bpo-10544: Deprecate "yield" in comprehensions and generator expressi… · python/cpython@73a7e9b (original) (raw)

`` @@ -183,8 +183,21 @@ by considering each of the :keyword:for or :keyword:if clauses a block,

183

`nesting from left to right, and evaluating the expression to produce an element

184

`each time the innermost block is reached.

185

186

Note that the comprehension is executed in a separate scope, so names assigned

187

to in the target list don't "leak" into the enclosing scope.

186

`` +

However, aside from the iterable expression in the leftmost :keyword:for clause,

187

the comprehension is executed in a separate implicitly nested scope. This ensures

188

that names assigned to in the target list don't "leak" into the enclosing scope.

189

+

190

`` +

The iterable expression in the leftmost :keyword:for clause is evaluated

191

directly in the enclosing scope and then passed as an argument to the implictly

192

`` +

nested scope. Subsequent :keyword:for clauses and any filter condition in the

193

`` +

leftmost :keyword:for clause cannot be evaluated in the enclosing scope as

194

they may depend on the values obtained from the leftmost iterable. For example:

195


``[x*y for x in range(10) for y in range(x, x+10)]``.

196

+

197

To ensure the comprehension always results in a container of the appropriate

198


type, ``yield`` and ``yield from`` expressions are prohibited in the implicitly

199

`` +

nested scope (in Python 3.7, such expressions emit :exc:DeprecationWarning

200

`` +

when compiled, in Python 3.8+ they will emit :exc:SyntaxError).

188

201

189

202

`` Since Python 3.6, in an :keyword:async def function, an :keyword:async for

190

203

`` clause may be used to iterate over a :term:asynchronous iterator.

`` @@ -198,6 +211,13 @@ or :keyword:await expressions it is called an

198

211

`suspend the execution of the coroutine function in which it appears.

199

212

`` See also :pep:530.

200

213

214

.. versionadded:: 3.6

215

Asynchronous comprehensions were introduced.

216

+

217

.. deprecated:: 3.7

218


 ``yield`` and ``yield from`` deprecated in the implicitly nested scope.

219

+

220

+

201

221

`.. _lists:

202

222

203

223

`List displays

`@@ -316,27 +336,42 @@ brackets or curly braces.

316

336

317

337

`Variables used in the generator expression are evaluated lazily when the

318

338

`` :meth:~generator.__next__ method is called for the generator object (in the same

319

`` -

fashion as normal generators). However, the leftmost :keyword:for clause is

320

immediately evaluated, so that an error produced by it can be seen before any

321

other possible error in the code that handles the generator expression.

322

`` -

Subsequent :keyword:for clauses cannot be evaluated immediately since they

323


may depend on the previous :keyword:`for` loop. For example: ``(x*y for x in

324


range(10) for y in bar(x))``.

339

fashion as normal generators). However, the iterable expression in the

340

`` +

leftmost :keyword:for clause is immediately evaluated, so that an error

341

produced by it will be emitted at the point where the generator expression

342

is defined, rather than at the point where the first value is retrieved.

343

`` +

Subsequent :keyword:for clauses and any filter condition in the leftmost

344

`` +

:keyword:for clause cannot be evaluated in the enclosing scope as they may

345

depend on the values obtained from the leftmost iterable. For example:

346


``(x*y for x in range(10) for y in range(x, x+10))``.

325

347

326

348

`The parentheses can be omitted on calls with only one argument. See section

327

349

`` :ref:calls for details.

328

350

351

To avoid interfering with the expected operation of the generator expression

352


itself, ``yield`` and ``yield from`` expressions are prohibited in the

353

implicitly defined generator (in Python 3.7, such expressions emit

354

`` +

:exc:DeprecationWarning when compiled, in Python 3.8+ they will emit

355

`` +

:exc:SyntaxError).

356

+

329

357

`` If a generator expression contains either :keyword:async for

330

358

`` clauses or :keyword:await expressions it is called an

331

359

`` :dfn:asynchronous generator expression. An asynchronous generator

332

360

`expression returns a new asynchronous generator object,

333

361

`` which is an asynchronous iterator (see :ref:async-iterators).

334

362

363

.. versionadded:: 3.6

364

Asynchronous generator expressions were introduced.

365

+

335

366

`.. versionchanged:: 3.7

336

367

` Prior to Python 3.7, asynchronous generator expressions could

337

368

`` only appear in :keyword:async def coroutines. Starting

338

369

` with 3.7, any function can use asynchronous generator expressions.

339

370

371

.. deprecated:: 3.7

372


 ``yield`` and ``yield from`` deprecated in the implicitly nested scope.

373

+

374

+

340

375

`.. _yieldexpr:

341

376

342

377

`Yield expressions

`@@ -364,6 +399,16 @@ coroutine function to be an asynchronous generator. For example::

364

399

` async def agen(): # defines an asynchronous generator function (PEP 525)

365

400

` yield 123

366

401

402


Due to their side effects on the containing scope, ``yield`` expressions

403

are not permitted as part of the implicitly defined scopes used to

404

implement comprehensions and generator expressions (in Python 3.7, such

405

`` +

expressions emit :exc:DeprecationWarning when compiled, in Python 3.8+

406

`` +

they will emit :exc:SyntaxError)..

407

+

408

.. deprecated:: 3.7

409

Yield expressions deprecated in the implicitly nested scopes used to

410

implement comprehensions and generator expressions.

411

+

367

412

`Generator functions are described below, while asynchronous generator

368

413

`functions are described separately in section

369

414

`` :ref:asynchronous-generator-functions.