ptimson ptimson - 6 months ago 450
Python Question

Mocking boto3 S3 client method Python

I'm trying to mock a singluar method from the boto3 s3 client object to throw and exception. But I need all other methods for this class to work as normal.

This is so I can test a singular Exception test when and error occurs performing a upload_part_copy

1st Attempt

import boto3
from mock import patch

with patch('botocore.client.S3.upload_part_copy', side_effect=Exception('Error Uploading')) as mock:
client = boto3.client('s3')
# Should return actual result
o = client.get_object(Bucket='my-bucket', Key='my-key')
# Should return mocked exception
e = client.upload_part_copy()


However this gives the following error:

ImportError: No module named S3


2nd Attempt

After looking at the botocore.client.py source code I found that it is doing something clever and the method
upload_part_copy
does not exist. I found that it seems to call
BaseClient._make_api_call
instead so I tried to mock that

import boto3
from mock import patch

with patch('botocore.client.BaseClient._make_api_call', side_effect=Exception('Error Uploading')) as mock:
client = boto3.client('s3')
# Should return actual result
o = client.get_object(Bucket='my-bucket', Key='my-key')
# Should return mocked exception
e = client.upload_part_copy()


This throws an exception but throws an exception... but on the
get_object
which I want to avoid.

Any ideas about how I can only throw the exception on the
upload_part_copy
method?

Answer

As soon as I posted on here I managed to come up with a solution. Here it is hope it helps :)

import botocore
from botocore.exceptions import ClientError
from mock import patch
import boto3

orig = botocore.client.BaseClient._make_api_call

def mock_make_api_call(self, operation_name, kwarg):
    if operation_name == 'UploadPartCopy':
        parsed_response = {'Error': {'Code': '500', 'Message': 'Error Uploading'}}
        raise ClientError(parsed_response, operation_name)
    return orig(self, operation_name, kwarg)

with patch('botocore.client.BaseClient._make_api_call', new=mock_make_api_call):
    client = boto3.client('s3')
    # Should return actual result
    o = client.get_object(Bucket='my-bucket', Key='my-key')
    # Should return mocked exception
    e = client.upload_part_copy()

Jordan Philips also posted a great solution using the the botocore.stub.Stubber class. Whilst a cleaner solution I was un-able to mock specific operations.

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