How do I download a file from S3 using boto only if the remote file is newer than a local copy?

Question

I'm trying to download a file from S3 using boto, but only if a local copy of the file is older than the remote file.

I'm using the header 'If-Modified-Since' and the code below:

#!/usr/bin/python
import os
import datetime
import boto
from boto.s3.key import Key

bucket_name = 'my-bucket'

conn = boto.connect_s3()
bucket = conn.get_bucket(bucket_name)

def download(bucket, filename):
    key = Key(bucket, filename)
    headers = {}
    if os.path.isfile(filename):
        print "File exists, adding If-Modified-Since header"
        modified_since = os.path.getmtime(filename)
        timestamp = datetime.datetime.utcfromtimestamp(modified_since)
        headers['If-Modified-Since'] = timestamp.strftime("%a, %d %b %Y %H:%M:%S GMT")
    try:
        key.get_contents_to_filename(filename, headers)
    except boto.exception.S3ResponseError as e:
        return 304
    return 200

print download(bucket, 'README')

The problem is that when the local file does not exist everything works well and the file is downloaded. When I run the script for the second time my function returns 304 as expected, but the file that was previously downloaded is deleted.

Answer 1

boto.s3.key.Key.get_contents_to_filename open file with wb mode; it truncate the file at the beginning of the function (boto/s3/key.py). In addition to that, it removes the file when an exception raised.

Instead of get_contents_to_filename, you can use get_contents_to_file with different open mode.

def download(bucket, filename):
    key = Key(bucket, filename)
    headers = {}
    mode = 'wb'
    updating = False
    if os.path.isfile(filename):
        mode = 'r+b'
        updating = True
        print "File exists, adding If-Modified-Since header"
        modified_since = os.path.getmtime(filename)
        timestamp = datetime.datetime.utcfromtimestamp(modified_since)
        headers['If-Modified-Since'] = timestamp.strftime("%a, %d %b %Y %H:%M:%S GMT")
    try:
        with open(filename, mode) as f:
            key.get_contents_to_file(f, headers)
            f.truncate()
    except boto.exception.S3ResponseError as e:
        if not updating:
            # got an error and we are not updating an existing file
            # delete the file that was created due to mode = 'wb'
            os.remove(filename)
        return e.status
    return 200

NOTE file.truncate is used to handle case where new file is smaller than previous one.