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Python 3.10 is now generally available.

google.appengine.ext.blobstore.fetch_data

Fetches data for a blob.

Fetches a fragment of a blob up to MAX_BLOB_FETCH_SIZE in length. Attempting to fetch a fragment that extends beyond the boundaries of the blob will return the amount of data from start_index until the end of the blob, which will be a smaller size than requested. Requesting a fragment that is entirely outside the boundaries of the blob will return an empty string. Attempting to fetch a negative index will raise an exception.

blob A BlobInfo, BlobKey, string, or Unicode representation of the BlobKey of the blob from which you want to fetch data.
start_index The start index of blob data to fetch. This value must not be negative.
end_index The end index (inclusive) of the blob data to fetch. This value must be greater than or equal to start_index.
rpc Optional UserRPC object.

A string that contains partial data of a blob. If the indexes are legal but outside of the boundaries of the blob, an empty string is returned.

TypeError If start_index or end_index are not indexes, or if blob is not a string, BlobKey or BlobInfo.
DataIndexOutOfRangeError If start_index is set to a value that is less than 0 or end_index is less than start_index.
BlobFetchSizeTooLargeError If the requested blob fragment is larger than MAX_BLOB_FETCH_SIZE.
BlobNotFoundError If the blob does not exist.