python - Track download progress of S3 file using boto3 and callbacks

Question

I am trying to download a text file from S3 using boto3.

Here is what I have written.

class ProgressPercentage(object):
    def __init__(self, filename):
        self._filename = filename
        self._size = float(os.path.getsize(filename))
        self._seen_so_far = 0
        self._lock = threading.Lock()

    def __call__(self, bytes_amount):
        # To simplify we'll assume this is hooked up
        # to a single filename.
        with self._lock:
            self._seen_so_far += bytes_amount
            percentage = round((self._seen_so_far / self._size) * 100,2)
            LoggingFile('{} is the file name. {} out of {} done. The percentage completed is {} %'.format(str(self._filename), str(self._seen_so_far), str(self._size),str(percentage)))
            sys.stdout.flush()

and I am calling it using

transfer.download_file(BUCKET_NAME,FILE_NAME,'{}{}'.format(LOCAL_PATH_TEMP , FILE_NAME),callback = ProgressPercentage(LOCAL_PATH_TEMP + FILE_NAME))

this is giving me a error that file is not present in the folder. Apparently when I already have a file with this name in the same folder it works but when I am downloading a fresh file , it errors out.

What is correction I need to make?

Answer 1

callback = ProgressPercentage(LOCAL_PATH_TEMP + FILE_NAME)) creates a ProgressPercentage object, runs its __init__ method, and passes the object as callback to the download_file method. This means the __init__ method is run before download_file begins.

In the __init__ method you are attempting to read the size of the local file being downloaded to, which throws an exception as the file does not exist since the download has yet to start. If you've already downloaded the file, then there's no problem since a local copy exists and its size can be read.

Of course, this is merely the cause of the exception you're seeing. You're using the _size property as the maximum value of download progress. However you're attempting to use the size of the local file. Until the file is completely downloaded, the local file system does not know how large the file is, it only knows how much space it takes up right now. This means as you download the file will gradually get bigger until it reaches its full size. As such, it doesn't really make sense to consider the size of the local file as the maximum size of the download. It may work in the case where you've already downloaded the file, but that isn't very useful.

The solution to your problem would be to check the size of the file you're going to download, instead of the size of the local copy. This ensures you're getting the actual size of whatever it is you're downloading, and that the file exists (as you couldn't be downloading it if it didn't). You can do this by getting the size of the remote file with head_object as follows

class ProgressPercentage(object):
    def __init__(self, client, bucket, filename):
        # ... everything else the same
        self._size = client.head_object(Bucket=bucket, Key=filename).ContentLength

    # ...

# If you still have the client object you could pass that directly 
# instead of transfer._manager._client
progress = ProgressPercentage(transfer._manager._client, BUCKET_NAME, FILE_NAME)
transfer.download_file(..., callback=progress)

As a final note, although you got the code from the Boto3 documentation, it didn't work because it was intended for file uploads. In that case the local file is the source and its existence guaranteed.