How do I watch a file for changes?

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...

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...

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)

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 ):
    print "Text added: %s"%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,
  win32con.FILE_SHARE_READ | win32con.FILE_SHARE_WRITE,
  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
a.read()

# Wait for new data and call ProcessNewData for each new chunk that's written
while 1:
  # Wait for a change to occur
  results = win32file.ReadDirectoryChangesW (
    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:
        newText = a.read()
        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!

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

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

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.

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.

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.

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.

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)

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

repyt ./app.py

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...
                file_to_read = open(file, 'r')
                value = file_to_read.read()
                print("value from file", value)
                file_to_read.seek(0)
                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:
                # Action on file not found
                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

import inotify.adapters
from datetime import datetime


LOG_FILE='/var/log/mysql/server_audit.log'


def main():
    start_time = datetime.now()
    while True:
        i = inotify.adapters.Inotify()
        i.add_watch(LOG_FILE)
        for event in i.event_gen(yield_nones=False):
            break
        del i

        with open(LOG_FILE, 'r') as f:
            for line in f:
                entry = line.split(',')
                entry_time = datetime.strptime(entry[0],
                                               '%Y%m%d %H:%M:%S')
                if entry_time > start_time:
                    start_time = entry_time
                    print(entry)


if __name__ == '__main__':
    main()

The easiest solution would get the two instances of the same file after an interval and Compare them. You Could try something like this

    while True:
        # Capturing the two instances models.py after certain interval of time
        print("Looking for changes in " + app_name.capitalize() + " models.py\nPress 'CTRL + C' to stop the program")
        with open(app_name.capitalize() + '/filename', 'r+') as app_models_file:
            filename_content = app_models_file.read()
        time.sleep(5)
        with open(app_name.capitalize() + '/filename', 'r+') as app_models_file_1:
            filename_content_1 = app_models_file_1.read()
        # Comparing models.py after certain interval of time
        if filename_content == filename_content_1:
            pass
        else:
            print("You made a change in " + app_name.capitalize() + " filename.\n")
            cmd = str(input("Do something with the file?(y/n):"))
            if cmd == 'y':
                # Do Something
            elif cmd == 'n':
                # pass or do something
            else:
                print("Invalid input")

Just to put this out there since no one mentioned it: there's a Python module in the Standard Library named filecmp which has this cmp() function that compares two files.

Just make sure you don't do from filecmp import cmp to not overshadow the built-in cmp() function in Python 2.x. That's okay in Python 3.x, though, since there's no such built-in cmp() function anymore.

Anyway, this is how its use looks like:

import filecmp
filecmp.cmp(path_to_file_1, path_to_file_2, shallow=True)

The argument shallow defaults to True. If the argument's value is True, then only the metadata of the files are compared; however, if the argument's value is False, then the contents of the files are compared.

Maybe this information will be useful to someone.

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.

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.

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:
            lines = f.readlines()
            while True:
                    line = f.readline()
                    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.