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How to use Wait functionality to check whether a key in a S3 bucket exists, using Boto3 and AWS Client?
When a user wants to use wait functionality to validate whether a key in a bucket exists or not in programming code.
Problem Statement − Use boto3 library in Python to check whether a key exists in a bucket, using waiters functionality. For example, use waiters to check whether a key test.zip exists in Bucket_1.
Approach/Algorithm to solve this problem
Step 1 − Import boto3 and botocore exceptions to handle exceptions.
Step 2 − bucket_name and key are two parameters in function.
Step 3 − Create an AWS session using boto3 library.
Step 4 − Create an AWS client for S3.
Step 5 − Now create the wait object for object_exists using get_waiter function
Step 6 − Now, use the wait object to validate whether key exists or not in a given bucket. By default, it checks in every 5 seconds until a successful state is reached. An error is returned after 20 failed checks. However, the user can define polling time and max attempts.
Step 7 − It returns None.
Step 8 − Handle the generic exception if something went wrong while checking the bucket.
Example
Use the following code to use waiter to check whether a key exists in a bucket or not −
import boto3
from botocore.exceptions import ClientError
def use_waiters_check_object_exists(bucket_name, key_name):
session = boto3.session.Session()
s3_client = session.client('s3')
try:
waiter = s3_client.get_waiter('object_exists')
waiter.wait(Bucket=bucket_name, Key = key_name,
WaiterConfig={
'Delay': 2, 'MaxAttempts': 5})
print('Object exists: ' + bucket_name +'/'+key_name)
except ClientError as e:
raise Exception( "boto3 client error in use_waiters_check_object_exists: " + e.__str__())
except Exception as e:
raise Exception( "Unexpected error in use_waiters_check_object_exists: " + e.__str__())
print(use_waiters_check_object_exists("Bucket_1","testfolder/test.zip"))
print(use_waiters_check_object_exists("Bucket_1","testfolder/test1.zip")
)Output
Object exists: Bucket_1/testfolder/test.zip None botocore.exceptions.WaiterError: Waiter ObjectExists failed: Max attempts exceeded "Unexpected error in use_waiters_check_object_exists: " + e.__str__()) Exception: Unexpected error in use_waiters_check_object_exists: Waiter ObjectExists failed: Max attempts exceed
For Bucket_1/testfolder/test.zip, the output is print statement and None. Since the response doesn’t return anything, it prints None.
For Bucket_1/testfolder/test1.zip, the output is an exception, as this object doesn’t exist.
In exception, it can be read that Max attempts exceeded.
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