bpo-10544: Deprecate "yield" in comprehensions and generator expressions. #4579

serhiy-storchaka · 2017-11-27T07:24:10Z

https://bugs.python.org/issue10544

ncoghlan · 2017-11-27T09:00:12Z

@serhiy-storchaka Do you mind if I just edit some of the docs wording directly? I think that will be easier than trying to describe the edits I would recommend.

serhiy-storchaka · 2017-11-27T09:27:01Z

I will be grateful if you do this.

- updated the comprehension scoping definition to use some of the words from the generator expression definition - added appropriate versionadded/changed directives -

serhiy-storchaka · 2017-11-27T10:18:56Z

Doc/reference/expressions.rst

@@ -201,6 +210,12 @@ or :keyword:`await` expressions it is called an
 suspend the execution of the coroutine function in which it appears.
 See also :pep:`530`.

+.. versionadded:: 3.6
+   Asynchronous comprehensions were introduced


Missed a period.

serhiy-storchaka · 2017-11-27T10:20:21Z

Doc/reference/expressions.rst

+and then passed as an argument to the implictly nested scope. Subsequent
+:keyword:`for` clauses cannot be evaluated in the enclosing scope since they
+may depend on the previous :keyword:`for` loop. For example:
+``[x*y for x in range(10) for y in bar(x)]``.


Maybe use more concrete real-world example? E.g. for y in range(x+1, 10).

I lifted this one directly from the generator expression docs, but changing it in both places would definitely be reasonable.

serhiy-storchaka · 2017-11-27T10:21:52Z

Doc/reference/expressions.rst

+.. versionadded:: 3.6
+   Asynchronous comprehensions were introduced
+
+.. versionchanged:: 3.7


Shouldn't be deprecated or deprecated-removed?

Aye, it should - I just forgot there was a specific directive for deprecations. I'll go through and fix all the cases.

ncoghlan · 2017-11-27T10:31:47Z

OK, I've made my documentation edits.

@gvanrossum I think it would be helpful if you could review the proposed language reference updates directly, or else nominate someone else to do so.

serhiy-storchaka · 2017-11-27T10:33:32Z

Doc/whatsnew/3.7.rst

@@ -563,7 +563,10 @@ Deprecated
  potentially returning a :term:`generator-iterator` object), while generator
  expressions won't attempt to interleave their implicit output with the output
  from any explicit yield expressions.
-  (Contributed by Serhiy Storchaka in :issue:`10544`.)
+


Trailing spaces.

serhiy-storchaka · 2017-11-27T10:43:43Z

Doc/reference/expressions.rst


-.. versionchanged:: 3.7
+.. deprecated:: 3.7
   ``yield`` and ``yield from`` deprecated in the implicitly nested scope


Missed a period.

serhiy-storchaka · 2017-11-27T11:33:44Z

Doc/whatsnew/3.7.rst

+  in comprehensions and generator expressions (aside from the outermost iterable
+  defined in the leftmost :keyword`for` clause). This ensures that comprehensions
+  always immediately return a container of the appropriate type (rather than
+  potentially returning a :term:`generator-iterator` object), while generator


Warning, treated as error:
/home/travis/build/python/cpython/Doc/whatsnew/3.7.rst:559:term not in glossary: generator-iterator

serhiy-storchaka · 2017-11-27T11:34:35Z

Doc/reference/expressions.rst

+   Asynchronous comprehensions were introduced.
+
+.. deprecated:: 3.7
+   ``yield`` and ``yield from`` deprecated in the implicitly nested scope


Missed a period.

gvanrossum

Here are some suggestions to tighten the docs. Thanks to both of you for taking care of this!

gvanrossum · 2017-11-27T16:29:38Z

Doc/reference/expressions.rst

@@ -183,8 +183,20 @@ by considering each of the :keyword:`for` or :keyword:`if` clauses a block,
 nesting from left to right, and evaluating the expression to produce an element
 each time the innermost block is reached.

-Note that the comprehension is executed in a separate scope, so names assigned
-to in the target list don't "leak" into the enclosing scope.
+However, aside from the leftmost :keyword:`for` clause, the comprehension is


I think "leftmost for clause" is still ambiguous. Does this include the in for a[<expr>] in ...? It should not, but I would assume it's part of "the for clause." Maybe say "the iterable in the leftmost for clause"?

Using just "iterable" ended up looking odd in context (since executing an iterable could be taken to mean iterating over it).

However, "the iterable expression in the leftmost :keyword:for clause" reads reasonably well to me, and I also made it explicit that the leftmost filter condition is evaluated in the nested scope.

It may be worth our while to explicitly define "outermost iterable" as a formal term in the glossary, but I think that would be better handled as a distinct docs RFE.

gvanrossum · 2017-11-27T16:32:49Z

Doc/whatsnew/3.7.rst

@@ -556,6 +556,18 @@ Other CPython Implementation Changes
 Deprecated
 ==========

+* Yield expressions (both ``yield`` and ``yield from`` clauses) are now deprecated
+  in comprehensions and generator expressions (aside from the outermost iterable
+  defined in the leftmost :keyword:`for` clause). This ensures that comprehensions


I'd drop "defined" here.

Based on the changes I made to the language reference, I replaced "the outermost iterable defined in" with "the iterable expression in" here.

- consistently use "leftmost" (never "outermost") - explicitly exclude the filter condition from eager evaluation - better align wording for comprehensions and generator expressions

gvanrossum · 2017-11-27T21:47:30Z

Docs LGTM now.

ncoghlan · 2017-11-27T22:02:28Z

Python/symtable.c

+                   to get a more accurate error report */
+                PyErr_Clear();
+                PyErr_SetObject(PyExc_SyntaxError, msg);
+                PyErr_SyntaxLocationObject(st->st_filename,


serhiy-storchaka · 2017-11-30T21:34:54Z

Ping.

serhiy-storchaka · 2017-12-01T06:57:32Z

Thank you for your documentation changes @ncoghlan!

…ons in Py3k mode. (pythonGH-4579)

…pressions in Py3k mode. (GH-4579) (#4676)

serhiy-storchaka added 3 commits November 26, 2017 01:04

bpo-10544: Disallow "yield" in comprehensions and generator expressions.

ca6cd26

Merge branch 'master' into deprecate-yield-in-comprehensions

0b8f666

Emit a warning istead of an error.

5e133a1

serhiy-storchaka added the type-feature A feature request or enhancement label Nov 27, 2017

the-knights-who-say-ni added the CLA signed label Nov 27, 2017

bedevere-bot added the awaiting merge label Nov 27, 2017

ncoghlan added 5 commits November 27, 2017 20:04

Reword language reference updates

bad2b05

- updated the comprehension scoping definition to use some of the words from the generator expression definition - added appropriate versionadded/changed directives -

Fix typo in markup changes

109fd32

Reword What's New notice

e89b0c5

Reword NEWS entry

802a362

Describe specific consequences in What's New entry

5b9498d

serhiy-storchaka commented Nov 27, 2017

View reviewed changes

ncoghlan and others added 3 commits November 27, 2017 20:33

Remove trailing whitespace

afde4b5

Incorporate Serhiy's review comments

c63cfb8

Add an explicit PyErr_Clear() and improve the comment.

65b7725

serhiy-storchaka commented Nov 27, 2017

View reviewed changes

Fix minor doc errors.

978e863

gvanrossum reviewed Nov 27, 2017

View reviewed changes

ncoghlan added 2 commits November 28, 2017 07:34

Clarify eager evaluation of leftmost iterable

5287acf

- consistently use "leftmost" (never "outermost") - explicitly exclude the filter condition from eager evaluation - better align wording for comprehensions and generator expressions

Tweak What's New wording

adb6268

ncoghlan reviewed Nov 27, 2017

View reviewed changes

ncoghlan merged commit 73a7e9b into python:master Dec 1, 2017

bedevere-bot removed the awaiting merge label Dec 1, 2017

serhiy-storchaka deleted the deprecate-yield-in-comprehensions branch December 1, 2017 06:23

serhiy-storchaka added a commit to serhiy-storchaka/cpython that referenced this pull request Dec 2, 2017

bpo-10544: Deprecate "yield" in comprehensions and generator expressi…

b9d155c

…ons in Py3k mode. (pythonGH-4579)

serhiy-storchaka added a commit that referenced this pull request Dec 2, 2017

[2.7] bpo-10544: Deprecate "yield" in comprehensions and generator ex…

65d1887

…pressions in Py3k mode. (GH-4579) (#4676)

serhiy-storchaka mentioned this pull request Feb 6, 2018

[2.7] bpo-10544: Deprecate "yield" in comprehensions and generator expressions in Py3k mode. (GH-4579) #4676

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bpo-10544: Deprecate "yield" in comprehensions and generator expressions. #4579

bpo-10544: Deprecate "yield" in comprehensions and generator expressions. #4579

serhiy-storchaka commented Nov 27, 2017 •

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bpo-10544: Deprecate "yield" in comprehensions and generator expressions. #4579

bpo-10544: Deprecate "yield" in comprehensions and generator expressions. #4579

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serhiy-storchaka commented Nov 27, 2017 • edited by bedevere-bot Loading

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