Python-ideas January 2009

python-ideas@python.org

44 participants
19 discussions

Re: [Python-ideas] min_fields argument to str.split()

by Vitor Bosshard

NOTE: It was brought to my attention that my last e-mail message arrived completely garbled. Here it is again, as I meant to send it. My Apologies to everyone. > > > > How about this? > > > >>>> from itertools import * > >>>> def fixlen(iterable, len, item=None): > > ... return islice(chain(iterable, repeat(item)), len) > > ... > >>>> list(fixlen([1, 2, 3], 5)) > > [1, 2, 3, None, None] > > > > I like this. Maybe an addition to itertools? > > +1 +1 from me too. Way better than the generator I proposed. Having this as a function also makes it applicable to much more than just one use case. If you see the following code: a,b,c,d = fixlen(str.split(),4) You immediately know it's not a regular unpacking operation; it will be the first place you look if something goes wrong. Vitor

12 years, 1 month

Fwd: standard library proposal for "tasklets" using generators

by Guillaume Chereau

Just realized i clicked "reply" instead of 'reply to all" ---------- Forwarded message ---------- From: Guillaume Chereau <charlie137(a)gmail.com> Date: Wed, Jan 21, 2009 at 12:00 PM Subject: Re: [Python-ideas] standard library proposal for "tasklets" using generators To: Guido van Rossum <guido(a)python.org> Thanks for the answer, I understand the concern here. I still think having such a library, if not as a standard python lib, but at least as an independent one, that could then be used by all the project using such pattern (twisted, kiwi, gobject, etc.) would be a good idea. I will try to contact those different projects for suggestions. I also have to say one of my secret hope was that the pypy interpreter could eventually understand this kind of 'tasklet' generators and generate C or other language code out of it, since pypy already has support for tasklets, but using a syntax that can't be interpreted by cpython. - Guillaume On Wed, Jan 21, 2009 at 10:56 AM, Guido van Rossum <guido(a)python.org> wrote: > The pattern of invoking all sorts of stuff that may wait indefinitely > using yield has been proposed before, but I find it creates rather > awkward code compared to using plain threads. Not something I'd like > to encourage by having standard decorators and other APIs around this > idea. I believe Twisted has something like this, and that's probably a > good place to isolate such patterns; in their case they really > want/need this, so they put up with the awkwardness. Putting the > necessary decorators/APIs in the language doesn't do much about the > awkwardness, so I don't think we should do this. > > On Tue, Jan 20, 2009 at 6:36 PM, Guillaume Chereau <charlie137(a)gmail.com> wrote: >> Hello, >> Thanks to the introduction of new generator features, as described in >> PEP 0342, it is possible to write what I would call "tasklet" (I am no >> sure what should be the term exactly). >> tasklet are a way to write callback based code that looks like thread. Example : >> >> (assuming we have a function sleep(sec, callback) that will call the >> callback after `sec` seconds) >> >> @tasklet >> task1(): >> print "sleep" >> yield Wait(sleep, 5) >> print "called after sleep" >> yield 10 # returned value >> >> @tasklet >> task2(): >> for i in range(10): >> value = yield task1() >> >> (the same thing using callback only would be quite difficult to read) >> >> The library should provide -the names are not very important- a >> Tasklet class, plus a tasklet decorator to turn any generator into a >> tasklet, plus a Wait function to turn a callback function into a >> tasklet, plus maybe a few other functions. >> >> In many cases this kind of library would be very useful. I am >> currently using something very similar for a project I am working on >> [0]. the kiwi project also have a similar library [1] (but the syntax >> is not really nice because they don't take advantage of the PEP 0342 >> features.) >> >> I guess more and more people may try to use similar library in the >> future, but the problem is that the implementation of it is really not >> trivial, so I would like to suggest having such a library in the >> standard set of python libraries. >> >> Do you think this is a good idea ? >> >> cheers, >> >> Guillaume >> >> [0] http://git.openmoko.org/?p=tichy.git;a=blob;f=tichy/tasklet.py >> [1] http://www.async.com.br/projects/kiwi/api/kiwi.tasklet.html >> >> -- >> http://charlie137.blogspot.com/ >> _______________________________________________ >> Python-ideas mailing list >> Python-ideas(a)python.org >> http://mail.python.org/mailman/listinfo/python-ideas >> > > > > -- > --Guido van Rossum (home page: http://www.python.org/~guido/) > -- http://charlie137.blogspot.com/ -- http://charlie137.blogspot.com/

12 years, 1 month

standard library proposal for "tasklets" using generators

by Guillaume Chereau

Hello, Thanks to the introduction of new generator features, as described in PEP 0342, it is possible to write what I would call "tasklet" (I am no sure what should be the term exactly). tasklet are a way to write callback based code that looks like thread. Example : (assuming we have a function sleep(sec, callback) that will call the callback after `sec` seconds) @tasklet task1(): print "sleep" yield Wait(sleep, 5) print "called after sleep" yield 10 # returned value @tasklet task2(): for i in range(10): value = yield task1() (the same thing using callback only would be quite difficult to read) The library should provide -the names are not very important- a Tasklet class, plus a tasklet decorator to turn any generator into a tasklet, plus a Wait function to turn a callback function into a tasklet, plus maybe a few other functions. In many cases this kind of library would be very useful. I am currently using something very similar for a project I am working on [0]. the kiwi project also have a similar library [1] (but the syntax is not really nice because they don't take advantage of the PEP 0342 features.) I guess more and more people may try to use similar library in the future, but the problem is that the implementation of it is really not trivial, so I would like to suggest having such a library in the standard set of python libraries. Do you think this is a good idea ? cheers, Guillaume [0] http://git.openmoko.org/?p=tichy.git;a=blob;f=tichy/tasklet.py [1] http://www.async.com.br/projects/kiwi/api/kiwi.tasklet.html -- http://charlie137.blogspot.com/

12 years, 1 month

With-expression

by Carl Johnson

From the department of doomed ideas: data = f.read() with open("file.txt") as f Has the idea of a with-expression already been preemptively shot down? I guess the objection is that the above could easily be rewritten as data = read_and_close("file.txt") if one defines the appropriate function. The counter-objection is that why do I need to write a micro-function when the with-statement already does what I want, except it takes up two lines when it would still be clear as one line. But then the counter-counter-objection is that you can write with open("file.txt") as f: data = f.read() today. To which the counter-counter-counter-objection is, yeah, but that looks cluttered and ugly in a way that the first example doesn't... Except the first example is sort of cluttered. I dunno, what do other people think about this? Doomed or super-doomed? -- Carl

12 years, 1 month

More things in the standard library equipped with __enter__ and __exit__

by Tarek Ziadé

Hello, I have entered this yesterday : http://bugs.python.org/issue4972. about the fact that ftplib.FTP could be used in a "with" loop. But I spotted that imaplib.IMAP4/IMAP4_SSL and smtplib.SMTP could be equipped as well, so an __exit__ call takes care of closing or quiting. This change could be done together with some examples in the documentation. If this sounds like a good idea, I can start a patch in #4972, Regards Tarek -- Tarek Ziadé | Association AfPy | www.afpy.org Blog FR | http://programmation-python.org Blog EN | http://tarekziade.wordpress.com/

12 years, 1 month

defaultdict factory argument

by Zac Burns

I think defaultdict would be more useful if one could initialize it with the option to have the key that is being looked up passed to default_factory. -- Zachary Burns (407)590-4814 Aim - Zac256FL Production Engineer (Digital Overlord) Zindagi Games

12 years, 1 month

allow tuples in a with statement?

by Sturla Molden

Sometimes multiple arguments in a with statement would be convinient. with open('in.txt','rt'), \ open('out.txt','wt'),\ open('err.txt','wt') \ as fstdin, fstdout, fstderr: But today this has to be written as: with open('in.txt','rt') as fstdin: with open('out.txt','wt') as fstdout: with open('err.txt','wt') as fstderr: with the effect of causing multiple redundant levels of intendation. Regards, Sturla Molden

12 years, 1 month

Make getpass.getpass use system password keyrings ?

by Tarek Ziadé

Hello, What about having an option in getpass to store and reuse passwords in system keyrings ? getpass(prompt[, stream]) would become: getpass(prompt[, stream, keyring]) where keyring would be a callable that can be use to retrieve the password from a keyring system and store it the first time. The getpass module could provide some keyring support for: - ssh-agent under Linux - keychain under Mac OS X - ... And let the developers use their own keyring system by providing a callable. Regards Tarek -- Tarek Ziadé | Association AfPy | www.afpy.org Blog FR | http://programmation-python.org Blog EN | http://tarekziade.wordpress.com/

12 years, 1 month

Add "while" clauses to generator expressions

by Gerald Britton

I've been using Python generators for a while now. e.g. a=(i for i in range(10)) a.next() a.next() ...etc. I also find the "if" clause handy: a = (i for i in range(10) if i%2==0) (I know that range(0,12,2) will do the same thing, but it's the idea I like, especially for more complex predicates.) I would like to know if anyone has thought of adding a "while" clause as well, like this: a = (i for i in range(100) while i <=50) (Again, this could be done with range(0,51) but then the predicate can be more complicated.) Why would this be helpful? Consider the "in ..." part of the generator. You could be referring to something that is ordered -- sorted names for example. Then you might want to stop your iterator when you reach "Morgan" so you would like to write: name = (n for n in names while n <= "Morgan") name.next() ...etc... Of course, name = (n for n in names if n <= "Morgan") will work, but it will look at every item in "names." Since "names" is sorted, this is a waste of time. Imagine you want to stop at "Baker". Your "if" clause will look at and discard most names in the list, assuming a normal distribution of English names. Now, you could do the same thing with a generator function: def leMorgan(names): i = 0 while names[i] <= "Morgan": yield names[i] i+=1 and use it like this: name=leMorgan(names) name.next() ...etc... but I think that adding a while clause to the generator expression is simpler and clearer (and it keeps it all in one place!) I know that this is functionally equivalent to the takewhile function in itertools and my motivation is the same. I just think that this could be done nicely within the context of the existing syntax. This is also convenient when the "in ..." clause refers to another (possibly infinite) generator. For a simple example, suppose I want to run through some natural numbers. I can write an infinte generator function like this: def genN(n=0): while 1: yield n n+=1 Then, I might use this in another generator: p = (n for n in genN() if prime(n) while n <= 100) to get the prime numbers under 100 (assuming I have a predicate "prime" that works as one would hope). Of course you could do this with range(101) instead of genN, but this is just an example to demonstrate the idea. Without the "while" clause, this will not work: p = (n for n in genN() if prime(n) if n <= 100) This will actually NEVER terminate, since EVERY item in genN() (which is an infinite generator) will be tested for <= 100. So...What do others think? Is this a loony idea? Is there a better way? Also, can anyone think of a similarly syntax to replicate the dropwhile function?

12 years, 1 month

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Python-ideas January 2009