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 ligne 1, dans <module>
ZeroDivisionError: division entière ou modulo par zéro
André
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 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...
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:
I've been doing some more hacking, and I now have a
working implementation of cofunctions, which I'll
upload somewhere soon.
I have also translated my yield-from examples to
use cofunctions. In the course of doing this, the
additional restrictions that cofunctions impose have
already proved their worth -- I forgot a cocall, and
it clearly told me so and pointed out exactly where
it had to go!
--
Greg
I would propose that an idiomatic way is created, for instance a
pragma or statement, which allows one to disambiguate the used python
version in a manner that is both obvious for the human reader, and as
well allows python to reject the script, should the wrong version of
the interpreter be present. I think this is a quite common
problem[1][2], which could be solved in a fairly easy and pragmatic
way.
Being a python beginner, I'm not well-adversed in the ways of
idiomatic python, so feel free to reject or improve on my syntactic
examples, these are merely meant to demonstrate my point.
Building on [3], I would suggest some syntax like this:
#!/usr/local/bin/python
# version: python-3
import os, sys
...
Alternatively, some syntax could be used that allows one to use basic
comparison operators, a range or even simple chained logical
statements.
#!/usr/local/bin/python
# version: [2.4 .. 3.0] and not 2.6.1
# or multiple clauses
# version: >= 2.4.2
# version: < 3.0
# version: not 2.6.1
# jpython-version: ...
# or multiple keys
# min-version: 2.4.2
# min-jpython-version: 2.4.4
# max-version: 2.6.1
import os, sys
...
This way it should be fairly obvious to the casual reader which python
version the program is intended to run under, and the python
interpreter can simply reject to parse/execute the program, should it
encounter an incompatible requirement.
All comments, thoughts and suggestions welcome.
References
[1] http://stackoverflow.com/questions/446052/python-best-way-to-check-for-py...
[2] http://stackoverflow.com/questions/1093322/how-do-i-check-what-version-of...
[3] http://www.python.org/dev/peps/pep-0263/
Hello list,
The documentation states that the default value of function parameter,
if mutable, can change it's default value at runtime due to be
evaluated only once on function object creation.
I would like to suggest the inclusion of an default language warning
when this kind of construction is used, as it's Python specific
behavior and can lead to "strange behavior" or misuse by programmers
that are migrating from other languages to Python.
This proposal was first open as a suggestion issue in bug track, but,
as a request from Mr. Peterson, I'm rewriting it to this list.
http://bugs.python.org/issue9646
Regards,
--
.:''''':.
.:' ` Sérgio Surkamp | Gerente de Rede
:: ........ sergio(a)gruposinternet.com.br
`:. .:'
`:, ,.:' *Grupos Internet S.A.*
`: :' R. Lauro Linhares, 2123 Torre B - Sala 201
: : Trindade - Florianópolis - SC
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:: +55 48 3234-4109
:
' http://www.gruposinternet.com.br
Hello,
when working on a test suite, no matter if writing new test cases from
scratch or modifying an existing one, the need of temporarily
excluding other tests and focus on that particular one is very common
(at least for me).
This is especially true when working on a test suite which contains a
lot of tests producing a very verbose output result or which takes a
lot to complete.
What I usually do in such cases is to scroll down until test_main()
function, manually change support.run_unittest() ilke this:
- support.run_unittest(TestCase1, TestCase2, TestCase3, TestCase4)
+ support.run_unittest(TestCase4)
...and then comment out all the test cases in TestCase4 class except
the one I'm interested in.
This is obviously not very flexible, then I thought that maybe
unittest (or test/support module, I'm not sure) could provide a
specific decorator for temporarily running only one specific test
class(es) or method(s).
The decorator could be used in 3 ways:
== use case #1 ==
@unittest.exclude_others
class TestCase1:
...
All TestCase1 test cases are run, TestCase2, TestCase3 and TestCase4
classes are excluded.
== use case #2 ==
class TestCase1:
@unittest.exclude_others
def test_something(self):
....
All TestCase1.* tests are excluded except "test_something".
TestCase2, TestCase3 and TestCase4 are run.
== use case #3 ==
@unittest.exclude_others
class TestCase1:
@unittest.exclude_others
def test_something(self):
....
Only TestCase1.test_something() is run (this is the most common use case).
Thoughts?
Kindest regards,
--- Giampaolo
http://code.google.com/p/pyftpdlib/http://code.google.com/p/psutil/
Hi,
A few days ago I submitted a feature request for the fnmatch module to
allow curly braces shell-like expansions:
http://bugs.python.org/issue9584
The patch I submitted was found to be incorrect, and I'm working on it
(I already have a patch that seems to be working and I need to test it
extensively).
However, some concerns were raised about the fnmatch module not being
the correct place to do that, if the goal is to mimic shell behavior.
Expansion would have to be done before, generating the list of
patterns that would then be given to fnmatch.
It was suggested that I took this issue on Python-ideas, so here we go.
What do people think? Should it be done in fnmatch, expanding the
curly braces to the regex? Should it be done before that?
If the latter, what would be an appropriate module to expand the
braces? glob? Another module?
FWIW (i.e not much), my opinion is leaning towards implementing it in glob.py:
- add '{' to the magic_check regex
- in glob1 (which is called when the pattern 'has_magic'), expand the
braces and then call fnmatch.filter() on each resulting pattern
That would respect more what is done in shells like Bash, and it makes
it also more straight-forward to implement.
Cheers,
--
Mathieu
The original proposal for introducing 'yield from' (which is still
in PEP 380's name) was to delegate a part of generator's work to another generator.
However, in later discussions, the focus totally shifted to
cooperative multitasking.
In the example code Greg has given in
http://mail.python.org/pipermail/python-ideas/2010-August/007927.html ,
there's not a single use case for delegating! 'yield from's essentially replace
normal function calls!
All Greg uses this stuff for is to manually switch between `threads'
simulating individual customers! The _programming_logic_ is plain function calls,
yields/cocalls are just a crutch (and quite an ugly one) to release a time slot.
So, in fact, this all `yield from/coroutine' effort is an attempt to
unseparatably mix two very loosely-related subjects:
- code delegation (sequence in which sequential/closely-related code is executed)
- execution scheduling (sequence in which parallel/loosely-related/unrelated code is executed)
in the form 'a function call _may_be_ a scheduling event at the same time'!
That's why it all feels so `clumsy/weird/twisted/fallacious' !
Cooperative threads must be switched somehow but choosing such a
quirky and implicit technique for that is completely against Python
Zen (violates about half of the items :^) )!
Instead, if it's cooperative multitasking you play with,
switching must be independent from other activity and as explicit as possible.
There's a technique just for that called 'fibers'
(MS fiber API: http://msdn.microsoft.com/en-us/library/ms682661.aspx ).
In short:
- ConvertThreadToFiber() decorates current thread as an initial fiber
- CreateFiber (*function, *parameter)->ID creates a new fiber
- SwitchToFiber(ID) switches execution to another fiber.
- ExitFiber() exits a fiber. In python, the function may just return
as in threads
As the goal of fibers is the same as threads, it's reasonable to
derive knowledge from there too, maybe duplicate the interface
where applicable.
And as cofunctions, these Fibers are just what
http://en.wikipedia.org/wiki/Coroutine describes. With interacting coroutines,
there is no execution stack anymore: there are 'calls' but no 'returns'!
It's essentially like passing messages back and forth as in win32k.
`Yield from's are still valid. But only as code delegation technique, i.e. a
shortcut to `for i in subgen(): yield i'.
The latter statement looks brief enough for me to discard the proposal
altogether. Send() and stuff doesn't look to fit in what
generators were originally intended to do - producing sequences with
arbitrary or unknown length.
On Aug 10, 2010 6:18 PM, "Greg Ewing" <greg.ewing(a)canterbury.ac.nz> wrote:
> Bruce Leban wrote:
>> Is there a pythonesque alternative?
>
> It's generally considered unpythonic to assign arbitary meanings
> to randomly chosen punctuation (decorator syntax notwithstanding!) ...
I quite agree. I hate random punctuation.
Suppose hypothetically that cocall is just one of a class of execution
modifiers that we might ultimately want to add. In that case maybe there's a
syntax (analogous to adding decorator syntax) that doesn't require new
keywords. There's yield, yield from and cocall. Are there more?
(I don't know. I'm just wondering.)
--- Bruce
(via android)
I think I may have found a way to eliminate the need for
the 'codef' keyword. I realised that although it's important
to prevent making a regular call to a cofunction, there's
no need to prevent making a cocall to an ordinary function.
In that case, it can just be treated like a normal call.
So I suggest the following changes:
1. The presence of 'cocall' inside a function causes it
to be a cofunction. There is no 'codef' keyword.
2. The 'cocall' construct checks whether the function
supports cocalls, and if so, proceeds as previously
specified. Otherwise, it calls the function normally
and returns the result.
(To allow for objects such as bound methods to wrap things
which could be cofunctions or not, the __cocall__ method
will be permitted to return NotImplemented as a way of
signalling that cocalls are not supported.)
Does this scheme sound any better?
--
Greg
