Automatic Restart

So far, our program freezes until the download stops. However, when the program ends we are not sure whether the file is stopped because it is completely downloaded or because an error occured. What if an error occured and we want to restart the download again? This is easy. We just pass restart_wait argument to Downloader.__init__():

dl = bitpit.Downloader(url, restart_wait=30)

This argument decides the time to wait before the downloader retries downloading when an error occures. It defaults to -1 if not given which means do not restart even after an error. Because we gave it the value 30 here, anytime an error happens, the downloader will wait for 30 seconds and then retry again. Try to download linux mint and shutdown your internet connection. Here is the output I got:

The file size is 1899528192
The speed is 0 B/s
The state changed to: start
The speed is 207.0622560278128 KB/s
The speed is 474.6406851817469 KB/s
The speed is 0 B/s
The state changed to: error
The file size is 1899528192
The speed is 0 B/s
The state changed to: start
The speed is 506.2438224533826 KB/s
The speed is 594.6743846283302 KB/s

You can see the state has changed to error after I shutdown my internet but the program did not terminate. After 30 seconds, the state changed again to start and the download continued. Now our program will only terminate when the download is successfully completed.

One last note, some connection errors are perminant. For instance, if you get a 404 NOT FOUND error, then no matter how many times you try, the error will keep happening. bitpit does not handle that and will keep trying to download regardless of the error. You can check the error that happened by looking at the Downloader.last_exception property. You will most probably get an exception from requests.exceptions module.

We have only changed 1 line in this lesson. Now our program so far has become:

import bitpit

def on_size_changed(downloader):
    print('The file size is', downloader.size)

def on_speed_changed(downloader):
    print('The speed is', *downloader.human_speed)

def on_state_changed(downloader, old_state):
    print('The state changed to:', downloader.state)

#will download this
url = 'https://www.python.org/static/img/python-logo.png'

#this is our downloader
dl = bitpit.Downloader(url, restart_wait=30)

#listen to signals
#print size as soon as it is known
dl.listen('size-changed', on_size_changed)

#print speed periodically
dl.listen('speed-changed', on_speed_changed)

#print state
dl.listen('state-changed', on_state_changed)

#start downloading and tell user download has started.
dl.start()
print('Download has started.')

#end of the main thread

We are getting closer to the end.

In Specify Path and Rate Limit, we will specify the path and name to save our file instead of saving it in the current directory with the default name. We will also start limiting the download speed instead of eating up all our internet bandwidth before my brother gets angry.