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  3. Developer Guide (Python SDK)
  4. Object Download
  5. Performing a Conditioned Download

Performing a Conditioned Download

When downloading an object, you can specify one or more conditions. Only when the conditions are met, the object will be downloaded. Otherwise, an error code will be returned and the download will fail.

You can set the following conditions:

Parameter

Description

Format

GetObjectHeader.if_modified_since

Returns the object if it is modified after the time specified by this parameter; otherwise, an error code is returned.

This parameter must conform to the HTTP time format specified in http://www.ietf.org/rfc/rfc2616.txt.

GetObjectHeader.if_unmodified_since

Returns the object if it remains unchanged since the time specified by this parameter; otherwise, an error code is returned.

This parameter must conform to the HTTP time format specified in http://www.ietf.org/rfc/rfc2616.txt.

GetObjectHeader.if_match

Returns the source object if its ETag is the same as the one specified by this parameter; otherwise, an error code is returned.

Character string

GetObjectHeader.if_none_match

Returns the source object if its ETag is different from the one specified by this parameter; otherwise, an error code is returned.

Character string

NOTE:
  • The ETag of an object is the MD5 check value of the object.
  • If a request includes if_unmodified_since or if_match and the specified condition is not met, the object download will fail with error code 412 Precondition Failed.
  • If a request includes if_modified_since or if_none_match and the specified condition is not met, the object download will fail with error code 304 Not Modified returned.

Sample code:

# Import the module.
from com.obs.client.obs_client import ObsClient
  
# Create an instance of ObsClient.
obsClient = ObsClient(
    access_key_id='*** Provide your Access Key ***',    
    secret_access_key='*** Provide your Secret Key ***',    
    server='yourdomainname'
)
from com.obs.models.get_object_header import GetObjectHeader
headers = GetObjectHeader()
headers.if_modified_since = 'Thu, 31 Dec 2015 16:00:00 GMT'

resp = obsClient.getObject('bucketname', 'objectkey', loadStreamInMemory=True, headers=headers)
if resp.status < 300:
    print('requestId:', resp.requestId)
    # Obtain the object content.
    print('buffer:', resp.body.buffer)
else:
    print('errorCode:', resp.errorCode)
    print('errorMessage:', resp.errorMessage)