New submission from Carl Bordum Hansen <c...@bordum.dk>:
Hi there, In ``Doc/faq/design.rst`` there is an explanation of why Python does not have assignment in expressions. This is dated since PEP 572 / Python 3.8. Online version: https://docs.python.org/3/faq/design.html#why-can-t-i-use-an-assignment-in-an-expression I suggest updating it to the attached file. `git diff`: ``` diff --git a/Doc/faq/design.rst b/Doc/faq/design.rst index e2d63a0323..e61284611d 100644 --- a/Doc/faq/design.rst +++ b/Doc/faq/design.rst @@ -149,7 +149,15 @@ to tell Python which namespace to use. 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: +In Python 3.8 and newer, you can use assignment in an expression with the +``:=`` operator (as described in :pep:`572`):: + + while line := f.readline(): + ... # do something with line + +For more than 25 years it was not possible to do assignments in expressions in +Python. Naturally, many people used to C or Perl would complain that they want +to use this C idiom: .. code-block:: c @@ -157,7 +165,7 @@ Many people used to C or Perl complain that they want to use this C idiom: // do something with line } -where in Python you're forced to write this:: +where in Python you would be forced to write this:: while True: line = f.readline() @@ -165,8 +173,10 @@ where in Python you're forced to write this:: 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: +The reason different operators are used for assignment and assignment in +expressions (``=`` and ``:=``, respectively), and why Python didn't allow +assignment in expressions for a long time is a common, hard-to-find bug in +those other languages, caused by this construct: .. code-block:: c @@ -180,11 +190,6 @@ hard-to-find bug in those other languages, caused by this construct: 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 ``` ---------- assignee: docs@python components: Documentation files: design.rst messages: 333063 nosy: carlbordum, docs@python priority: normal severity: normal status: open title: Update design FAQ about assignment expression type: enhancement versions: Python 3.8 Added file: https://bugs.python.org/file48025/design.rst _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue35666> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
