watcher.py

"""
watcher.py
A nifty little script for responsive coding ala Brackets, but in python

It watches a directory for file changes, then executes an aribtrary python script.
Imports and large data loads can be cached at setup so that the remaining code can execute very fast.

See more about intended use case here:
http://stackoverflow.com/questions/32997698/auto-run-a-python-script-without-reloading-dependencies

The imported module must have methods for setup() and update().
Setup can return a dictionary object that will be used to populate arguments for update.

Ex:

def setup():
	foo = set_up_some_stuff()
	return {
		'foo' : foo,
		'bar' = "bar, naturally",
	}

def update(foo, bar):
	print foo, bar


"""

import sys
import datetime
import time
import traceback
import json
import argh
import importlib

from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler

class MyHandler(FileSystemEventHandler):

    def __init__(self, module, module_data):
        super(MyHandler, self).__init__()

        self.module = module
        self.module_data = module_data

    def on_modified(self, event):
        now = datetime.datetime.now()
        if hasattr(self, 'last_ts'):
            if (now - self.last_ts).total_seconds() < 1:
                # print "too soon, doing nothing"
                return
        
        print "Handling a new event..."
        self.last_ts = now

        try:
            reload(self.module)
            self.module.update(**self.module_data)

        except Exception, err:
            print "===== Ran into an error ====="
            print(traceback.format_exc())


@argh.arg('module_name')
@argh.arg('-p', '--file_path', default='.')
@argh.arg('-c', '--config_file', default=None)
def main(module_name, **kwargs):
    print 'Launching watcher...'

    if kwargs['config_file']:
        config = json.load(file(kwargs['config_file']))
    else:
        config = {}

    print 'Importing and setting up module', module_name
    module = importlib.import_module(module_name)
    module_data = module.setup(**config)

    print 'Launching event_handler and observer to watch for file changes...'
    event_handler = MyHandler(module, module_data)

    observer = Observer()
    observer.schedule(event_handler, kwargs['file_path'])
    observer.start()

    try:
        while True:
            time.sleep(1)
    except KeyboardInterrupt:
        observer.stop()

    observer.join()

argh.dispatch_command(main)