Read a file line by line from S3 using boto?

问题:

I have a csv file in S3 and I'm trying to read the header line to get the size (these files are created by our users so they could be almost any size). Is there a way to do this using boto? I thought maybe I could us a python BufferedReader, but I can't figure out how to open a stream from an S3 key. Any suggestions would be great. Thanks!

回答1:

It appears that boto has a read() function that can do this. Here's some code that works for me:

>>> import boto
>>> from boto.s3.key import Key
>>> conn = boto.connect_s3('ap-southeast-2')
>>> bucket = conn.get_bucket('bucket-name')
>>> k = Key(bucket)
>>> k.key = 'filename.txt'
>>> k.open()
>>> k.read(10)
'This text '

The call to read(n) returns the next n bytes from the object.

Of course, this won't automatically return "the header line", but you could call it with a large enough number to return the header line at a minimum.

回答2:

You may find https://pypi.python.org/pypi/smart_open useful for your task.

From documentation:

for line in smart_open.smart_open('s3://mybucket/mykey.txt'):
    print line

回答3:

Here's a solution which actually streams the data line by line:

from io import TextIOWrapper
from gzip import GzipFile
...

# get StreamingBody from botocore.response
response = s3.get_object(Bucket=bucket, Key=key)
# if gzipped
gzipped = GzipFile(None, 'rb', fileobj=response['Body'])
data = TextIOWrapper(gzipped)

for line in data:
    # process line

回答4:

With boto3 you can access a raw stream and read line by line. Just note raw stream is a private property for some reason

s3 = boto3.resource('s3', aws_access_key_id='xxx', aws_secret_access_key='xxx')
obj = s3.Object('bucket name', 'file key')

obj.get()['Body']._raw_stream.readline() # line 1
obj.get()['Body']._raw_stream.readline() # line 2
obj.get()['Body']._raw_stream.readline() # line 3...

回答5:

Using boto3:

s3 = boto3.resource('s3')
obj = s3.Object(BUCKET, key)
for line in obj.get()['Body']._raw_stream:
    # do something with line

回答6:

If you want to read multiple files (line by line) with a specific bucket prefix (i.e., in a "subfolder") you can do this:

s3 = boto3.resource('s3', aws_access_key_id='<key_id>', aws_secret_access_key='<access_key>')

    bucket = s3.Bucket('<bucket_name>')
    for obj in bucket.objects.filter(Prefix='<your prefix>'):
        for line in obj.get()['Body'].read().splitlines():
            print(line.decode('utf-8'))

Here lines are bytes so I am decoding them; but if they are already a string, you can skip that.

回答7:

The most dynamic and low cost way to read the file is to read each byte until you find the number of lines you need.

line_count = 0
line_data_bytes = b''

while line_count < 2 :

    incoming = correlate_file_obj['Body'].read(1)
    if incoming == b'\n':
        line_count = line_count + 1

    line_data_bytes = line_data_bytes + incoming

logger.debug("read bytes:")
logger.debug(line_data_bytes)

line_data = line_data_bytes.split(b'\n')

You won't need to guess about header size if the header size can change, you won't end up downloading the whole file, and you don't need 3rd party tools. Granted you need to make sure the line delimeter in your file is correct and you are reading the right number of bytes to find it.