Mailman 3 December 2011 - Python-ideas

@classproperty, @abc.abstractclasspropery, etc.
by K. Richard Pixley Dec. 16, 2020

Dec. 16, 2020

There's a whole matrix of these and I'm wondering why the matrix is currently sparse rather than implementing them all. Or rather, why we can't stack them as: class foo(object): @classmethod @property def bar(cls, ...): ... Essentially the permutation are, I think: {'unadorned'|abc.abstract}{'normal'|static|class}{method|property|non-callable attribute}. concreteness implicit first arg type name comments {unadorned} {unadorned} method def foo(): exists … [View More]now {unadorned} {unadorned} property @property exists now {unadorned} {unadorned} non-callable attribute x = 2 exists now {unadorned} static method @staticmethod exists now {unadorned} static property @staticproperty proposing {unadorned} static non-callable attribute {degenerate case - variables don't have arguments} unnecessary {unadorned} class method @classmethod exists now {unadorned} class property @classproperty or @classmethod;@property proposing {unadorned} class non-callable attribute {degenerate case - variables don't have arguments} unnecessary abc.abstract {unadorned} method @abc.abstractmethod exists now abc.abstract {unadorned} property @abc.abstractproperty exists now abc.abstract {unadorned} non-callable attribute @abc.abstractattribute or @abc.abstract;@attribute proposing abc.abstract static method @abc.abstractstaticmethod exists now abc.abstract static property @abc.staticproperty proposing abc.abstract static non-callable attribute {degenerate case - variables don't have arguments} unnecessary abc.abstract class method @abc.abstractclassmethod exists now abc.abstract class property @abc.abstractclassproperty proposing abc.abstract class non-callable attribute {degenerate case - variables don't have arguments} unnecessary I think the meanings of the new ones are pretty straightforward, but in case they are not... @staticproperty - like @property only without an implicit first argument. Allows the property to be called directly from the class without requiring a throw-away instance. @classproperty - like @property, only the implicit first argument to the method is the class. Allows the property to be called directly from the class without requiring a throw-away instance. @abc.abstractattribute - a simple, non-callable variable that must be overridden in subclasses @abc.abstractstaticproperty - like @abc.abstractproperty only for @staticproperty @abc.abstractclassproperty - like @abc.abstractproperty only for @classproperty --rich [View Less]

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Specify number of items to allocate for array.array() constructor
by Sven Rahmann Feb. 22, 2020

Feb. 22, 2020

At the moment, the array module of the standard library allows to create arrays of different numeric types and to initialize them from an iterable (eg, another array). What's missing is the possiblity to specify the final size of the array (number of items), especially for large arrays. I'm thinking of suffix arrays (a text indexing data structure) for large texts, eg the human genome and its reverse complement (about 6 billion characters from the alphabet ACGT). The suffix array is a long int … [View More]

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i18n and Python tracebacks
by Andre Roberge Oct. 28, 2012

Oct. 28, 2012

I think it would be a good idea if Python tracebacks could be translated into languages other than English - and it would set a good example. For example, using French as my default local language, instead of >>> 1/0 Traceback (most recent call last): File "<stdin>", line 1, in <module> ZeroDivisionError: integer division or modulo by zero I might get something like >>> 1/0 Suivi d'erreur (appel le plus récent en dernier) : Fichier "<stdin>", à la … [View More]

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Re: [Python-ideas] Cofunctions - Back to Basics
by Mark Shannon Oct. 21, 2012

Oct. 21, 2012

Greg Ewing wrote: > Mark Shannon wrote: > >> Why not have proper co-routines, instead of hacked-up generators? > > What do you mean by a "proper coroutine"? > A parallel, non-concurrent, thread of execution. It should be able to transfer control from arbitrary places in execution, not within generators. Stackless provides coroutines. Greenlets are also coroutines (I think). Lua has them, and is implemented in ANSI C, so it can be done portably. See: http://www.jucs.… [View More]

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Cofunctions PEP - Revision 4
by Greg Ewing Oct. 13, 2012

Oct. 13, 2012

Here's an updated version of the PEP reflecting my recent suggestions on how to eliminate 'codef'. PEP: XXX Title: Cofunctions Version: $Revision$ Last-Modified: $Date$ Author: Gregory Ewing <greg.ewing(a)canterbury.ac.nz> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 13-Feb-2009 Python-Version: 3.x Post-History: Abstract ======== A syntax is proposed for defining and calling a special type of generator called a 'cofunction'. It is designed to provide a … [View More]streamlined way of writing generator-based coroutines, and allow the early detection of certain kinds of error that are easily made when writing such code, which otherwise tend to cause hard-to-diagnose symptoms. This proposal builds on the 'yield from' mechanism described in PEP 380, and describes some of the semantics of cofunctions in terms of it. However, it would be possible to define and implement cofunctions independently of PEP 380 if so desired. Specification ============= Cofunction definitions ---------------------- A cofunction is a special kind of generator, distinguished by the presence of the keyword ``cocall`` (defined below) at least once in its body. It may also contain ``yield`` and/or ``yield from`` expressions, which behave as they do in other generators. From the outside, the distinguishing feature of a cofunction is that it cannot be called the same way as an ordinary function. An exception is raised if an ordinary call to a cofunction is attempted. Cocalls ------- Calls from one cofunction to another are made by marking the call with a new keyword ``cocall``. The expression :: cocall f(*args, **kwds) is evaluated by first checking whether the object ``f`` implements a ``__cocall__`` method. If it does, the cocall expression is equivalent to :: yield from f.__cocall__(*args, **kwds) except that the object returned by __cocall__ is expected to be an iterator, so the step of calling iter() on it is skipped. If ``f`` does not have a ``__cocall__`` method, or the ``__cocall__`` method returns ``NotImplemented``, then the cocall expression is treated as an ordinary call, and the ``__call__`` method of ``f`` is invoked. Objects which implement __cocall__ are expected to return an object obeying the iterator protocol. Cofunctions respond to __cocall__ the same way as ordinary generator functions respond to __call__, i.e. by returning a generator-iterator. Certain objects that wrap other callable objects, notably bound methods, will be given __cocall__ implementations that delegate to the underlying object. Grammar ------- The full syntax of a cocall expression is described by the following grammar lines: :: atom: cocall | <existing alternatives for atom> cocall: 'cocall' atom cotrailer* '(' [arglist] ')' cotrailer: '[' subscriptlist ']' | '.' NAME Note that this syntax allows cocalls to methods and elements of sequences or mappings to be expressed naturally. For example, the following are valid: :: y = cocall self.foo(x) y = cocall funcdict[key](x) y = cocall a.b.c[i].d(x) Also note that the final calling parentheses are mandatory, so that for example the following is invalid syntax: :: y = cocall f # INVALID New builtins, attributes and C API functions -------------------------------------------- To facilitate interfacing cofunctions with non-coroutine code, there will be a built-in function ``costart`` whose definition is equivalent to :: def costart(obj, *args, **kwds): try: m = obj.__cocall__ except AttributeError: result = NotImplemented else: result = m(*args, **kwds) if result is NotImplemented: raise TypeError("Object does not support cocall") return result There will also be a corresponding C API function :: PyObject *PyObject_CoCall(PyObject *obj, PyObject *args, PyObject *kwds) It is left unspecified for now whether a cofunction is a distinct type of object or, like a generator function, is simply a specially-marked function instance. If the latter, a read-only boolean attribute ``__iscofunction__`` should be provided to allow testing whether a given function object is a cofunction. Motivation and Rationale ======================== The ``yield from`` syntax is reasonably self-explanatory when used for the purpose of delegating part of the work of a generator to another function. It can also be used to good effect in the implementation of generator-based coroutines, but it reads somewhat awkwardly when used for that purpose, and tends to obscure the true intent of the code. Furthermore, using generators as coroutines is somewhat error-prone. If one forgets to use ``yield from`` when it should have been used, or uses it when it shouldn't have, the symptoms that result can be extremely obscure and confusing. Finally, sometimes there is a need for a function to be a coroutine even though it does not yield anything, and in these cases it is necessary to resort to kludges such as ``if 0: yield`` to force it to be a generator. The ``cocall`` construct address the first issue by making the syntax directly reflect the intent, that is, that the function being called forms part of a coroutine. The second issue is addressed by making it impossible to mix coroutine and non-coroutine code in ways that don't make sense. If the rules are violated, an exception is raised that points out exactly what and where the problem is. Lastly, the need for dummy yields is eliminated by making it possible for a cofunction to call both cofunctions and ordinary functions with the same syntax, so that an ordinary function can be used in place of a cofunction that yields zero times. Record of Discussion ==================== An earlier version of this proposal required a special keyword ``codef`` to be used in place of ``def`` when defining a cofunction, and disallowed calling an ordinary function using ``cocall``. However, it became evident that these features were not necessary, and the ``codef`` keyword was dropped in the interests of minimising the number of new keywords required. The use of a decorator instead of ``codef`` was also suggested, but the current proposal makes this unnecessary as well. It has been questioned whether some combination of decorators and functions could be used instead of a dedicated ``cocall`` syntax. While this might be possible, to achieve equivalent error-detecting power it would be necessary to write cofunction calls as something like :: yield from cocall(f)(args) making them even more verbose and inelegant than an unadorned ``yield from``. It is also not clear whether it is possible to achieve all of the benefits of the cocall syntax using this kind of approach. Prototype Implementation ======================== An implementation of an earlier version of this proposal in the form of patches to Python 3.1.2 can be found here: http://www.cosc.canterbury.ac.nz/greg.ewing/python/generators/cofunctions.h… If this version of the proposal is received favourably, the implementation will be updated to match. Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: [View Less]

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Re: [Python-ideas] PEP x: Static module/package inspection
by anatoly techtonik March 29, 2012

March 29, 2012

(reposting this from Google Group once more as the previous post missed Mailing List, because I was not subscribed in Mailman) *Static module/package inspection* Abstract: - static: without execution (as opposed to dynamic) - module/package: .py or __init__.py file - inspection: get an overview of the contents *What should this do?* * * The proposal to add a mechanism to Python interpreter to get an outline of module/package contents without importing or executing module/package. The … [View More]outline includes names of classes, functions, variables. It also should contain values for variables that could be provided without sophisticated calculations (e.g. a string, integer, but probably not expressions as it may lead to security leaks). *Why?* * * *user story PEPx.001:* As a Python package maintainer, I find it bothersome to repeatedly write bolierplate code (e.g. setup.py) to package my single file module. The reason I should write setup.py is to provide version and description info. This info is already available in my module source code. So I need to either copy/paste the info from the module manually, or to import (and hence execute) my module during packaging and installation, which I don't want either, because modules are often installed with root privileges. With this PEP, packing tool will be able to extract meta information from my module without executing it or without me manually copying version fields into some 'package configuration file'. *user story PEPx.002:* As a Python Application developer, I find it really complicated to provide plugin extension subsystem for my users. Users need a mechanism to switch between different versions of the plugin, and this mechanism is usually provided by external tool such as setuptools to manage and install multiple versions of plugins in local Python package repository. It is rather hard to create an alternative approach, because you are forced to maintain external meta-data about your plugin modules even in case it is already available inside the module. With this PEP, Python Application will be able to inspect meta-data embedded inside of plugins before choosing which version to load. This will also provide a standard mechanism for applications to check modules returned by packaging tools without executing them. This will greatly simplify writing and debugging custom plugins loaders on different platforms. *Feedback goal* At this stage I'd like to a community response to two separate questions: 1. If everybody feels this functionality will be useful for Python 2. If the solution is technically feasible [View Less]

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Idea: Google Groups web interface for Ideas
by anatoly techtonik Jan. 4, 2012

Jan. 4, 2012

As you may know, the python-ideas list is opened only to subscribers. This is inconvenient, because: 1. it requires three step subscription process 2. it is impossible to post a reply to existing thread/idea There is a web-interface in Google Groups at https://groups.google.com/forum/#!forum/python-ideas that can solve problems above and provide some more nifty features such as embedded search. But there comes another problem that messages posted through the group doesn't end in list, … [View More]

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Re: [Python-ideas] [Python-Dev] hello, new dict addition for new eve ?
by julien tayon Jan. 4, 2012

Jan. 4, 2012

Hello, I had tough time finding a connection to internet :) @Arnaud + Terry Thx, code corrected to have the right terminology. at least my code really test for the good rules :) But being precise matter, so that I changed all the terms to fit in. @nathan << is a symbol indicating an asymetry for left and right value, and I carefully tried to have a symetrical behaviour with left & right : associativity, distribution, commutativity. The whole point about using a symbol, is that it … [View More]follow rules. That is the reason why consistent_addition class is all about testing linear algebraic rules (hopefully the right way, and now, I hope with the right terminology). But I have reason to think that there are more than way to do addition. Linear algebrae is the most used and intuitive one for non developer, and developer think of sets operations as more intuitive. I guess, it all come down to one point. Which is what is a dict as a mathematical object. If there is more than one choice, which has the most benefits. Should there be more than one definition ? @Guido & Eric In my book there are rules for sets. The question is a dict the same as a vector or as a set ? since sets are using logical operators for operations that are roughly sets operations why not use & ^ | for sets operation on dict ? it would be pretty consistent with sets operations. ( @jakkob ) In this case I would expect inconsistencies error when doing dict( a = 1 ) | dict( a = 2 , b = 2 ) (thus key-> value are seen like an atomic element of the set, unless a "value collision" operator is given, and this collision operator would recursively and naturely apply to the descendants) because I expect both a | b == b | a and a + b = b + a The problem would be indeed list + list vectorish behaviour is my prefered of course. but if we shift the record algebrae addition to another symbol (let's assume << ) we resolve the conflict. But then we have a problem of retro compatibility. We then may be tempted to use & | ^ (since it would be used for dict) and then we have a problem : should [ 1 , 2 ] & [ 3 , 2 ] mean set addition or applying & to all the element of the same rank ? I'm pretty stucked myself in trying to be consistent. Just for the record, why dit you not use "." (dot) for concatenation ? I know it is typographically unreadable on a messy screen, but is there a better reason ? @terry & Nathan Regarding Counter. on https://github.com/jul/ADictAdd_iction/blob/master/demonstrate.py#L120 you may notice that I do have all the property of counter, but counter does not as -this dict- aggregate values and (sub) totals at the same time. It is very convenient for map reduce since it does some re-reduce operations at reduce time. smart, no ? <:o) @terry Sorry for making assertive and carefree statements regarding strong resentment. My bad. @bruce, truth is aim at similarity cosinus for dict, and I imagine a map reduce on dict representing values of a model you want ideal = dict( blue = 1 , height = 180, weight = 70, wage = 500 ) in the filter before the map reduce you would want to filter all records close to this one by using similarity cosinus in this way : filter( lambda model : cos( ideal, model) > .7 , all_model ) http://en.wikipedia.org/wiki/Cosine_similarity of course I will try to advocate dot product, and metrics. As a result my real goal is to make people consider dict as vectors of path to value => value not sets of key => value To solve issues such as weighted decisions and non linear choices (based on trigger or a value belonging to a set) I can fairly easily concevie a projector (which would be a class transforming a vector in a vector, but not with a a matrix, but with computed rules ). @Mark Jansen +1 are you telepath ? Yes a key collision operator would be nice : given for instance an apache log, I will have segments of path, and I may want keys to be the accumulated graph of a user on a website. so my collision rule might be : (key being the referer value the page visited afterwards) dict( a = b, a = c ) + dict( b = e ) = dict( a = dict( b = e ), a = c) I thought of it, but I really loved the conservation rule. And I feared people would think of it as too complicated. Keep It Simple I learnt :) @all Wrapping everything to logicial sense. I was thinking of supersets of object (thus out of the stdlib) that would have different algebrae and could be used as casts on object to redifine + - * / cos & | ^ and for different objects would behave consistently ex : vector( dict ) vector( list ) vector( string ) would make dict & list & string behave like vectors thus mainly supporting elementwise operations sets( dict ) , sets( string ) ... would make dict string be sets of elements ... (with the original dict addition design of GvR et al) Each superset would have a unit test for the operation verifying that behaviour is consistent. (associativity, distributivity, ...) what about this solution ? -- Friendly jul [View Less]

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Re: [Python-ideas] [Python-Dev] hello, new dict addition for new eve ?
by Guido van Rossum Dec. 31, 2011

Dec. 31, 2011

On Sat, Dec 31, 2011 at 2:16 AM, julien tayon <julien(a)tayon.net> wrote: > Dear All :) > > 2011/12/30 Eric Snow <ericsnowcurrently(a)gmail.com>: > > On Fri, Dec 30, 2011 at 10:02 AM, Guido van Rossum <guido(a)python.org> > wrote: > >> What I meant is similar to set union on the keys, where if a key exists > in > >> both dicts, the value in the result is equal to one of the values in the > >> operands (and if the value is the … [View More]

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Re: [Python-ideas] [Python-Dev] hello, new dict addition for new eve ?
by Guido van Rossum Dec. 30, 2011

Dec. 30, 2011

Hi Julien, Don't despair! I have tried to get people to warm up to dict addition too -- in fact it was my counter-proposal at the time when we were considering adding sets to the language. I will look at your proposal, but I have a point of order first: this should be discussed on python-ideas, not on python-dev. I have added python-ideas to the thread and moved python-dev to Bcc, so followups will hopefully all go to python-ideas. --Guido On Fri, Dec 30, 2011 at 7:26 AM, julien tayon <… [View More]

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Re: [Python-ideas] Idea: Google Groups web interface for Ideas
by anatoly techtonik Dec. 28, 2011

Dec. 28, 2011

On Tuesday, December 27, 2011 10:53:56 PM UTC+3, RunThePun wrote: > > On Tue, Dec 27, 2011 at 7:01 PM, anatoly techtonik <tech...(a)gmail.com>wrote: > >> As you may know, the python-ideas list is opened only to subscribers. >> This is inconvenient, because: >> 1. it requires three step subscription process >> 2. it is impossible to post a reply to existing thread/idea >> >> There is a web-interface in Google Groups at https://groups.google.com/… [View More]

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Should stdlib provide something to properly handle SIGINT/SIGTERM signals?
by Giampaolo Rodolà Dec. 28, 2011

Dec. 28, 2011

Since this is a problem which occurs very often, I'd like to hear your opinion as to whether something like this might find some place in the python stdlib (signal module maybe?). Please read the discussion included in this recipe: http://code.activestate.com/recipes/577997/ It should provide a description of the problem and the general use case. Thanks in advance for your comments, --- Giampaolo http://code.google.com/p/pyftpdlib/ http://code.google.com/p/psutil/

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defaultattrgetter
by John O'Connor Dec. 22, 2011

Dec. 22, 2011

Pythons: What are your thoughts on the concept of a `defaultattrgetter`? It would be to operator.attrgetter what getattr(foo, x, default) is to getattr(foo, x). I dont like that attrgetter requires the attribute to exist or else the getter be wrapped in a try catch with respect to how defaultdict solves roughly the same problem for dictionary keys. The semantics could be something such as: from operator import defaultattrgetter _x = defaultattrgetter({'x': 0}) _y = defaultattrgetter({'y': 1})… [View More]

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Adding "Typed" collections/iterators to Python
by Nathan Rice Dec. 21, 2011

Dec. 21, 2011

I believe it would be a good idea in instances where it is known that a collection of a single type is going to be returned, to return a subclass with type information and type specific methods "mixed in". You could provide member methods as collection methods that operate in a vectorized manner, returning a new collection or iterator with the results much like the mathematical functions in NumPy. This would also give people a reliable method to make functions operate on both scalar and … [View More]vector values. I believe this could be implemented without needing subclasses for everything under the sun with a generic collection "type contract" mix-in. If a developer wanted to provide additional type specific collection/iterator methods they would of course need to subclass that. To avoid handcuffing people with types (which is definitely un-pythonic) and maintain backwards compatibility, the standard collection modification methods could be hooked so that if an object of an incorrect type is added, a warning is raised and the collection gracefully degrades by removing mixed-in type information and methods. Additionally, a method could be provided that lets the user "terminate the contract" causing the collection to degrade without a warning. I have several motivations for this: -- Performing a series of operations using comprehensions or map tends to be highly verbose in an uninformative way. Compare the current method with what would be possible using "typed" collections: L2 = [X(e) for e in L1] L3 = [Y(e) for e in L2] vs L2 = X(L1) # assuming X has been updated to work in both vector/scalar L3 = Y(L2) # context... L2 = [Z(Y(X(e))) for e in L1] vs L2 = Z(Y(X(L1))) L2 = [e.X().Y().Z() for e in L1] vs L2 = L1.X().Y().Z() # assuming vectorized versions of member methods #are folded into the collection via the mixin. -- Because collections are type agnostic, it is not possible to place methods on them that are type specific. This leads to a lot of cases where python forces you to read inside out or a the syntax gets very disjoint in general. A good example of this is: "\n".join(l.capitalize() for l in my_string.split("\n")) which could reduce to something far more readable, such as: my_string.split("\n").capitalize().join_items("\n") Besides the benefits to basic language usability (in my opinion) there are tool and development benefits: -- The additional type information would simplify static analysis and provide cues for optimization (I'm looking at pypy here; their list strategies play to this perfectly) -- The warning on "violating the contract" and without first terminating it would be a helpful tool in catching and debugging errors. I have some thoughts on syntax and specifics that I think would work well, however I wanted to solicit more feedback before I go too far down that path. Nathan [View Less]

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Columnize in module "cmd"
by Tom Zhou Dec. 21, 2011

Dec. 21, 2011

Hi~ alls recently, i focus on the module "cmd", and find some confused things-- the function named "columnize". Why we need a multiloop as "for nrows .. for col .. for row.." ?? i think we can make a easier method, for example, first, find out the maxlen str in list, and use its length as the standard size to format the list. Ok, maybe i ignore something, so please give me some hints. --- thanks tom

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Re: [Python-ideas] Adding "Typed" collections/iterators to Python
by Nathan Rice Dec. 20, 2011

Dec. 20, 2011

>> L2 = [X(e) for e in L1]>> L3 = [Y(e) for e in L2]>> vs>> L2 = X(L1) # assuming X has been updated to work in both vector/scalar>> L3 = Y(L2) # context...>>> L = ['a', 'bc', ['ada', 'a']]>> What is len(L)? 3 or [1, 2, 2] or [1, 2, [3, 1]]?>>>> L2 = [Z(Y(X(e))) for e in L1]>> vs>> L2 = Z(Y(X(L1)))>>>> L2 = [e.X().Y().Z() for e in L1]>> vs>> L2 = L1.X().Y().Z() # assuming vectorized versions of member … [View More]

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context managers used to allocate resources can be abused - a solution
by Alon Horev Dec. 18, 2011

Dec. 18, 2011

Hello, while I was implementing a connection pool i've noticed a pitfall of our beloved with statements: with pool.get_connection() as conn: ....conn.execute(...) conn.execute(...) # the connection has been returned to the pool and does not belong to the user! a proposed solution: with protected_context(pool.get_connection()) as conn: ....conn.execute(...) conn.execute(...) # raises OutOfContextError() with protected_context(file("/tmp/bla.txt", "w")) as f: ....file.write("blo") file.write(… [View More]

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Python Isn't Perfect: adding a 'gotchas' section to the tutorial
by Richard Prosser Dec. 16, 2011

Dec. 16, 2011

Although I love Python there are some aspects of the language design which are disappointing and which can even lead to problems in some cases. A classic example is a mutable default argument having the potential to produce unexpected side-effects, as a consequence of the non-intuitive scoping rules. Another awkward 'feature' is the requirement for a trailing comma in singleton tuples, due I believe to the use of expression parentheses rather than (say) the use of special brackets like … [View More]

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contextlib.maybe
by Julian Berman Dec. 16, 2011

Dec. 16, 2011

Twice recently I've found myself wanting to write the following code: def fn(a_file=None): responsible_for_closing = False if a_file is None: a_file = open(a_default_location) responsible_for_closing = True do_stuff(a_file) if responsible_for_closing: a_file.close() which can be written slightly shorter I know, but it's still a tiny bit messy and repetitive. What I'd prefer to write is something more like: def fn(… [View More]

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list / array comprehensions extension
by Alexander Heger Dec. 15, 2011

Dec. 15, 2011

Hi, in IDL/GDL, if at have an array x = [1,2,3] ?I can simply construct a new array by y = [0,x] which would give [0,1,2,3] I know you can do this case with x[0:0] = [0] but obviously more complicated cases are conceivable, common for me, in fact x=[1,2,3] y=[5,6,7] z=[0,x,4,y,8] result [0,1,2,3,4,5,6,7,8] or maybe even z=[0,x[0:2],5] and so forth. On the other hand, if you have a function you can pass the list of arguments to another function def f(*args): ? g(*args) which expands the list … [View More]

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An Alternate Suite Delineation Syntax For Python? (was Re: [Python-Dev] [PATCH] Adding braces to __future__)
by Nick Coghlan Dec. 15, 2011

Dec. 15, 2011

A more appropriate way to phrase this suggestion would be one that clearly states the problems with the status quo and explores them, rather than assuming an answer, and indulging in a long rambling diatribe against those that will shout you down for adding nothing new to the discussion. Step 1: post to the right mailing list (i.e. python-ideas, not python-dev) Step 2: don't assume the conclusion in the subject line. Ask a question, don't state an imperative. For example: "An Alternate Suite … [View More]Delineation Syntax For Python?" Step 3: respect the time of others, by trimming your message to its core essence For example: An Alternate Suite Delineation Syntax For Python? ================================================= Python's whitespace based delineation of suites is one of its greatest strengths. It aligns what the human reader perceives with what the computer is actually executing, reducing the frequency of semantic errors due to mismatches between the use of separate block delimiters and the human readable indentation. However, this benefit comes at quite a high price: it is effectively impossible to embed arbitrary Python statements into any environment where leading whitespace is *not* significant, including Python's own expression syntax. It can be argued that this restriction has led directly to the introduction of "expression friendly" variants of several Python top level constructs (for example, lambda expressions, conditional expressions and as a contributing factor in creating the various forms of comprehension). It is also one of the reasons Python-based templating languages almost always create their own custom syntax - embedding Python's own whitespace sensitive statement syntax into environments where leading whitespace is either ignored or forms a significant part of the template output is a formidable challenge. In other languages, this kind of issue is handled by using explicit suite and statement delimiters (often braces and semi-colons, respectively) to allow full suites to be used as expressions. While Python uses braces for another purpose (dictionary and set definitions), it is already the case that semi-colons (';') can be used as statement terminators, both optionally at the end of any simple statement, and also to combine multiple simple statements into a single larger statement (e.g. "x += y; print(x)"). It seems that this existing feature could be combined with a brace-based notation to create an unambiguous "suite expression" syntax that would enjoy the same semantics as ordinary Python suites (i.e. doesn't create a new scope, doesn't directly affect control flow), but allows *all* Python statements to be embedded inside expressions. Currently, the character sequence "{:" is a Syntax Error: you are attempting to end a compound statement header line while an opening brace remains unmatched, or else trying to build a dictionary without specifying the key value. This creates an opportunity to re-use braces for a suite expression syntax without conflicting with their use for set and dictionary construction. Specifically, it should be possible to create a variant of the top-level Python syntax that: 1. Explicitly delimits suites using the notation "{:" to open the suite and "}" to end it 2. Requires the use of ";" to separate simple statements (i.e. newline characters would not end a statement, since we would be inside an expression) 3. Requires that all subordinate suites also be suite expressions (i.e. leading whitespace would not be significant, since we would be inside an expression) 4. Returns the value of the last statement in the suite as the result of the suite expression (since return statements would affect the containing scope) 5. Anonymous class and function definitions would be permitted in a suite expression (but accepting only a suite expression instead of an ordinary suite) (Ruby block notation and C's comma expressions are pretty much direct inspiration for the above list) Some examples: Raise expressions: x = y if y is not None else {: raise ValueError("y must not be None!")} Try expressions: x = {: try {: y.hello} except AttributeError {: "world!"}} With expressions: data = {: with open(fname) as f {: f.read()}} Embedded assignments: if {: m = pat.search(data); m is not None}: # do something with m else: # No match! In-order conditional expressions: {: if a {:b} else {:c}} One-liner accumulator function: def acc(n) {: s=n; return {: def (i) {: nonlocal s; s += i; return s}}} Cheers, Nick. P.S. I actually think the above idea is kinda cryptic and ugly. It is, however, extraordinarily powerful, especially when it comes to embedding Python code inside other environments that may not be friendly to significant leading whitespace. Applying the excellent set of criteria suggested by Mike Meyer: 1. What's the use case? Cleanly embedding arbitrary Python statements inside environments that are not generally sensitive to leading whitespace, such as templating languages, Python expressions and Python strings. 2. Does it make such code more readable? Yes, I believe it does, by providing "one obvious way to do it". Currently there are a mish-mash of alternatives out there (or else people give up and use something else entirely, like Javascript). 3. Does it encourage writing unreadable code? Perhaps. You certainly end up with a lot of braces, colons and semi-colons floating around. However, that's also something that can be addressed by style guides - if people are using the embedded syntax when the indented syntax would work fine, the problem isn't really due to the embedded syntax, it's due to people not caring about maintainability. -- Nick Coghlan | ncoghlan(a)gmail.com | Brisbane, Australia [View Less]

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Re: [Python-ideas] Inconsistent script/console behaviour
by Nick Coghlan Dec. 15, 2011

Dec. 15, 2011

(moved thread to python-ideas, where it belongs. Messing with 20 years of precedent is *not* a topic for python-dev) On Thu, Dec 15, 2011 at 6:58 PM, anatoly techtonik <techtonik(a)gmail.com> wrote: >> You can't fix this without completely changing the way the interactive >> console treats blank lines. > > But the fix doesn't require changing the way interactive console treats > blank lines at all. It only requires to finish current block when a dedented > … [View More]line is encountered and not throwing obviously confusing SyntaxError. At the > very least it should not say it is SyntaxError, because the code is pretty > valid Python code. If it appears to be invalid "Python Console code" - the > error message should say that explicitly. That would be a correct > user-friendly fix for this UX issue, but I'd still like the behavior to be > fixed - i.e. "allow dedented lines end current block in console without > SyntaxError". Right now I don't see the reasons why it is not possible. > > Please speak code when replying about use cases/examples that will be broken > - I didn't quite get the problem with "global scope if" statements. There are two steps to what the interactive console does: Step 1: accept a block of code. Step 2: compile and execute it in single statement mode (in order to get the result of any expressions for display) It is the *first* step that is terminated by a blank line. That step understands almost nothing about Python syntax - just enough to figure out whether or not it should be asking for a continuation line. (IIRC, it pretty much just does bracket matching and "if I have seen a ':' character, ask for continuation lines until one of them is blank". So yes, fixing example 1 *does* require significant changes to the way the interactive interpreter works. The interactive interpreter currently operates on individual statements. For compound statements, those statements are terminated by a blank line. If you don't terminate them correctly, then it will attempt to interpret two statements as one, and hence you will get UnexpectedDedentError. To fix example 1, the interactive interpreter would have to be redesigned to use exec mode instead, so it can accept multiple statements at once. That's not impossible, but is a lot of effort to address a relatively minor inconvenience (the inability to tolerate blank lines *within* a suite significantly more annoying in my experience, but even harder to fix than example 1 would be). Cheers, Nick. -- Nick Coghlan | ncoghlan(a)gmail.com | Brisbane, Australia [View Less]

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Fwd: Transportable indent level markers. >>>===<<<
by Andre Roberge Dec. 15, 2011

Dec. 15, 2011

On Wed, Dec 14, 2011 at 9:08 PM, Ron Adam <ron3200(a)gmail.com> wrote: > On Wed, 2011-12-14 at 18:53 -0500, Terry Reedy wrote: > > On 12/13/2011 10:53 AM, Guido van Rossum wrote: > > > On Mon, Dec 12, 2011 at 9:24 PM, Terry Reedy<tjreedy(a)udel.edu> wrote: > > >> On 12/12/2011 5:07 PM, Ron Adam wrote: > > >> > > >>> To make that work, we need to use unique character sequences that > aren't > > >>> already … [View More]

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Re: [Python-ideas] Python Isn't Perfect: adding a 'gotchas' section to the tutorial
by Richard Prosser Dec. 12, 2011

Dec. 12, 2011

Ned, I accept your comment about the tutorial - I wasn't sure about that. Thanks. However I still feel that there are some aspects of the language which are not in the true spirit of Python (i.e. 'intuitive'). The discussion about default mutable types is one of these. It seems to me that the essential problem is that of assignment in general, which (I believe) creates a reference on the LHS to the object on the RHS, rather than having a copy operation to make the two objects completely … [View More]separate. That can be confusing in other contexts, not just with default parameters. If I am to write a 'gotchas' FAQ or whatever then I would like to understand the reasoning behind such design decisions but I can't find any 'deep' explanations at present - just several posts about people being puzzled! A similar comment applies to the lack of type declarations. So if you or anyone else can explain exactly why such odditties are implemented I would be grateful. Unfortunately it is almost certainly too late to propose fixes (if appropriate) for such quirks in Python 3 but at least I should be able provide arguments as to why things are done the way they are. Richard On 11 December 2011 01:21, <python-ideas-request(a)python.org> wrote: > Message: 2 > Date: Sat, 10 Dec 2011 13:34:15 -0500 > From: Ned Batchelder <ned(a)nedbatchelder.com> > To: Richard Prosser <richard.prosser(a)mail.com> > Cc: python-ideas(a)python.org > Subject: Re: [Python-ideas] Python Isn't Perfect: adding a 'gotchas' > section to the tutorial > Message-ID: <4EE3A627.8010002(a)nedbatchelder.com> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 12/10/2011 9:16 AM, Richard Prosser wrote: > > Although I love Python there are some aspects of the language design > > which are disappointing and which can even lead to problems in some > cases. > > > > A classic example is a mutable default argument having the potential > > to produce unexpected side-effects, as a consequence of the > > non-intuitive scoping rules. > > > > Another awkward 'feature' is the requirement for a trailing comma in > > singleton tuples, due I believe to the use of expression parentheses > > rather than (say) the use of special brackets like chevrons. > > > > Something that I personally wish for is the ability to declare > > variable types 'up front' but that facility is missing from Python. > > > One of these things is not like the others. Mutable default arguments, > and singleton tuples are surprises that make sense once you understand > things on a deeper level. It makes sense to call them out as a > "gotcha": a common stumbling block for learners. But "no type > declarations" is not a wart, it's a fundamental feature of the language > that is immediately apparent from the first lesson. > > --Ned. > [View Less]

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Avoiding nested for try..finally: atexit for functions?
by Nikolaus Rath Dec. 12, 2011

Dec. 12, 2011

Hello, I often have code of the form: def my_fun(): allocate_res1() try: # do stuff allocate_res2() try: # do stuff allocate_res3() try: # do stuff finally: cleanup_res3() finally: cleanup_res2() finally: cleanup_res1() return With increasing number of managed resources, the indentation becomes really annoying, there is lots of line noise, and I don't like … [View More]

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