Mailman 3 September 2019 - Python-ideas

syntax for dicts with arguments
by Mikhail V 26 Sep '19

26 Sep '19

Idea: how about an alternative syntax specifically for argument list definitions. To be able to write down a dict holding argument list in simpler form, namely with a function-like syntax: mystyle = dict ** (linestyle="dotted", marker="s", color= (0.1,0.1,0.0), markersize=4) as a synonym for the common dict:mystyle= {"linestyle": "dotted", marker": "s", "color": (0.1,0.1, 0.0), "markersize": "4"} (The example is from Mathplotlib plot line style.) Pros: same look as in functon arguments, thus less visual confusion and less re-typing, With such syntax I could simply copy-paste, say, examples from library docs to separate dict "mystyle" and ready to insert the dict into other function making use of dict unpackong. If need to move some argument from "mystyle" to function call, no re-typing needed. Also it looks significantly cleaner with less quotes and less colons. Mikhail

2 1

Add a time-based synchronization primitive
by Viktor Roytman 25 Sep '19

25 Sep '19

The asyncio and threading modules include a number of synchronization primitives. In particular, a Semaphore allows you to limit the number of concurrent tasks if you need to stay under some capacity constraint. However, none of the existing primitives provide for rate limiting, as in making sure there are no more than n tasks started per second. I believe this Stack Overflow question shows that adding such a primitive would be useful: https://stackoverflow.com/questions/35196974/aiohttp-set-maximum-number-of-… The asker clearly wants rate limiting, but the answers provided limit concurrency instead. I found an excellent answer by Martijn Pieters, which includes an implementation of the leaky bucket algorithm, here https://stackoverflow.com/a/45502319/1475412 The AsyncLeakyBucket is used in exactly the same way as a Semaphore.

2 1

Link: Bidirectional Aliasing in Python
by Nutchanon Ninyawee 25 Sep '19

25 Sep '19

Hi, Python community I would like to discuss with you about the idea of "bidirectional Aliasing". I am not good at English wording. I will try my best to communicate the idea Link is a language feature that allows multiple variable names to always refer to the same underlying object define in a namespace. For now, if the variable a link with b. I will denote as a >< b or link('a', 'b') a = 2 a >< alpha # link 'a' with 'alpha' b = 3 print(b**alpha) # print 9 alpha = 3 print(b**a) # print 27 This also able to apply to function name, class property and method. More in detail on this link https://dev.to/circleoncircles/python-ideas-link-bidirectional-aliasing-in-… Look forward to hearing from you Nutchanon

8 20

Add a slot for "keys" in tp_as_mapping
by Serhiy Storchaka 24 Sep '19

24 Sep '19

Most C API uses type slots instead looking up in the type's dictionary for methods. But there are few methods used in basic operations which do not have corresponding slots. For example "keys", "__trunc__" and "__length_hint__". In particularly, "keys" is checked in dict.__init__() and dict.update() with a positional argument to distinguish a mapping from a sequence of key-value pairs. If there is such method, it is called in PyDict_Merge(). Actually "keys" is looked up twice in these operations. PyDict_Merge() is called every time when you have a call like `f(a=1, **kw)` (keyword arguments with var-keyword arguments). It is optimized for dicts, but if kw is something different (for example OrderedDict or mappingproxy) the keys() method is called. So would not be worth to add slots for keys (and maybe for values and items) to the tp_as_mapping structure?

2 2

Multiple items for each loop in comprehension
by 보성 최 22 Sep '19

22 Sep '19

How about allowing to create multiple items for each loop in comprehension? I'm not sure this grammar is available, though. for example: [loop for set] s = set() for i in range(10): s.add(i) s.add(i * 10) [current comprehension] s = { x for i in range(10) for x in (i, i * 10) } [suggested comprehension] s = { i, i * 10 for i in range(10) } [loop for dict] dic = {} for i in range(10): dic[i] = i * 10 dic[i * 10] = i [current comprehension] dic = { k: v for i in range(10) for k, v in ((i, i * 10), (i * 10, i)) } [suggested comprehension] dic = { i: i * 10, i * 10: i for i in range(10) }

2 1

Need a core developer sponsor for a PEP
by Philippe Prados 20 Sep '19

20 Sep '19

Hello, I would like to publish a new PEP (see here <https://github.com/pprados/peps/blob/master/pep-9999.rst>). I discussed this idea here <https://mail.python.org/archives/list/python-ideas@python.org/thread/FCTXGD…> . The PEP1 <https://www.python.org/dev/peps/pep-0001/> explains that I must have a sponsor. Who can help me ? Thanks Philippe Prados

7 11

Discussions-To PEP-0604: Complementary syntax for Union[] and Optional[]
by Philippe Prados 20 Sep '19

20 Sep '19

Hello, I just publish the PEP-0604 <https://www.python.org/dev/peps/pep-0604/> for discussions (https://www.python.org/dev/peps/pep-0604/). Because this proposition is between python and typing, and because the discussion has already been made here, I start a new discussion <https://mail.python.org/archives/list/typing-sig@python.org/thread/D5HCB4NT…> in typing-sig(a)python.org. Your remarks are welcome. Philippe Prados

1 0

Skip modules by default in star-import
by Serhiy Storchaka 19 Sep '19

19 Sep '19

From the description of the import statement in the language reference [1]: > The *public names* defined by a module are determined by checking the module's > namespace for a variable named ``__all__``; if defined, it must be a sequence > of strings which are names defined or imported by that module. The names > given in ``__all__`` are all considered public and are required to exist. If > ``__all__`` is not defined, the set of public names includes all names found > in the module's namespace which do not begin with an underscore character > (``'_'``). ``__all__`` should contain the entire public API. It is intended > to avoid accidentally exporting items that are not part of the API (such as > library modules which were imported and used within the module). As an example of names which should be excluded from the public API the reference mentions names of modules which were imported and used within the module. And they are perhaps the most common cases. It is often imported module names are the only non-underscored names which are imported by a star-import by accident. For example, modules re and sys were used in Lib/idlelib/editor.py but there were no imports for them. This worked because they were imported implicitly by `from tkinter import *`. [1] There is other case when a module name can be occurred in the module namespace: if it is a submodule. For example: >>> from xml.etree import * >>> ElementTree Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'ElementTree' is not defined >>> ElementPath Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'ElementPath' is not defined >>> import xml.etree.ElementTree >>> from xml.etree import * >>> ElementTree <module 'xml.etree.ElementTree' from '/home/serhiy/py/cpython3.8/Lib/xml/etree/ElementTree.py'> >>> ElementPath <module 'xml.etree.ElementPath' from '/home/serhiy/py/cpython3.8/Lib/xml/etree/ElementPath.py'> Names imported by a `from xml.etree import *` are depended on importing a xml.etree's submodule ElementTree. I do not think this is good. I propose to change the rule for determining the set of public names if `__all__` is not defined. In addition to underscored names I propose to exclude names of modules. In these rare cases where modules should be imported in a star import it is not difficult to introduce `__all__`. [1] https://docs.python.org/3/reference/simple_stmts.html#the-import-statement [2] https://bugs.python.org/issue29446

9 16

Simpler syntax for async generators
by George Fischhof 18 Sep '19

18 Sep '19

Hi All, While I was reading the documentation of asyn generators, which shows the following example: async def ticker(delay, to): """Yield numbers from 0 to *to* every *delay* seconds.""" for i in range(to): yield i await asyncio.sleep(delay) It's came to my mind that it could could be simpler. We could use just async yield and all the other stuff could be done in the background. Like if we use the yield in function, that function becomes a generator example: def my_async_generator(): for i in range(5): async yield i What do you think about this? Would this be feasible? BR, __george__

8 10

High level interface for loop.run_in_executor?
by Antoine Pietri 17 Sep '19

17 Sep '19

Hi, I noticed that in 3.7 a lot of high level functions have been added to the asyncio API, but nothing to start blocking functions as non-blocking threads. You still have to call get_event_loop() then await loop.run_in_executor(None, callable). Would an API like this make sense? async def run_thread(callable, *args, *, loop=None, executor=None, timeout=None) Then it could just be used like this: await asyncio.run_thread(my_blocking_read, 4096, timeout=10) This API could wrap the run_in_executor in an asyncio.Task, that way you wouldn't have to await it to start the thread. There's evidence that this has confused people in the past: https://stackoverflow.com/questions/54263558/is-asyncio-run-in-executor-spe… Any thoughts? -- Antoine Pietri

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Python-ideas September 2019