[PyQt] graphical file tail

John Lee jjl at pobox.com
Tue Jun 25 00:25:25 BST 2013

On Mon, 24 Jun 2013, Eric Frederich wrote:

> I'm trying to tail several files graphically.
> I have been trying to find a way to tail several files in a GUI
> without much luck at all.
> I get errors from tail saying broken pipe.
> I get PyQt errors saying underlying C++ objects have been destroyed.
> I get other Qt errors saying that threads are still running when the
> application exits
> etc....
> The implementation posted below seems to suffer from the following errors.
> Not all the time.  It depends.
> QThread: Destroyed while thread is still running
> QWaitCondition::wakeAll(): mutex lock failure:

You're not calling .wait on the threads, and you probably want to give 
e.g. your QObjects a parent where you can, so that Qt manages the lifetime 
of the wrapped C++ objects (e.g. pass in self to the parent arg of the 
QTextBrowser constructor).

But, I recommend doing it a different way: I find event-based code easier 
to think about than threads.  So, if you can use Qt5, you might want to do 
away with the threads and the tail subprocess and replace them with 
QFileSystemWatcher.  Use event-based code instead of the threads (I'm not 
talking about Qt events, I just mean hook up to the signals that that 
class emits and process a little input at a time so as to avoid blocking 
the UI, using QTimer where needed).  I say Qt5 because Qt4 isn't very 
friendly to this way of working since it uses threads in the 
implementation of QFileSystemWatcher.

Caveat: QFileSystemWatcher still has its problems, but the ones discussed 
at the URL below are more convenience issues than fundamental problems: I 
just found I had to experiment a bit to see when the different signals got 


Caveat #2: I imagine QFileSystemWatcher does not support filesystems like 
NFS, at least on Linux kernels with inotify support (but don't take my 
word for it, check the source).


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