On , I learnt ...

Python tests using moto should be explicit about AWS regions

Consider this test that creates an S3 bucket using moto:

import moto

def test_s3_bucket_creation():
    with moto.mock_s3():

This was failing locally for me with a botocore.exceptions.ClientError exception:

An error occurred (IllegalLocationConstraintException) when calling the
CreateBucket operation: The unspecified location constraint is incompatible for
the region specific endpoint this request was sent to.

but was passing in a remote CI environment and on other people’s machines.


The problem is that boto3 will take its default AWS region from your ~/.aws/config file and mine was set to Dublin:

region = eu-west-1

Remove this and the test passes.

Default AWS regions

If there is no default region configured then boto3 will default to us-east-1 when creating S3 buckets which, crucially, allows buckets to be created in a simpler process.

For other regions, S3 buckets need to be created with explicit location constraint configuration:

region = "eu-west-1"
client = boto3.client('s3', region_name=region)
    CreateBucketConfiguration={"LocationConstraint": region}

When creating S3 buckets in tests, it’s better to use this formulation where the region is explicitly specified.

Moral of the story

Beware of boto3 as it can take configuration from outside of your project boundaries.