How can I use boto to stream a file out of Amazon S3 to Rackspace Cloudfiles?

The Key object in boto, which represents on object in S3, can be used like an iterator so you should be able to do something like this:

>>> import boto
>>> c = boto.connect_s3()
>>> bucket = c.lookup('garnaat_pub')
>>> key = bucket.lookup('Scan1.jpg')
>>> for bytes in key:
...   write bytes to output stream

Or, as in the case of your example, you could do:

>>> shutil.copyfileobj(key, rsObject.stream())

I figure at least some of the people seeing this question will be like me, and will want a way to stream a file from boto line by line (or comma by comma, or any other delimiter). Here's a simple way to do that:

def getS3ResultsAsIterator(self, aws_access_info, key, prefix):        
    s3_conn = S3Connection(**aws_access)
    bucket_obj = s3_conn.get_bucket(key)
    # go through the list of files in the key
    for f in bucket_obj.list(prefix=prefix):
        unfinished_line = ''
        for byte in f:
            byte = unfinished_line + byte
            #split on whatever, or use a regex with re.split()
            lines = byte.split('\n')
            unfinished_line = lines.pop()
            for line in lines:
                yield line

@garnaat's answer above is still great and 100% true. Hopefully mine still helps someone out.

Other answers in this thread are related to boto, but S3.Object is not iterable anymore in boto3. So, the following DOES NOT WORK, it produces an TypeError: 's3.Object' object is not iterable error message:

s3 = boto3.session.Session(profile_name=my_profile).resource('s3')
s3_obj = s3.Object(bucket_name=my_bucket, key=my_key)

with io.FileIO('sample.txt', 'w') as file:
    for i in s3_obj:
        file.write(i)

In boto3, the contents of the object is available at S3.Object.get()['Body'] which is an iterable since version 1.9.68 but previously wasn't. Thus the following will work for the latest versions of boto3 but not earlier ones:

body = s3_obj.get()['Body']
with io.FileIO('sample.txt', 'w') as file:
    for i in body:
        file.write(i)

So, an alternative for older boto3 versions is to use the read method, but this loads the WHOLE S3 object in memory which when dealing with large files is not always a possibility:

body = s3_obj.get()['Body']
with io.FileIO('sample.txt', 'w') as file:
    for i in body.read():
        file.write(i)

But the read method allows to pass in the amt parameter specifying the number of bytes we want to read from the underlying stream. This method can be repeatedly called until the whole stream has been read:

body = s3_obj.get()['Body']
with io.FileIO('sample.txt', 'w') as file:
    while file.write(body.read(amt=512)):
        pass

Digging into botocore.response.StreamingBody code one realizes that the underlying stream is also available, so we could iterate as follows:

body = s3_obj.get()['Body']
with io.FileIO('sample.txt', 'w') as file:
    for b in body._raw_stream:
        file.write(b)

While googling I've also seen some links that could be use, but I haven't tried:

• WrappedStreamingBody
• Another related thread
• An issue in boto3 github to request StreamingBody is a proper stream - which has been closed!!!