------------+-----------------------------------+
+| ``or``                       | Boolean OR                        |
++------------------------------+-----------------------------------+
+| ``and``                      | Boolean AND                       |
++------------------------------+-----------------------------------+
+| ``not`` ``x``                | Boolean NOT                       |
++------------------------------+-----------------------------------+
+| ``in``, ``not in``,          | Comparisons, including membership |
+| ``is``, ``is not``, ``<``,   | tests and identity tests          |
+| ``<=``, ``>``, ``>=``,       |                                   |
+| ``!=``, ``==``               |                                   |
++------------------------------+-----------------------------------+
+| ``|``                        | Bitwise OR                        |
++------------------------------+-----------------------------------+
+| ``^``                        | Bitwise XOR                       |
++------------------------------+-----------------------------------+
+| ``&``                        | Bitwise AND                       |
++------------------------------+-----------------------------------+
+| ``<<``, ``>>``               | Shifts                            |
++------------------------------+-----------------------------------+
+| ``+``, ``-``                 | Addition and subtraction          |
++------------------------------+-----------------------------------+
+| ``*``, ``@``, ``/``, ``//``, | Multiplication, matrix            |
+| ``%``                        | multiplication, division,         |
+|                              | remainder                         |
++------------------------------+-----------------------------------+
+| ``+x``, ``-x``, ``~x``       | Positive, negative, bitwise NOT   |
++------------------------------+-----------------------------------+
+| ``**``                       | Exponentiation                    |
++------------------------------+-----------------------------------+
+| ``await``                    | Await expression                  |
++------------------------------+-----------------------------------+
+| ``x[index]``,                | Subscription, slicing,            |
+| ``x[index:index]``,          | call, attribute reference         |
+| ``x(arguments...)``,         |                                   |
+| ``x.attribute``              |                                   |
++------------------------------+-----------------------------------+
+| ``(expressions...)``,        | Binding or tuple display,         |
+| ``[expressions...]``,        | list display,                     |
+| ``{key: value...}``,         | dictionary display,               |
+| ``{expressions...}``         | set display                       |
++------------------------------+-----------------------------------+
+
+See `Grammar Updates`_ section for details.
+
+
+Examples of "await" expressions
+'''''''''''''''''''''''''''''''
+
 Valid syntax examples:
 
 ================================== ==================================
@@ -197,7 +264,6 @@
 ``await -coro()``                  ``await (-coro())``
 ================================== ==================================
 
-
 See `Grammar Updates`_ section for details.
 
 

-- 
Repository URL: https://hg.python.org/peps