[Python-checkins] (no subject)
Stéphane Wirtel
webhook-mailer at python.org
Wed Sep 11 11:12:14 EDT 2019
To: python-checkins at python.org
Subject: bpo-35224: Additional documentation for Assignment Expressions
(GH-15935) (GH-15967)
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https://github.com/python/cpython/commit/be2aa58fdc29cf13aabff6d6712e7853e94e=
88f8
commit: be2aa58fdc29cf13aabff6d6712e7853e94e88f8
branch: 3.8
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.co=
m>
committer: St=C3=A9phane Wirtel <stephane at wirtel.be>
date: 2019-09-11T17:12:09+02:00
summary:
bpo-35224: Additional documentation for Assignment Expressions (GH-15935) (GH=
-15967)
Add or update assignment expression documentation for:
- FAQ - Design
- Reference - Expressions
- Reference - Lexical Analysis
https://bugs.python.org/issue35224
Automerge-Triggered-By: @matrixise
(cherry picked from commit 6357c95716d89ac1f80587fbc4133df8d2e8396c)
Co-authored-by: Emily Morehouse <emily at cuttlesoft.com>
files:
M Doc/faq/design.rst
M Doc/reference/expressions.rst
M Doc/reference/lexical_analysis.rst
M Doc/whatsnew/3.8.rst
diff --git a/Doc/faq/design.rst b/Doc/faq/design.rst
index e2d63a0323da..81c0f474ac16 100644
--- a/Doc/faq/design.rst
+++ b/Doc/faq/design.rst
@@ -149,66 +149,15 @@ to tell Python which namespace to use.
Why can't I use an assignment in an expression?
-----------------------------------------------
=20
-Many people used to C or Perl complain that they want to use this C idiom:
+Starting in Python 3.8, you can!
=20
-.. code-block:: c
+Assignment expressions using the walrus operator `:=3D` assign a variable in=
an
+expression::
=20
- while (line =3D readline(f)) {
- // do something with line
- }
-
-where in Python you're forced to write this::
-
- while True:
- line =3D f.readline()
- if not line:
- break
- ... # do something with line
-
-The reason for not allowing assignment in Python expressions is a common,
-hard-to-find bug in those other languages, caused by this construct:
-
-.. code-block:: c
-
- if (x =3D 0) {
- // error handling
- }
- else {
- // code that only works for nonzero x
- }
-
-The error is a simple typo: ``x =3D 0``, which assigns 0 to the variable ``x=
``,
-was written while the comparison ``x =3D=3D 0`` is certainly what was intend=
ed.
-
-Many alternatives have been proposed. Most are hacks that save some typing =
but
-use arbitrary or cryptic syntax or keywords, and fail the simple criterion f=
or
-language change proposals: it should intuitively suggest the proper meaning =
to a
-human reader who has not yet been introduced to the construct.
-
-An interesting phenomenon is that most experienced Python programmers recogn=
ize
-the ``while True`` idiom and don't seem to be missing the assignment in
-expression construct much; it's only newcomers who express a strong desire to
-add this to the language.
-
-There's an alternative way of spelling this that seems attractive but is
-generally less robust than the "while True" solution::
-
- line =3D f.readline()
- while line:
- ... # do something with line...
- line =3D f.readline()
-
-The problem with this is that if you change your mind about exactly how you =
get
-the next line (e.g. you want to change it into ``sys.stdin.readline()``) you
-have to remember to change two places in your program -- the second occurren=
ce
-is hidden at the bottom of the loop.
-
-The best approach is to use iterators, making it possible to loop through
-objects using the ``for`` statement. For example, :term:`file objects
-<file object>` support the iterator protocol, so you can write simply::
+ while chunk :=3D fp.read(200):
+ print(chunk)
=20
- for line in f:
- ... # do something with line...
+See :pep:`572` for more information.
=20
=20
=20
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 73a2f271ca54..d9db33a78568 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -1784,6 +1784,8 @@ precedence and have a left-to-right chaining feature as=
described in the
+-----------------------------------------------+---------------------------=
----------+
| Operator | Description =
|
+=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D+=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D+
+| ``:=3D`` | Assignment expression =
|
++-----------------------------------------------+---------------------------=
----------+
| :keyword:`lambda` | Lambda expression =
|
+-----------------------------------------------+---------------------------=
----------+
| :keyword:`if <if_expr>` -- :keyword:`!else` | Conditional expression =
|
diff --git a/Doc/reference/lexical_analysis.rst b/Doc/reference/lexical_analy=
sis.rst
index 7e1e17edb2d8..c0e13b53698e 100644
--- a/Doc/reference/lexical_analysis.rst
+++ b/Doc/reference/lexical_analysis.rst
@@ -887,7 +887,7 @@ The following tokens are operators:
=20
=20
+ - * ** / // % @
- << >> & | ^ ~
+ << >> & | ^ ~ :=3D
< > <=3D >=3D =3D=3D !=3D
=20
=20
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index aefe9b53746f..c2455f487b14 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -122,8 +122,6 @@ See :pep:`572` for a full description.
=20
(Contributed by Emily Morehouse in :issue:`35224`.)
=20
-.. TODO: Emily will sprint on docs at PyCon US 2019.
-
=20
Positional-only parameters
--------------------------
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