Python-ideas July 2020

python-ideas@python.org

93 participants
61 discussions

by Jimmy Thrasibule

Hi, In Python, there are multiple [compound statements](https://docs.python.org/3/reference/compound_stmts.html) with the `else` keyword. For example: ``` for x in iterable: if x == sentinel: break else: print("Sentinel not found.") ``` or: ``` try: do_something_sensitive() except MyError: print("Oops!") else: print("We're all safe.") ``` In my situation, I would like to mix the `with` statement with `else`. In this case I would like that if no exception is raised within the `with` to run the `else` part. For example: ``` with my_context(): do_something_sensitive() else: print("We're all safe.") ``` Now imagine that in my `try .. except` block I have some heavy setup to do before `do_something_sensitive()` and some heavy cleanup when the exception occurs. I'd like my context manager to do the preparation work, execute the body, and cleanup. Or execute my else block only if there is no exception. Is there already a way to accomplish this in Python or can this be a nice to have? Regards, Jimmy

8 months, 1 week

What about having a .get(index, default) method for arrays like we have for dicts?

by Daniel.

When I need to traverse nested dicts, is a common pattern to do somedict.get('foo', {}).get('bar', {}) But there is no such equivalent for arrays, wouldn't be nice if we can follow somedict.get('foo', {}).get('bar', []).get(10) ... ? What I do in this case is surround with try/except IndexError and set some variable to None, or wrap the try/except block in a function. But this is noise in my opinion, I just want to follow the same reasoning that I have with dicts: I'm searching for some path deep nested, and want a default if not found. What do you think? Regards -- “If you're going to try, go all the way. Otherwise, don't even start. ..." Charles Bukowski

8 months, 3 weeks

Aside RE: Re: macaddress or networkaddress module

by Steve Barnes

As an aside I have a perfect example to back up what Paul is saying below. I work for a large corporation where developers are permitted to install python modules on their development machines, (subject to some licence restrictions), but the proxy settings required to get to PyPi vary from user to user, site to site, day to day (sometimes hour to hour) and connection type to connection type, e.g. If you are connected via WiFi you need to use a different proxy to that needed if you are on a wired connection and on a VPN (we have more than one in use) different again. To help with this I have put together a script that will try pip, if it fails will check the current location for pac.pac, get and parse it for proxies, correct a common incompatibility and then try each discovered proxy and if sill none work fall back to a list of common proxies. Once it finds a working setting it advises the user what to use in the current situation. Obviously, since the script is intended to work out how to get to PyPi, I needed to stick 100% to the standard libraries. -----Original Message----- From: Paul Moore <p.f.moore(a)gmail.com> Sent: 06 April 2020 09:11 To: Stephen J. Turnbull <turnbull.stephen.fw(a)u.tsukuba.ac.jp> Cc: Pete Wicken <petewicken(a)gmail.com>; Python-Ideas <python-ideas(a)python.org> Subject: [Python-ideas] Re: macaddress or networkaddress module On Mon, 6 Apr 2020 at 04:14, Stephen J. Turnbull <turnbull.stephen.fw(a)u.tsukuba.ac.jp> wrote: > That's OK for developers on their "own" machines, but I think there > are still a lot of folks who need approval from corporate (though I > haven't heard from Paul Moore on that theme in quite a while, maybe > it's gotten better?) Howdy :-) It's actually got a bit worse, to the point where I don't bother trying to fight the system as much any more, so things have gone quieter because of that :-) But yes, I still believe that there are important use cases for code that only uses the stdlib, and "not being able to get to the internet" is a valid use case that we shouldn't just dismiss. On the other hand, with regard to the comment you were replying to: > Up to around a decade ago, installing third-party libraries > was a huge mess, but nowadays, PyPI works, Python comes with pip > pre-installed, and telling developers they need internet access > when they first start a new project isn’t considered onerous. I think all of that is true. What I think is more important, though, is around things that aren't so much "projects" as "scripts", and that *distributing* such scripts that depend on 3rd party libraries is still problematic. Here, I'm not talking about making full-fledged packages, or py2exe style fully bundled apps, but "here's this monitoring script I wrote, let's ship it across 10 production systems and why don't you try it out on your dev boxes?" There's a *significant* step change in complexity between doing that if the script depends on just the stdlib, versus if it depends on a 3rd party module. And we don't have a really good answer to that yet (zipapps, and tools like shiv that wrap zipapps, are pretty good in practice, but they are not well known or commonly used). Paul _______________________________________________ Python-ideas mailing list -- python-ideas(a)python.org To unsubscribe send an email to python-ideas-leave(a)python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/5BN5D… Code of Conduct: http://python.org/psf/codeofconduct/

9 months, 1 week

default parameter in fuctions to clean up flow

by Peter Moore

I have had a long standing unanswered question on on stackoverflow: is it possible to pass a function to a default parameter so that you could do in essence things like this. def time_diff(target_time, curr_time= lambda : datetime.now() ): return curr_time - target_time this would be an syntactical improvement over this style where you have if statement to initialize a missing parameter. def time_diff(target_time, curr_time=None): if curr_time == None: curr_time = datetime.datetime.now() return curr_time - target_time

9 months, 1 week

PEP 472 -- Support for indexing with keyword arguments

by Andras Tantos

Hello! I'm not sure I'm addressing the right audience here, so please direct me to the appropriate channel if that's the case... My name is Andras Tantos and I'm working on a Python library desscribing HW designs. I came across this problem of __getitem__ and co. not supporting kwargs. Apparently this extension was proposed and rejected as PEP 472. Apart from my use-case, which is arguably a corner-case and not worth modifying the language for, I believe there are two important use-cases that are worth considering with the latest improvements in the language: 1. With the recent type-hint support, the feature could be made way more descriptive if this PEP got implemented. For example, instead of doing the following: def func(in: Dict[str, int]) one could write: def func(in: Dict[key=str, value=int]) 2. It would also make 'generic classes' much cleaner to implement, similar to the way type-hints look. Consider the following code: class_Generic(object): Specializations = [] @classmethod def__getitem__(cls, *args): name = f"Generic_{len(cls.Specializations)}" Specialized = type(name, (cls,), {"specials": tuple(args)}) cls.Specializations.append(Specialized) returnSpecialized def__init__(self, value= None): self.value = value def__str__(self): ifhasattr(self, "specials"): return(f"[{type(self)}- "+ ",".join(str(special) forspecial inself.specials) + f"] - {self.value}") else: return(f"[{type(self)}- GENERIC"+ f"] - {self.value}") Generic = _Generic() #g = Generic() - fails because of no specialization is given s1 = Generic[12]() s2 = Generic[42]("Hi!") print(s1) print(s2) Running this simple example results in: python3 -i python_test.py [<class '__main__.Generic_0'> - 12] - None [<class '__main__.Generic_1'> - 42] - Hi! You can see how the specialized parameters got passed as well as the ones to '__init__'. Obviously, in real code the idea would be to filter generic parameters and set up 'Specialized' with the right set of methods and arguments. Now, without kwargs support for __getitem__, it's impossible to pass named arguments to the specialization list, which greatly limits the usability of this notation. I don't know how convincing these arguments and use-cases are for you, but could you advise me about how to start the 'ball rolling' to drum-up support for re-activating this PEP? Thanks again, Andras Tantos

9 months, 1 week

Function suggestion: itertools.one()

by Noam Yorav-Raphael

Hello, There's a simple function that I use many times, and I think may be a good fit to be added to itertools. A function that gets an iterator, and if it has exactly one element returns it, and otherwise raises an exception. This is very useful for cases where I do some sort of query that I expect to get exactly one result, and I want an exception to be raised if I'm wrong. For example: jack = one(p for p in people if p.id == '1234') sqlalchemy already has such a function for queries: https://docs.sqlalchemy.org/en/13/orm/query.html#sqlalchemy.orm.query.Query… more-itertools has this exact function: https://more-itertools.readthedocs.io/en/latest/api.html#more_itertools.one Here is a simple implementation: def one(iterable): it = iter(iterable) try: first = next(it) except StopIteration: raise ValueError("Iterator is empty") try: second = next(it) except StopIteration: return first else: raise ValueError("Iterator has more than one item") I brought this up on python-dev, but it should be discussed here. Here is the discussion: https://mail.python.org/archives/list/python-dev@python.org/thread/D52MPKLI… Brett Cannon said that this idea has been brought up at least twice before. I found: https://mail.python.org/archives/list/python-ideas@python.org/thread/FTJ6JR… https://mail.python.org/archives/list/python-ideas@python.org/thread/REYDJF… The first thread hasn't reached any operative conclusion. The second thread was very long, and seemed to focus mostly on another function, first(), that doesn't check if there is more than one item. Joao S. O Bueno said that it passed "general approval". I think that perhaps a new discussion, focused just on one (no pun intended) function in the itertools module may reach a conclusion. It was suggested that instead of an additional function, one can use iterator unpacking: jack, = (p for p in people if p.id == '1234') or [jack] = (p for p in people if p.id == '1234') I still think that having a one() function would be useful, since: 1. I think it spells the intention more clearly. It is not symbols that you need to understand their meaning in order to understand that I expect the iterable to have exactly one item, it's spelled in code. 2. The exception would be easier to understand, since errors in tuple unpacking usually mean something else. 3. The one() function allows you to use the result inside an expression without assigning it to a variable. Therefore, I think it encourages writing better code. It's very easy to write: print([p for p in people if p.id == '1234][0]) (which has the problem of not verifying the assumption that there's no more than one result), and I find it easier to replace _[0] with one(_) than to be required to name a new variable, and instead of having an operation on the iterable, change the way I'm assigning to it. WDYT? Cheers, Noam

9 months, 1 week

Idea: Extend "for ... else ..." to allow "for ... if break ..." else

by Jonathan Fine

There's been a discussion of the "for ... else ..." construction in Python. Here's a suggestion as to how we might improve matters, and perhaps usefully extend the language. I hope it might benefit some, without harming anyone else. Aside: I've not read the whole of the previous discussion, and it may be that some or all of what I say has already been expressed there. So apologies for any missing credits to others. The errors, I'm responsible for. Here's what happens at present. for i in items: LOOP_BODY else: print('exit via StopIteration') I find the following clearer (where 'if break' is a new language 'compound keyword'). for i in items: LOOP_BODY if break: pass else: print('exit via StopIteration') This would now allow us to write for i in items: LOOP_BODY if break: print('exit via break') else: print('exit via StopIteration') Sometimes a loop has multiple break commands, which have different purposes. So a further extension might be for i in items: LOOP_BODY if break left: print('exit via break left') elif break right: print('exit via break right') elif break: print('exit via some other break') else: print('exit via StopIteration') To allow this to work, we extend the language to allow commands such as break left break right as well as the usual break Here the identifiers 'left' and 'right' look like variable names, but in fact they are labels. The compiler can, and I think should, produce a hard-coded jump from command break right to the code block under elif break right: This is the basic idea, keeping the syntax close to Python as it now is (but of course adding to the semantics). Some programmers, and I think I'm one, might want a clearer distinction between for i in items: LOOP_BODY if break: do_something and for i in items: LOOP_BODY if broke: do_something For this reason I tentatively suggest instead for i in items: LOOP_BODY case: break left: print('exit via break left') break right: print('exit via break right') break: print('exit via some other break') else: print('exit via StopIteration') Some people have asked a built-in means to detect, at the end of the iteration, whether theitems was empty to begin with. The syntax could easily provide this, for example via case: zero: print('no items to process') However, if we adopt this perhaps we'd also like to retain the simpler for ... if break ... else ... construct. I hope this helps. -- Jonathan

9 months, 2 weeks

Improve handling of Unicode quotes and hyphens

by Steve Barnes

Hi All, Apologies if this has already been discussed to death. Python 3 allows Unicode characters in strings and identifiers but the actual quotation marks are only accepted in plain ASCII, i.e. the following all successfully initialise strings: ``` S1 = "Double Quoted" # Opened and closed with chr(34)0x22 S2 = 'Single Quoted' # Opened and closed with chr(39)0x27 ``` But the following all result in an error - "SyntaxError: invalid character in identifier": ``` S1 = "Double Quoted" # Opened with \u201c and closed with \u201d S2 = 'Single Quoted' # Opened with \u2018 and closed with \u2019 ``` To the experienced eye, and depending on the character font used, it is "obvious" what the problem is! The wrong quotation marks were used. The big problem, especially for beginners, is that the same keys were typed, just in the "wrong" editor or even the wrong editor mode or context I have found that in Outlook if the font is FixSys or I am replying to a plain text email it is fine but otherwise it is "helpful" - unfortunately, especially on Windows, "wrong" editors abound and include, but are not limited to, MS-Outlook, MS-Word, some online editing environments such as Quora. On top of that is the helpful substitution of a m-hyphen for minus when you press space a word later so: A = 3 - 2 # With a space syntax error due to \u2013 A = 3 - 2 # No Space or CR after I last typed it is OK as 0x2d Use cases that catch people out: 1. Sending a code snipped by email using Outlook 2. User manuals written in MS-Word - (many peoples work environment) 3. Articles published on Quora - people expect to be able to copy and paste the code for some reason. I am sure that many us have encountered these issues or similar. What can be done? 1. Persuade Microsoft, and others, to stop being so helpful by default - good luck with that! 2. Tell all users that they need to use a "proper" editor or IDE - This seems like adding an additional barrier to new & casual users. 3. Better yet tell them to use a "proper" OS like .... - At the very least many of us have to use Windows at work. 4. Start accepting hyphens as minus & Unicode quotation marks - this would be the ideal answer for pasted code but has a lot of possible things to iron out such as do we require that the quotes match and are in the typographically correct order. It is also quite a big & complex change to the python interpreter. 5. Normalise the input to the python interpreter (at least for these characters and possibly a few others) so that entering or reading from a file S1 = "Double Quoted" becomes S1 = "Double Quoted", etc. - this should be a easier change to the interpreter but, from a purist point of view, could be said to make us as bad as the others because we are not honouring what the user entered. 6. Change the error message "SyntaxError: invalid character in identifier" to include which character and it's Unicode value so that it becomes "SyntaxError: invalid character 0x201c " in identifier" - this is almost certainly the easiest change and fits well with explicit is better than implicit but still leaves it to the user to correct the erroneous input (which could be argued is both good and bad). I would like to suggest that an incremental approach might be the best - clarifying the existing error message being the thing that should not break anything and either substituting for problem characters or processing them "properly" as a later enhancement. Steve Barnes

9 months, 2 weeks

Re: Idea: Extend "for ... else ..." to allow "for ... if break ..." else

by Jonathan Fine

Hi Rob You wrote: So: You're asking that the bytecode generated for the for-loop depends > on something that happens (or not) after the end of the for-loop body > (which could be arbitrarily long). > > I speak from ignorance, but I suspect that even with the new parser, > which I am reliably informed can make the tea and implement world peace, > that would be asking a lot. > I'm also speaking from ignorance. However, here I'm the optimist. Ignorance is bliss. You're the pessimist. The devil is in the details. Until we have knowledge and wisdom on this, I think we need both points of view. So I'm grateful for your https://en.wikipedia.org/wiki/Grain_of_salt (American English, pinch of salt in British English). I hope it makes my dish taste better! -- Jonathan

9 months, 2 weeks

How to prevent shared memory from being corrupted ?

by Vinay Sharma

Problem: Currently, let’s say I create a shared_memory segment using mulitprocessing.shared_memory.SharedMemory <https://docs.python.org/3.10/library/multiprocessing.shared_memory.html> in Process 1 and open the same in Process 2. Then, I try to write some data to the shared memory segment using both the processes, so for me to prevent any race condition (data corruption), either these operations must be atomic, or I should be able to lock / unlock shared memory segment, which I cannot at the moment. I earlier posted a solution <https://mail.python.org/archives/list/python-ideas@python.org/thread/X4AKFF…> to this problem, which received positive response, but there weren’t many responses to it, despite the fact this problem makes shared_memory practically unusable if there are simultaneous writes. So, the purpose of this post is to have discussion about the solution of the same. Some Solutions: 1. Support for shared semaphores across unrelated processes to lock/unlock shared memory segment. --> More Details <https://mail.python.org/archives/list/python-ideas@python.org/thread/X4AKFF…> 2. Let the first bit in the shared memory segment be the synchronisation bit used for locking/unlocking. --> A process can only lock the shared memory segment if this bit is unset, and will set this bit after acquiring lock, and similarly unset this bit after unlocking. Although set/unset operations must be atomic. Therefore, the following tools can be used: type __sync_add_and_fetch (type *ptr, type value, ...) type __sync_sub_and_fetch (type *ptr, type value, ...) type __sync_or_and_fetch (type *ptr, type value, ...) Documentation of these can be found at below links: https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html#g_t_005f_00… <https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html#g_t_005f_00…> https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html#g_t_005f_… <https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html#g_t_005f_…> Any other ideas/solutions are very welcome.

9 months, 2 weeks

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Python-ideas July 2020