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pep-0492: Add operator precedence table for await expression.

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Yury Selivanov 2015-04-29 18:07:18 -04:00
parent a09fa75d7d
commit 32b06ca706
1 changed files with 70 additions and 4 deletions
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pep-0492.txt
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@ -164,13 +164,80 @@ It is a ``TypeError`` to pass anything other than an *awaitable* object
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 @@ Expression Should be written as
``await -coro()`` ``await (-coro())``
================================== ==================================
See `Grammar Updates`_ section for details.