Signed POST request

Creating form fields for a signed Amazon S3 POST request

import boto3
from flask import current_app
from pontus import AmazonS3SignedRequest

session = boto3.session.Session(
    aws_access_key_id=current_app.config.get('AWS_ACCESS_KEY_ID'),
    aws_secret_access_key=current_app.config.get('AWS_SECRET_ACCESS_KEY'),
    region_name=current_app.config.get('AWS_REGION_NAME')
)
bucket = session.resource('s3').Bucket('testbucket')

signed_request = AmazonS3SignedRequest(
    key_name=u'my/file.jpg',
    mime_type=u'image/jpeg',
    bucket=bucket,
    session=session
)

signed_request.form_fields

# {
#     'x-amz-algorithm': 'AWS4-HMAC-SHA256',
#     'x-amz-credential': 'your-aws-access-key-id/date/region-name/s3/aws4_request',
#     'x-amz-date': 'date',
#     'x-amz-signature': 'generated-signature',
#     'success_action_status': '201',
#     'acl': 'public-read',
#     'Content-Type': 'image/png',
#     'key': u'f6c157e1-1a1a-4418-99fe-3362dcf7b1ea/images/my-image.jpg',
#     'policy': 'generated-policy-document'
# }

These form fields can be used to POST files to Amazon S3 as described in Amazon’s documentation.

Amazon S3 file validation

Validating that an image file is less than 2MB and is of image/jpeg MIME type.

import boto3
from flask import current_app
from pontus import AmazonS3FileValidator
from pontus.validators import FileSize, MimeType

session = boto3.session.Session(
    aws_access_key_id=current_app.config.get('AWS_ACCESS_KEY_ID'),
    aws_secret_access_key=current_app.config.get('AWS_SECRET_ACCESS_KEY')
)
bucket = session.resource('s3').Bucket('testbucket')

validator = AmazonS3FileValidator(
    key_name='images/my-image.jpg',
    bucket=bucket,
    validators=[FileSize(max=2097152), MimeType('image/jpeg')],
    session=session
)

if validator.validate():
    # File is <2MB image/jpeg
    pass
else:
    # File was invalid, printing errors
    print validator.errors

Validators can either be instances of a class inheriting pontus.validators.BaseValidator or callable functions that take one parameter obj, which is a boto.S3.Object instance.

from pontus.exceptions import ValidationError
from pontus.validators import BaseValidator

def name_starts_with_images(obj):
    if not obj.key.startswith('images/'):
        raise ValidationError()

# OR

class NameStartsWith(BaseValidator):
    def __init__(self, starts_with_str):
        self.starts_with_str = starts_with_str

    def __call__(self, obj):
        if not obj.key.startswith(starts_with_str):
            raise ValidationError()

name_starts_with_images = NameStartsWith('images/')