Streaming download using OSS SDK for Python 1.0-Object Storage Service(OSS)-阿里云帮助中心

Streaming download retrieves an object in chunks rather than loading it entirely into memory. Use it for large objects or when you need to pipe the data directly to a file or another object.

All three examples below use bucket.get_object(), which returns a file-like object that is also iterable. The destination determines which pattern to apply: read into memory, write to a local file, or copy to another object in the same bucket.

Prerequisites

Before you begin, make sure you have:

An OSS bucket containing the object to download
The oss:GetObject permission on that bucket. For setup instructions, see Attach a custom policy to a RAM user
The OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables set with your AccessKey credentials

Usage notes

The examples use the public endpoint for the China (Hangzhou) region (https://oss-cn-hangzhou.aliyuncs.com). If you access OSS from another Alibaba Cloud service in the same region, use the internal endpoint instead. See Regions and endpoints.
To create an OSSClient instance using a custom domain name or Security Token Service (STS), see Initialization.

Read into memory

Call object_stream.read() to load the full object content into memory. After reading, verify data integrity by comparing the client-side and server-side cyclic redundancy check (CRC) values.

This pattern loads the entire object into memory at once. For large objects, use the "Save to a local file" pattern below instead.

# -*- coding: utf-8 -*-
import oss2
from oss2.credentials import EnvironmentVariableCredentialsProvider

# Load credentials from environment variables (OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET)
auth = oss2.ProviderAuthV4(EnvironmentVariableCredentialsProvider())

# Replace the endpoint with the one for your bucket's region
endpoint = "https://oss-cn-hangzhou.aliyuncs.com"

# The region parameter is required for V4 signatures
region = "cn-hangzhou"

bucket = oss2.Bucket(auth, endpoint, "yourBucketName", region=region)

# get_object returns a file-like object; read() loads the full content into memory
# The full path must not include the bucket name
object_stream = bucket.get_object('exampleobject.txt')
print(object_stream.read())

# read() triggers CRC calculation -- always verify after calling it
if object_stream.client_crc != object_stream.server_crc:
    print("The CRC checksum between client and server is inconsistent!")

Save to a local file

Use shutil.copyfileobj() to write the stream directly to a local file without loading it into memory. This is the preferred pattern for large objects.

import shutil
import oss2
from oss2.credentials import EnvironmentVariableCredentialsProvider

# Load credentials from environment variables (OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET)
auth = oss2.ProviderAuthV4(EnvironmentVariableCredentialsProvider())

# Replace the endpoint with the one for your bucket's region
endpoint = "https://oss-cn-hangzhou.aliyuncs.com"

# The region parameter is required for V4 signatures
region = "cn-hangzhou"

bucket = oss2.Bucket(auth, endpoint, "yourBucketName", region=region)

# The full path must not include the bucket name
object_stream = bucket.get_object('exampleobject.txt')

# shutil.copyfileobj streams data chunk by chunk -- no full in-memory load
# If the local file exists, it is overwritten; if not, it is created
# If no path is specified, the file is saved to the working directory of the project
with open('D:\\localpath\\examplefile.txt', 'wb') as local_fileobj:
    shutil.copyfileobj(object_stream, local_fileobj)

Copy to another object

Because get_object() returns an iterable, pass it directly to put_object() to stream the data into a new object in the same bucket.

import oss2
from oss2.credentials import EnvironmentVariableCredentialsProvider

# Load credentials from environment variables (OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET)
auth = oss2.ProviderAuthV4(EnvironmentVariableCredentialsProvider())

# Replace the endpoint with the one for your bucket's region
endpoint = "https://oss-cn-hangzhou.aliyuncs.com"

# The region parameter is required for V4 signatures
region = "cn-hangzhou"

bucket = oss2.Bucket(auth, endpoint, "yourBucketName", region=region)

# The full path must not include the bucket name
object_stream = bucket.get_object('exampleobject.txt')

# Pass the iterable stream directly to put_object -- no intermediate file needed
bucket.put_object('exampleobjectnew.txt', object_stream)

What's next

For the complete sample code, see the GitHub example.
For the underlying API details, see GetObject.