# python – How do I watch a file for changes?

## The Question :

337 people think this question is useful

I have a log file being written by another process which I want to watch for changes. Each time a change occurs I’d like to read the new data in to do some processing on it.

What’s the best way to do this? I was hoping there’d be some sort of hook from the PyWin32 library. I’ve found the win32file.FindNextChangeNotification function but have no idea how to ask it to watch a specific file.

If anyone’s done anything like this I’d be really grateful to hear how…

 I should have mentioned that I was after a solution that doesn’t require polling.

 Curses! It seems this doesn’t work over a mapped network drive. I’m guessing windows doesn’t ‘hear’ any updates to the file the way it does on a local disk.

• on Linux one could use strace monitoring write calls for this
• @simao’s answer uses python-watchdog. Python-Watchdog has great documentation –> here is a link to the [“QuickStart”] documentation which provides a minimal code example that watches the current working directory.

80 people think this answer is useful

Have you already looked at the documentation available on http://timgolden.me.uk/python/win32_how_do_i/watch_directory_for_changes.html? If you only need it to work under Windows the 2nd example seems to be exactly what you want (if you exchange the path of the directory with one of the files you want to watch).

Otherwise, polling will probably be the only really platform-independent option.

Note: I haven’t tried any of these solutions.

294 people think this answer is useful

Did you try using Watchdog?

Python API library and shell utilities to monitor file system events.

### Directory monitoring made easy with

• A cross-platform API.
• A shell tool to run commands in response to directory changes.

Get started quickly with a simple example in Quickstart

101 people think this answer is useful

If polling is good enough for you, I’d just watch if the “modified time” file stat changes. To read it:

os.stat(filename).st_mtime



(Also note that the Windows native change event solution does not work in all circumstances, e.g. on network drives.)

import os

class Monkey(object):
def __init__(self):
self._cached_stamp = 0
self.filename = '/path/to/file'

def ook(self):
stamp = os.stat(self.filename).st_mtime
if stamp != self._cached_stamp:
self._cached_stamp = stamp
# File has changed, so do something...



49 people think this answer is useful

If you want a multiplatform solution, then check QFileSystemWatcher. Here an example code (not sanitized):

from PyQt4 import QtCore

@QtCore.pyqtSlot(str)
def directory_changed(path):
print('Directory Changed!!!')

@QtCore.pyqtSlot(str)
def file_changed(path):
print('File Changed!!!')

fs_watcher = QtCore.QFileSystemWatcher(['/path/to/files_1', '/path/to/files_2', '/path/to/files_3'])

fs_watcher.connect(fs_watcher, QtCore.SIGNAL('directoryChanged(QString)'), directory_changed)
fs_watcher.connect(fs_watcher, QtCore.SIGNAL('fileChanged(QString)'), file_changed)



29 people think this answer is useful

It should not work on windows (maybe with cygwin ?), but for unix user, you should use the “fcntl” system call. Here is an example in Python. It’s mostly the same code if you need to write it in C (same function names)

import time
import fcntl
import os
import signal

FNAME = "/HOME/TOTO/FILETOWATCH"

def handler(signum, frame):
print "File %s modified" % (FNAME,)

signal.signal(signal.SIGIO, handler)
fd = os.open(FNAME,  os.O_RDONLY)
fcntl.fcntl(fd, fcntl.F_SETSIG, 0)
fcntl.fcntl(fd, fcntl.F_NOTIFY,
fcntl.DN_MODIFY | fcntl.DN_CREATE | fcntl.DN_MULTISHOT)

while True:
time.sleep(10000)



20 people think this answer is useful

Check out pyinotify.

inotify replaces dnotify (from an earlier answer) in newer linuxes and allows file-level rather than directory-level monitoring.

13 people think this answer is useful

Well after a bit of hacking of Tim Golden’s script, I have the following which seems to work quite well:

import os

import win32file
import win32con

path_to_watch = "." # look at the current directory
file_to_watch = "test.txt" # look for changes to a file called test.txt

def ProcessNewData( newData ):

# Set up the bits we'll need for output
ACTIONS = {
1 : "Created",
2 : "Deleted",
3 : "Updated",
4 : "Renamed from something",
5 : "Renamed to something"
}
FILE_LIST_DIRECTORY = 0x0001
hDir = win32file.CreateFile (
path_to_watch,
FILE_LIST_DIRECTORY,
None,
win32con.OPEN_EXISTING,
win32con.FILE_FLAG_BACKUP_SEMANTICS,
None
)

# Open the file we're interested in
a = open(file_to_watch, "r")

# Throw away any exising log data

# Wait for new data and call ProcessNewData for each new chunk that's written
while 1:
# Wait for a change to occur
hDir,
1024,
False,
win32con.FILE_NOTIFY_CHANGE_LAST_WRITE,
None,
None
)

# For each change, check to see if it's updating the file we're interested in
for action, file in results:
full_filename = os.path.join (path_to_watch, file)
#print file, ACTIONS.get (action, "Unknown")
if file == file_to_watch:
if newText != "":
ProcessNewData( newText )



It could probably do with a load more error checking, but for simply watching a log file and doing some processing on it before spitting it out to the screen, this works well.

Thanks everyone for your input – great stuff!

12 people think this answer is useful

For watching a single file with polling, and minimal dependencies, here is a fully fleshed-out example, based on answer from Deestan (above):

import os
import sys
import time

class Watcher(object):
running = True
refresh_delay_secs = 1

# Constructor
def __init__(self, watch_file, call_func_on_change=None, *args, **kwargs):
self._cached_stamp = 0
self.filename = watch_file
self.call_func_on_change = call_func_on_change
self.args = args
self.kwargs = kwargs

# Look for changes
def look(self):
stamp = os.stat(self.filename).st_mtime
if stamp != self._cached_stamp:
self._cached_stamp = stamp
# File has changed, so do something...
print('File changed')
if self.call_func_on_change is not None:
self.call_func_on_change(*self.args, **self.kwargs)

# Keep watching in a loop
def watch(self):
while self.running:
try:
# Look for changes
time.sleep(self.refresh_delay_secs)
self.look()
except KeyboardInterrupt:
print('\nDone')
break
except FileNotFoundError:
pass
except:
print('Unhandled error: %s' % sys.exc_info()[0])

# Call this function each time a change happens
def custom_action(text):
print(text)

watch_file = 'my_file.txt'

# watcher = Watcher(watch_file)  # simple
watcher = Watcher(watch_file, custom_action, text='yes, changed')  # also call custom action function
watcher.watch()  # start the watch going



8 people think this answer is useful

Check my answer to a similar question. You could try the same loop in Python. This page suggests:

import time

while 1:
where = file.tell()
if not line:
time.sleep(1)
file.seek(where)
else:
print line, # already has newline



Also see the question tail() a file with Python.

7 people think this answer is useful

Simplest solution for me is using watchdog’s tool watchmedo

From https://pypi.python.org/pypi/watchdog I now have a process that looks up the sql files in a directory and executes them if necessary.

watchmedo shell-command \
--patterns="*.sql" \
--recursive \
.



6 people think this answer is useful

Well, since you are using Python, you can just open a file and keep reading lines from it.

f = open('file.log')



If the line read is not empty, you process it.

line = f.readline()
if line:
// Do what you want with the line



You may be missing that it is ok to keep calling readline at the EOF. It will just keep returning an empty string in this case. And when something is appended to the log file, the reading will continue from where it stopped, as you need.

If you are looking for a solution that uses events, or a particular library, please specify this in your question. Otherwise, I think this solution is just fine.

6 people think this answer is useful

Here is a simplified version of Kender’s code that appears to do the same trick and does not import the entire file:

# Check file for new data.

import time

f = open(r'c:\temp\test.txt', 'r')

while True:

if not line:
time.sleep(1)
print 'Nothing New'
else:
print 'Call Function: ', line



6 people think this answer is useful

This is another modification of Tim Goldan’s script that runs on unix types and adds a simple watcher for file modification by using a dict (file=>time).

usage: whateverName.py path_to_dir_to_watch

#!/usr/bin/env python

import os, sys, time

def files_to_timestamp(path):
files = [os.path.join(path, f) for f in os.listdir(path)]
return dict ([(f, os.path.getmtime(f)) for f in files])

if __name__ == "__main__":

path_to_watch = sys.argv[1]
print('Watching {}..'.format(path_to_watch))

before = files_to_timestamp(path_to_watch)

while 1:
time.sleep (2)
after = files_to_timestamp(path_to_watch)

added = [f for f in after.keys() if not f in before.keys()]
removed = [f for f in before.keys() if not f in after.keys()]
modified = []

for f in before.keys():
if not f in removed:
if os.path.getmtime(f) != before.get(f):
modified.append(f)

if removed: print('Removed: {}'.format(', '.join(removed)))
if modified: print('Modified: {}'.format(', '.join(modified)))

before = after



4 people think this answer is useful

As you can see in Tim Golden’s article, pointed by Horst Gutmann, WIN32 is relatively complex and watches directories, not a single file.

I’d like to suggest you look into IronPython, which is a .NET python implementation. With IronPython you can use all the .NET functionality – including

System.IO.FileSystemWatcher



Which handles single files with a simple Event interface.

2 people think this answer is useful
ACTIONS = {
1 : "Created",
2 : "Deleted",
3 : "Updated",
4 : "Renamed from something",
5 : "Renamed to something"
}
FILE_LIST_DIRECTORY = 0x0001

def __init__(self, threadID, fileName, directory, origin):
self.fileName = fileName
self.daemon = True
self.dir = directory
self.originalFile = origin
def run(self):
startMonitor(self.fileName, self.dir, self.originalFile)

def startMonitor(fileMonitoring,dirPath,originalFile):
hDir = win32file.CreateFile (
dirPath,
FILE_LIST_DIRECTORY,
None,
win32con.OPEN_EXISTING,
win32con.FILE_FLAG_BACKUP_SEMANTICS,
None
)
# Wait for new data and call ProcessNewData for each new chunk that's
# written
while 1:
# Wait for a change to occur
hDir,
1024,
False,
win32con.FILE_NOTIFY_CHANGE_LAST_WRITE,
None,
None
)
# For each change, check to see if it's updating the file we're
# interested in
for action, file_M in results:
full_filename = os.path.join (dirPath, file_M)
#print file, ACTIONS.get (action, "Unknown")
if len(full_filename) == len(fileMonitoring) and action == 3:
#copy to main file
...



1 people think this answer is useful

This is an example of checking a file for changes. One that may not be the best way of doing it, but it sure is a short way.

Handy tool for restarting application when changes have been made to the source. I made this when playing with pygame so I can see effects take place immediately after file save.

When used in pygame make sure the stuff in the ‘while’ loop is placed in your game loop aka update or whatever. Otherwise your application will get stuck in an infinite loop and you will not see your game updating.

file_size_stored = os.stat('neuron.py').st_size

while True:
try:
file_size_current = os.stat('neuron.py').st_size
if file_size_stored != file_size_current:
restart_program()
except:
pass



In case you wanted the restart code which I found on the web. Here it is. (Not relevant to the question, though it could come in handy)

def restart_program(): #restart application
python = sys.executable
os.execl(python, python, * sys.argv)



Have fun making electrons do what you want them to do.

1 people think this answer is useful

Here’s an example geared toward watching input files that write no more than one line per second but usually a lot less. The goal is to append the last line (most recent write) to the specified output file. I’ve copied this from one of my projects and just deleted all the irrelevant lines. You’ll have to fill in or change the missing symbols.

from PyQt5.QtCore import QFileSystemWatcher, QSettings, QThread
from ui_main_window import Ui_MainWindow   # Qt Creator gen'd

class MainWindow(QMainWindow, Ui_MainWindow):
def __init__(self, parent=None):
QMainWindow.__init__(self, parent)
Ui_MainWindow.__init__(self)
self._fileWatcher = QFileSystemWatcher()
self._fileWatcher.fileChanged.connect(self.fileChanged)

def fileChanged(self, filepath):
QThread.msleep(300)    # Reqd on some machines, give chance for write to complete
# ^^ About to test this, may need more sophisticated solution
with open(filepath) as file:
lastLine = list(file)[-1]
destPath = self._filemap[filepath]['dest file']
with open(destPath, 'a') as out_file:               # a= append
out_file.writelines([lastLine])



Of course, the encompassing QMainWindow class is not strictly required, ie. you can use QFileSystemWatcher alone.

1 people think this answer is useful

Seems that no one has posted fswatch. It is a cross-platform file system watcher. Just install it, run it and follow the prompts.

I’ve used it with python and golang programs and it just works.

0 people think this answer is useful

You can also use a simple library called repyt, here is an example:

repyt ./app.py



0 people think this answer is useful

related @4Oh4 solution a smooth change for a list of files to watch;

import os
import sys
import time

class Watcher(object):
running = True
refresh_delay_secs = 1

# Constructor
def __init__(self, watch_files, call_func_on_change=None, *args, **kwargs):
self._cached_stamp = 0
self._cached_stamp_files = {}
self.filenames = watch_files
self.call_func_on_change = call_func_on_change
self.args = args
self.kwargs = kwargs

# Look for changes
def look(self):
for file in self.filenames:
stamp = os.stat(file).st_mtime
if not file in self._cached_stamp_files:
self._cached_stamp_files[file] = 0
if stamp != self._cached_stamp_files[file]:
self._cached_stamp_files[file] = stamp
# File has changed, so do something...
print("value from file", value)
if self.call_func_on_change is not None:
self.call_func_on_change(*self.args, **self.kwargs)

# Keep watching in a loop
def watch(self):
while self.running:
try:
# Look for changes
time.sleep(self.refresh_delay_secs)
self.look()
except KeyboardInterrupt:
print('\nDone')
break
except FileNotFoundError:
pass
except Exception as e:
print(e)
print('Unhandled error: %s' % sys.exc_info()[0])

# Call this function each time a change happens
def custom_action(text):
print(text)
# pass

watch_files = ['/Users/mexekanez/my_file.txt', '/Users/mexekanez/my_file1.txt']

# watcher = Watcher(watch_file)  # simple

if __name__ == "__main__":
watcher = Watcher(watch_files, custom_action, text='yes, changed')  # also call custom action function
watcher.watch()  # start the watch going



0 people think this answer is useful

The best and simplest solution is to use pygtail: https://pypi.python.org/pypi/pygtail

from pygtail import Pygtail
import sys

while True:
for line in Pygtail("some.log"):
sys.stdout.write(line)



0 people think this answer is useful

If you’re using windows, create this POLL.CMD file

@echo off
:top
xcopy /m /y %1 %2 | find /v "File(s) copied"
timeout /T 1 > nul
goto :top



then you can type “poll dir1 dir2” and it will copy all the files from dir1 to dir2 and check for updates once per second.

The “find” is optional, just to make the console less noisy.

This is not recursive. Maybe you could make it recursive using /e on the xcopy.

0 people think this answer is useful

Since I have it installed globally, my favorite approach is to use nodemon. If your source code is in src, and your entry point is src/app.py, then it’s as easy as:

nodemon -w 'src/**' -e py,html --exec python src/app.py



… where -e py,html lets you control what file types to watch for changes.

-3 people think this answer is useful

I don’t know any Windows specific function. You could try getting the MD5 hash of the file every second/minute/hour (depends on how fast you need it) and compare it to the last hash. When it differs you know the file has been changed and you read out the newest lines.

-6 people think this answer is useful

I’d try something like this.

    try:
f = open(filePath)
except IOError:
print "No such file: %s" % filePath
raw_input("Press Enter to close window")
try:
while True:
try:
if not line:
time.sleep(1)
else:
functionThatAnalisesTheLine(line)
except Exception, e:
# handle the exception somehow (for example, log the trace) and raise the same exception again
raw_input("Press Enter to close window")
raise e
finally:
f.close()



The loop checks if there is a new line(s) since last time file was read – if there is, it’s read and passed to the functionThatAnalisesTheLine function. If not, script waits 1 second and retries the process.