python - What is the difference between the bucket object LastModified date and the bucket object metadata LastModified date?

Question

I'm accessing bucket objects to get a subset of files and their last modified date to see if there's been any updates since files were last ingested into our system. I noticed a discrepancy in how we are accessing the modified date and how I think it should be done, but I don't know which date is correct or what each date actually represent.

I've created a dummy example below to illustrate the two methods I'm getting dates and where they are different. The first one uses list_objects_v2 and accesses the value in data['Contents']['LastModified']. The second method uses head_object and accesses the value in data['Metadata']['last-modified']. For the sake of example, assume they are both the same object foo/baz.

import boto3

# Method 1 - using list_objects_v2
client = boto3.client('s3', region_name='us-west-2')
paginator = client.get_paginator('list_objects_v2')
operation_parameters = {'Bucket': 'my-bucket',
                        'Prefix': 'foo/baz'}
page_iterator = paginator.paginate(**operation_parameters)
filtered_iterator = page_iterator.search("Contents[?Size > `100`][]")
for key_data in filtered_iterator:
    print(key_data['LastModified'])

• This will return a date like 2020-03-01

import boto3

# Method 2 - using head_object
client = boto3.client('s3', region_name='us-west-2')
obj_head = client.head_object(Bucket='my-bucket', Key='foo/baz')
print(obj_head['Metadata']['last-modified'])

• This will return a different date like 2020-02-01

import boto3

# Bonus method 3 - using head_object BUT not using 'Metadata'
client = boto3.client('s3', region_name='us-west-2')
obj_head = client.head_object(Bucket='my-bucket', Key='foo/baz')
print(obj_head['LastModified'])

• This will return the same date as Method 1 using list_objects_v2

What is the cause of the discrepancy? What does each date represent? What is the correct way to access the timestamp an object was last modified?

question from:https://stackoverflow.com/questions/65910164/what-is-the-difference-between-the-bucket-object-lastmodified-date-and-the-bucke

Answer 1

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