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@cuttlesoft.com>
diff --git a/Doc/faq/design.rst b/Doc/faq/design.rst
index e2d63a0..81c0f47 100644
--- a/Doc/faq/design.rst
+++ b/Doc/faq/design.rst
@@ -149,66 +149,15 @@
Why can't I use an assignment in an expression?
-----------------------------------------------
-Many people used to C or Perl complain that they want to use this C idiom:
+Starting in Python 3.8, you can!
-.. code-block:: c
+Assignment expressions using the walrus operator `:=` assign a variable in an
+expression::
- while (line = readline(f)) {
- // do something with line
- }
+ while chunk := fp.read(200):
+ print(chunk)
-where in Python you're forced to write this::
-
- while True:
- line = 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 = 0) {
- // error handling
- }
- else {
- // code that only works for nonzero x
- }
-
-The error is a simple typo: ``x = 0``, which assigns 0 to the variable ``x``,
-was written while the comparison ``x == 0`` is certainly what was intended.
-
-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 for
-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 recognize
-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 = f.readline()
- while line:
- ... # do something with line...
- line = 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 occurrence
-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::
-
- for line in f:
- ... # do something with line...
+See :pep:`572` for more information.
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 73a2f27..d9db33a 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -1784,6 +1784,8 @@
+-----------------------------------------------+-------------------------------------+
| Operator | Description |
+===============================================+=====================================+
+| ``:=`` | 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_analysis.rst
index 7e1e17e..c0e13b5 100644
--- a/Doc/reference/lexical_analysis.rst
+++ b/Doc/reference/lexical_analysis.rst
@@ -887,7 +887,7 @@
+ - * ** / // % @
- << >> & | ^ ~
+ << >> & | ^ ~ :=
< > <= >= == !=
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index aefe9b5..c2455f4 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -122,8 +122,6 @@
(Contributed by Emily Morehouse in :issue:`35224`.)
-.. TODO: Emily will sprint on docs at PyCon US 2019.
-
Positional-only parameters
--------------------------