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Ralsina.Me — Roberto Alsina's website

Posts about qt

Qt Mac Tips

My team has been work­ing on port­ing some PyQt stuff to Mac OS­X, and we have run in­to sev­er­al Qt bugs, sad­ly. Here are two, and the work­arounds we found.

Na­tive di­alogs are bro­ken.

Us­ing QFile­Di­a­log.ge­tEx­ist­ingDi­rec­to­ry we no­ticed the fol­low­ing symp­tom­s:

  • If you do noth­ing, the di­a­log went away on its own af­ter about 20 sec­ond­s.

  • Af­ter you used it on­ce, it may pop up and dis­­ap­­pear im­me­di­ate­­ly. Or not.

So­lu­tion: use the Don­tUse­N­a­tive­Di­a­log op­tion.

Wid­gets in QTreeWid­getItems don't scrol­l.

When you use Wid­gets in­side the items of a QTreeWid­get (which I know, is not a com­mon case, but hey, it hap­pen­s), the wid­gets don't scroll with the item­s.

Solution: use the -graph­ic­ssys­tem raster options. You can even inject them into argv if the platform is darwin.

The Future of PyQt by Example

Three years ago, I start­ed a se­ries of long posts called "PyQt by Ex­am­ple". It reached five posts be­fore I aban­doned for a se­ries of rea­sons that don't mat­ter any­more. That se­ries is com­ing back start­ing next week, rewrit­ten, im­proved and ex­tend­ed.

It will do so in a new site, and the "old" posts will be re­tired to an ar­chive page. Why? Well, the tech­nolo­gies used in some of them are ob­so­lete or don't quite work nowa­days. So, the new ver­sions will be the pre­ferred ones.

And while I am not promis­ing any­thing, I have enough writ­ten to make this some­thing quite longer, more nice­ly lay­out­ed, more in­ter­est­ing and make it cov­er more ground. BUT, while do­ing some checks on the traf­fic sta­tis­tics for the old post­s, some things popped out.

This was very popular

About 60% of my site's traf­fic goes to those five post­s. Out of about 1200 posts over 12 years, 60% of the view­ers go to the 0.4% of the pages. That is a lot.

It's a long tail

The traf­fic has not de­creased in three years. If any­thing, it has in­creased

https://p.twimg.com/Aw0MHhoCAAAXmro.png:large

A long and tall tail.

So, all this means there is a de­sire for PyQt doc­u­men­ta­tion that is not sat­is­fied. I am not sur­prised: PyQt is great, and the rec­om­mend­ed book is not free, so there is bound to be a lot of de­mand.

And, here's the not-­so-rosy bit: I had un­ob­tru­sive, rel­e­van­t, out­-of-the-way-but-vis­i­ble ads in those pages for more than two years. Of the 70000 unique vis­i­tors, not even one clicked on an ad. Don't wor­ry, I was not ex­pect­ing to get mon­ey out of them (although I would love to some day col­lect a $100 check in­stead of hav­ing google hold my mon­ey for me ad eter­num).

But re­al­ly? Not even one ad click? In more than two years, thou­sands of peo­ple? I have to won­der if I just at­tract cheap peo­ple ;-)

Ubuntu One APIs by Example (part 1)

One of the nice things about work­ing at Canon­i­cal is that we pro­duce open source soft­ware. I, specif­i­cal­ly, work in the team that does the desk­top clients for Ubun­tu One which is a re­al­ly cool job, and a re­al­ly cool piece of soft­ware. How­ev­er, one thing not enough peo­ple know, is that we of­fer damn nice APIs for de­vel­op­er­s. We have to, since all our client code is open source, so we need those APIs for our­selves.

So, here is a small tu­to­ri­al about us­ing some of those APIs. I did it us­ing Python and PyQt for sev­er­al rea­son­s:

  • Both are great tools for pro­­to­­typ­ing

  • Both have good sup­­port for the re­quired stuff (D­Bus, HTTP, OAu­th)

  • It's what I know and en­joy. Since I did this code on a sun­­day, I am not go­ing to use oth­­er things.

Hav­ing said that, there is noth­ing python-spe­cif­ic or Qt-spe­cif­ic in the code. Where I do a HTTP re­quest us­ing Qt­Net­work, you are free to use lib­soup, or what­ev­er.

So, on to the nuts and bolt­s. The main pieces of Ubun­tu One, from a in­fra­struc­ture per­spec­tive, are Ubun­tu SSO Clien­t, that han­dles us­er reg­is­tra­tion and login, and Sync­Dae­mon, which han­dles file syn­chro­niza­tion.

To in­ter­act with them, on Lin­ux, they of­fer DBus in­ter­faces. So, for ex­am­ple, this is a frag­ment of code show­ing a way to get the Ubun­tu One cre­den­tials (this would nor­mal­ly be part of an ob­jec­t's __init__):

# Get the session bus
bus = dbus.SessionBus()

:
:
:

# Get the credentials proxy and interface
self.creds_proxy = bus.get_object("com.ubuntuone.Credentials",
                        "/credentials",
                        follow_name_owner_changes=True)

# Connect to signals so you get a call when something
# credential-related happens
self.creds_iface = dbus.Interface(self.creds_proxy,
    "com.ubuntuone.CredentialsManagement")
self.creds_proxy.connect_to_signal('CredentialsFound',
    self.creds_found)
self.creds_proxy.connect_to_signal('CredentialsNotFound',
    self.creds_not_found)
self.creds_proxy.connect_to_signal('CredentialsError',
    self.creds_error)

# Call for credentials
self._credentials = None
self.get_credentials()

You may have no­ticed that get_­cre­den­tials does­n't ac­tu­al­ly re­turn the cre­den­tial­s. What it does is, it tells Sync­Dae­mon to fetch the cre­den­tial­s, and then, when/if they are there, one of the sig­nals will be emit­ted, and one of the con­nect­ed meth­ods will be called. This is nice, be­cause it means you don't have to wor­ry about your app block­ing while Sync­Dae­mon is do­ing all this.

But what's in those meth­ods we used? Not much, re­al­ly!

def get_credentials(self):
    # Do we have them already? If not, get'em
    if not self._credentials:
        self.creds_proxy.find_credentials()
    # Return what we've got, could be None
    return self._credentials

def creds_found(self, data):
    # Received credentials, save them.
    print "creds_found", data
    self._credentials = data
    # Don't worry about get_quota yet ;-)
    if not self._quota_info:
        self.get_quota()

def creds_not_found(self, data):
    # No credentials, remove old ones.
    print "creds_not_found", data
    self._credentials = None

def creds_error(self, data):
    # No credentials, remove old ones.
    print "creds_error", data
    self._credentials = None

So, ba­si­cal­ly, self­._­cre­den­tials will hold a set of cre­den­tial­s, or None. Con­grat­u­la­tion­s, we are now logged in­to Ubun­tu One, so to speak.

So, let's do some­thing use­ful! How about ask­ing for how much free space there is in the ac­coun­t? For that, we can't use the lo­cal APIs, we have to con­nect to the server­s, who are, af­ter al­l, the ones who de­cide if you are over quo­ta or not.

Ac­cess is con­trolled via OAuth. So, to ac­cess the API, we need to sign our re­quest­s. Here is how it's done. It's not par­tic­u­lar­ly en­light­en­ing, and I did not write it, I just use it:

def sign_uri(self, uri, parameters=None):
    # Without credentials, return unsigned URL
    if not self._credentials:
        return uri
    if isinstance(uri, unicode):
        uri = bytes(iri2uri(uri))
    print "uri:", uri
    method = "GET"
    credentials = self._credentials
    consumer = oauth.OAuthConsumer(credentials["consumer_key"],
                                   credentials["consumer_secret"])
    token = oauth.OAuthToken(credentials["token"],
                             credentials["token_secret"])
    if not parameters:
        _, _, _, _, query, _ = urlparse(uri)
        parameters = dict(cgi.parse_qsl(query))
    request = oauth.OAuthRequest.from_consumer_and_token(
                                        http_url=uri,
                                        http_method=method,
                                        parameters=parameters,
                                        oauth_consumer=consumer,
                                        token=token)
    sig_method = oauth.OAuthSignatureMethod_HMAC_SHA1()
    request.sign_request(sig_method, consumer, token)
    print "SIGNED:", repr(request.to_url())
    return request.to_url()

And how do we ask for the quo­ta us­age? By ac­cess­ing the http­s://one.ubun­tu.­com/api/quo­ta/ en­try point with the prop­er au­tho­riza­tion, we would get a JSON dic­tio­nary with to­tal and used space. So, here's a sim­ple way to do it:

    # This is on __init__
    self.nam = QtNetwork.QNetworkAccessManager(self,
        finished=self.reply_finished)

:
:
:

def get_quota(self):
    """Launch quota info request."""
    uri = self.sign_uri(QUOTA_API)
    url = QtCore.QUrl()
    url.setEncodedUrl(uri)
    self.nam.get(QtNetwork.QNetworkRequest(url))

Again, see how get_quo­ta does­n't re­turn the quo­ta? What hap­pens is that get_quo­ta will launch a HTTP re­quest to the Ubun­tu One server­s, which will, even­tu­al­ly, re­ply with the da­ta. You don't want your app to block while you do that. So, QNet­workAc­cess­Man­ag­er will call self­.re­ply_fin­ished when it gets the re­spon­se:

def reply_finished(self, reply):
    if unicode(reply.url().path()) == u'/api/quota/':
        # Handle quota responses
        self._quota_info = json.loads(unicode(reply.readAll()))
        print "Got quota: ", self._quota_info
        # Again, don't worry about update_menu yet ;-)
        self.update_menu()

What else would be nice to have? How about get­ting a call when­ev­er the sta­tus of sync­dae­mon changes? For ex­am­ple, when sync is up to date, or when you get dis­con­nect­ed? Again, those are DBus sig­nals we are con­nect­ing in our __init__:

self.status_proxy = bus.get_object(
    'com.ubuntuone.SyncDaemon', '/status')
self.status_iface = dbus.Interface(self.status_proxy,
    dbus_interface='com.ubuntuone.SyncDaemon.Status')
self.status_iface.connect_to_signal(
    'StatusChanged', self.status_changed)

# Get the status as of right now
self._last_status = self.process_status(
    self.status_proxy.current_status())

And what's sta­tus_changed?

def status_changed(self, status):
    print "New status:", status
    self._last_status = self.process_status(status)
    self.update_menu()

The pro­cess_s­ta­tus func­tion is bor­ing code to con­vert the in­fo from sync­dae­mon's sta­tus in­to a hu­man-read­able thing like "Sync is up­-­to-­date". So we store that in self­._last_s­ta­tus and up­date the menu.

What menu? Well, a QSys­tem­Tray­I­con's con­text menu! What you have read are the main pieces you need to cre­ate some­thing use­ful: a Ubun­tu One tray app you can use in KDE, XFCE or open­box. Or, if you are on uni­ty and in­stall sni-qt, a Ubun­tu One app in­di­ca­tor!

http://ubuntuone.com/7iXTbysoMM9PIUS9Ai4TNn

My Ubun­tu One in­di­ca­tor in ac­tion.

You can find the source code for the whole ex­am­ple app at my u1-­toys project in launch­pad and here is the full source code (miss­ing some icon re­sources, just get the re­po)

Com­ing soon(ish), more ex­am­ple app­s, and cool things to do with our APIs!

PyQt Quickie: command line parsing

So, you are writ­ing a PyQt ap­p, and you want it to sup­port com­mand line ar­gu­ments. So you do some­thing like this:

opt_parser = OptionParser()
opt_parser.add_option("-q", dest="quickly", action="store_true",
    help="Do it quickly (default=False)")
(options, args) = opt_parser.parse_args(sys.argv)
app = QApplication(sys.argv)
:
:
:

Or maybe even QAp­pli­ca­tion([]). Ok, you are doing it wrong. And this is wrong in most tutorials, too. Why? Because Qt (and thus PyQt) supports a bunch of useful command line options already. So if you do it like in the first listing, and pass "-style=oxygen" or whatever, one of the following will happen.

  1. Op­t­­Pars­er is go­ing to tell you it's not a valid op­­tion and abort

  2. You will ig­nore the op­­tion and not do any­thing use­­ful with it

  3. You will have your own -style op­­tion and do two things with it

All three out­comes are less than ide­al.

The right way to do this is:

opt_parser = OptionParser()
opt_parser.add_option("-q", dest="quickly", action="store_true",
    help="Do it quickly (default=False)")
app = QApplication(sys.argv)
(options, args) = opt_parser.parse_args(app.arguments())
:
:
:

This way, you give PyQt a chance to process the options it recognizes, and, then, you get to handle the rest, because app.arguments() has all Qt options removed.

The bad side of this is, you will make --help slightly slower, since it will have to build a QApplication to do nothing, and you will have undocumented options. Solution for both problems left as an exercise.

To write, and to write what.

Some of you may know I have writ­ten about 30% of a book, called "Python No Muerde", avail­able at http://no­muerde.net­man­ager­s.­com.ar (in span­ish on­ly).That book has stag­nat­ed for a long time.

On the oth­er hand, I wrote a very pop­u­lar se­ries of post­s, called PyQt by Ex­am­ple, which has (y­ou guessed it) stag­nat­ed for a long time.

The main prob­lem with the book was that I tried to cov­er way too much ground. When com­plete, it would be a 500 page book, and that would in­volve writ­ing half a dozen ex­am­ple app­s, some of them in ar­eas I am no ex­pert.

The main prob­lem with the post se­ries is that the ex­am­ple is lame (a TO­DO ap­p!) and ex­pand­ing it is bor­ing.

¡So, what bet­ter way to fix both things at on­ce, than to merge them!

I will leave Python No Muerde as it is, and will do a new book, called PyQt No Muerde. It will keep the tone and lan­guage of Python No Muerde, and will even share some chap­ter­s, but will fo­cus on de­vel­op­ing a PyQt app or two, in­stead of the much more am­bi­tious goals of Python No Muerde. It will be about 200 pages.

I have ac­quired per­mis­sion from my su­pe­ri­ors (my wife) to work on this project a cou­ple of hours a day, in the ear­ly morn­ing. So, it may move for­ward, or it may not. This is, as usu­al, an ex­per­i­men­t, not a prom­ise.