Conditional download - Object Storage Service - Alibaba Cloud Documentation Center

You can specify one or more conditions to download objects. If the specified conditions are met, the object is downloaded. If the specified conditions are not met, an error is returned and the object is not downloaded.

Usage notes

In this topic, the public endpoint of the China (Hangzhou) region is used. If you want to access OSS by using other Alibaba Cloud services in the same region as OSS, use an internal endpoint. For more information about the regions and endpoints supported by OSS, see Regions and endpoints.
In this topic, an OSSClient instance is created by using an OSS endpoint. If you want to create an OSSClient instance by using custom domain names or Security Token Service (STS), see Initialization.
To perform conditional download, you must have oss:GetObject permission. For more information, see Attach a custom policy to a RAM user.

Examples

The following sample code provides an example on how to perform conditional download:

import oss2
from oss2.credentials import EnvironmentVariableCredentialsProvider

# Obtain access credentials from environment variables. Before you run the sample code, make sure that the OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables are configured. 
auth = oss2.ProviderAuth(EnvironmentVariableCredentialsProvider())
# Specify the endpoint of the region in which the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the endpoint to https://oss-cn-hangzhou.aliyuncs.com. 
# Specify the name of the bucket. Example: examplebucket. 
bucket = oss2.Bucket(auth, 'https://oss-cn-hangzhou.aliyuncs.com', 'examplebucket')
# Specify the full path of the object. Do not include the bucket name in the full path. Example: exampledir/exampleobject.txt. For more information about the naming conventions for objects, see Object naming conventions. 
object_name = 'exampledir/exampleobject.txt'

headers = dict()
# If the time that is specified in the request is earlier than the actual modified time of the object, the object is downloaded. Otherwise, the error code "304 Not modified" is returned. 
headers['If-Modified-Since'] = 'Mon, 13 Dec 2021 14:47:53 GMT'
# If the time that is specified in the request is the same as or later than the actual modified time of the object, the object is downloaded. Otherwise, the error code 412 Precondition failed is returned. 
# headers['If-Unmodified-Since'] = 'Mon, 13 Dec 2021 14:47:53 GMT'
# If the ETag value that is specified in the request matches the ETag value of the object, the object is downloaded. Otherwise, the error code "412 Precondition failed" is returned. 
# headers['If-Match'] = 'DC21493F505BA3739562D8CC452C****'
# If the ETag value that is specified in the request does not match the ETag value of the object, the object is downloaded. Otherwise, the error code "304 Not modified" is returned. 
# headers['If-None-Match'] = 'DC21493F505BA3739562D8CC452C****'
object_stream = bucket.get_object(object_name, headers=headers)
print(object_stream.read())

References

For more information about the API operation that you can call to perform conditional download, see GetObject.