[PyQt] QFileSystemWatcher triggering too many signals

[Giuseppe Corbelli]

On 21/01/2013 08:49, Lars Beiderbecke wrote:
> Hello,
>
> I'm trying to watch a file for changes using QFileSystemWatcher:
>
> class MainWindow(QtGui.QMainWindow):
>
>      def __init__(self,parent,args=None):
>          QtGui.QMainWindow.__init__(self)
>          self.count = 0
>          self.watcher = QtCore.QFileSystemWatcher();
>          self.watcher.addPath("myfile");
>          self.connect(self.watcher, QtCore.SIGNAL("fileChanged(const
> QString&)"), self.check)
>
>      def check(self, file = "*"):
>          self.count += 1
>          print "Modified:", str(file), " #", self.count
>
> This works, more or less, as for every
>
> $ touch myfile
>
> I get exactly one invocation of check().  If the changes are larger,
> however, I get an excessive amount of invocations.  Some examples:
>
> $ dd if=/dev/urandom of=sample bs=1k count=100
> $ cp sample myfile
> ->  4 invocations
> $ vi myfile  # replacing first char, then :wq
> ->  21 invocations
> $ joe myfile  # ditto
> ->  11 invocations
> $ touch myfile
> ->  1 invocations
> $ cat sample>  watched
> ->  3 invocations
> $ cat sample sample>  watched
> ->  6 invocations
>
> Now I realize that editors may do weird things to files, but cp and
> cat? Is this behavior considered normal?

No answer on top of my mind, but I'd run all the commands (cp, vi, jo, cat) 
under strace and see all the related syscalls. Won't be surprised if the 
*notify system has some hooks related to common syscalls.

-- 
             Giuseppe Corbelli
WASP Software Engineer, Copan Italia S.p.A
Phone: +390303666318  Fax: +390302659932
E-mail: giuseppe.corbelli at copanitalia.com