Fabric/Boto: boto.exception.NoAuthHandlerFound

In our Fabric code we make use of Boto to connect to the EC2 API and pull back various bits of information and the first time anyone tries to use it they end up with the following stack trace:

File "/Library/Python/2.7/site-packages/fabric/main.py", line 717, in main
    *args, **kwargs
  File "/Library/Python/2.7/site-packages/fabric/tasks.py", line 332, in execute
    results['<local-only>'] = task.run(*args, **new_kwargs)
  File "/Library/Python/2.7/site-packages/fabric/tasks.py", line 112, in run
    return self.wrapped(*args, **kwargs)
  File "/Users/mark/projects/forward-puppet/ec2.py", line 131, in running
    instances = instances_by_zones(running_instances(region, role_name))
  File "/Users/mark/projects/forward-puppet/ec2.py", line 19, in running_instances
    ec2conn = ec2.connect_to_region(region)
  File "/Library/Python/2.7/site-packages/boto/ec2/__init__.py", line 57, in connect_to_region
    for region in regions(**kw_params):
  File "/Library/Python/2.7/site-packages/boto/ec2/__init__.py", line 39, in regions
    c = EC2Connection(**kw_params)
  File "/Library/Python/2.7/site-packages/boto/ec2/connection.py", line 94, in __init__
    validate_certs=validate_certs)
  File "/Library/Python/2.7/site-packages/boto/connection.py", line 936, in __init__
    validate_certs=validate_certs)
  File "/Library/Python/2.7/site-packages/boto/connection.py", line 548, in __init__
    host, config, self.provider, self._required_auth_capability())
  File "/Library/Python/2.7/site-packages/boto/auth.py", line 633, in get_auth_handler
    'Check your credentials' % (len(names), str(names)))
boto.exception.NoAuthHandlerFound: No handler was ready to authenticate. 1 handlers were checked. ['QuerySignatureV2AuthHandler'] Check your credentials

We haven’t told Boto about our AWS credentials and I’ve come across two ways of providing them:

As environment variables

export AWS_ACCESS_KEY_ID="aws_access_key_id"
export AWS_SECRET_ACCESS_KEY="aws_secret_access_key"

[Credentials]
aws_access_key_id = aws_access_key_id
aws_secret_access_key = aws_secret_access_key