How can you know when a file has been modified within a running Python?

Question

I have a Python/PyQt application that loads many file references into memory and informs the user of information (text) within each file. It even allow the user to open an editor (vim in my case) using Popen. I would like the program, which lists all the files it opened in a QTreeView, to mark a checkbox in a column with the mtime if newer then the start of the application. I know how to get the mtime of a file. What I want to do is know if the file has been updated (modified - Windows OS) and change the checkbox state to reflect the fact that the file is now modified (dirty). It seems to me that I could do this by running a separate Thread (or QDialog with nothing displayed?) But this would require a process to continually be doing a getmtime on a list of files and communicating with the MainWindow. It could be that the polling loop has some sleep in it so as to not take up too many cycles, but this presents other issues. I would really like it for the OS to emit a file_change, but I do not think that is a likely solution. Any thoughts?

Answer 1

There's watchdog, which monitors for filesystem changes on a few platforms, including windows.

If you want to do it yourself, the mechanism in use here, for the non-polling monitoring on windows, is the ReadDirectoryChangesW API function

Alternatively, there's the FindFirstChangeNotification API call, which is a bit less complicated for some use cases.

Here is a good write up about using all of these routines on windows, in python.

Answer 2

The suggestion to use FindFirstChange from otherchirps sent me in the direction I was looking for. My end answer was to use QFileSystemWatcher from PyQt4. This provided the features I wanted and closer integration with Qt. The following is an example of how to watch files, and directories using QFileSystemWatcher. Now I can extend the paths/files to check and ask which ones are being monitored. This lacks a lot of error checking, but it is a stub to start work from.

import sys
from PyQt4 import QtCore
from PyQt4 import QtGui
from os import curdir

class MainWindow(QtGui.QMainWindow):
    path_Sig = QtCore.SIGNAL('pathsAdd()')
    def __init__(self):
        super(MainWindow, self).__init__()
        self.path = ['./test.dat', 'local1.bdf', 'Sub/sub1.bdf']
        button = QtGui.QPushButton('Paths')
        self.label = QtGui.QLabel(curdir)
        widget = QtGui.QWidget()
        layout = QtGui.QVBoxLayout(widget)
        layout.addWidget(button)
        layout.addWidget(self.label)
        self.setCentralWidget(widget)
        self.connect(button, QtCore.SIGNAL('clicked()'), self.setPaths)

    @QtCore.pyqtSlot(list)
    def setPaths(self):
        self.emit(QtCore.SIGNAL('path_Sig'), self.path)
        self.myPaths(self.path)
    @QtCore.pyqtSlot(list)
    def myPaths(self, path_list):
        self.label.setText(str().join(path_list))
    @QtCore.pyqtSlot("QString")
    def dirChanged(self, results):
        self.label.setText("Dir  {} changed".format(results))
    @QtCore.pyqtSlot("QString")
    def fileChanged(self, results):
        self.label.setText("File {} changed".format(results))

class FileWatcher(QtCore.QFileSystemWatcher):
    these_Sig = QtCore.SIGNAL('myPaths()')
    def __init__(self, paths=None):
        super(FileWatcher, self).__init__()
        try:
            self.addPaths(paths)
        except:
            self.addPath(curdir)

    @QtCore.pyqtSlot()
    def filesReport(self):
        self.emit(QtCore.SIGNAL('these_Sig'), self.files())
    @QtCore.pyqtSlot(list)
    def pathsAdd(self, path_list):
        self.addPaths(path_list)
        self.filesReport()

 if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    app.setApplicationName('FileWatcherDemo')
    window = MainWindow()
    window.show()
    filewatcher = FileWatcher()
    QtCore.QObject.connect(filewatcher, QtCore.SIGNAL('directoryChanged(QString)'), window, QtCore.SLOT('dirChanged(QString)'))
    QtCore.QObject.connect(filewatcher, QtCore.SIGNAL('fileChanged(QString)'), window, QtCore.SLOT('fileChanged(QString)'))
    QtCore.QObject.connect(filewatcher, QtCore.SIGNAL('these_Sig'), window.myPaths)
    QtCore.QObject.connect(window, QtCore.SIGNAL('path_Sig'), filewatcher.pathsAdd)
    sys.exit(app.exec_())