lobotomy 0.1.4

Boto3 low-level client mocking library.

Lobotomy

• Installation
• Tl;dr Usage
• Usage
  • Configuration Files
  • Test Patching
  • Command Line Interface
• Advanced Usage
  • Key Prefixes
  • Patching Targets

The lo•boto•my library allows one to mock the low-level boto3 client libraries efficiently, especially in more complex scenario testing situations, using configuration-based response definitions. The benefit is a separation of the configuration from the test execution, which cleans up the test invocation process.

Installation

lobotomy is available on pypi and installable via pip:

$ pip install lobotomy

or via poetry as a development dependency in a project:

$ poetry add lobotomy -D

Tl;dr Usage

Create a configuration file in YAML, TOML, or JSON format with a root clients key. Beneath that key add the desired client calls mocked responses by service as shown here for a session.client('s3').get_object() mocked response:

clients:
  s3:
    get_object:
      Body: 'The contents of my S3 file.'
      LastModified: '2020-12-01T01:02:03Z'

Then in the test notice that the lobotomy patching process handles the rest, including casting the configuration values specified in the call above into the more complex data types returned for the specific call.

import lobotomy
import pathlib
import boto3
import datetime

my_directory = pathlib.Path(__file__).parent

@lobotomy.Patch(my_directory.joinpath('test_lobotomy.yaml'))
def test_lobotomy(lobotomized: lobotomy.Lobotomy):
    """
    Should return the mocked get_object response generated from the
    configuration data specified in the lobotomy patch above. By default
    the Patch(...) applies to the boto3.Session object, so calling 
    boto3.Session() will create a lobotomy.Session instead of a normal
    boto3.Session. From there the low-level client interface is designed
    to match normal usage.
    """
    s3_client = boto3.Session().client('s3')
    
    # Lobotomy will validate that you have specified the required keys
    # in your request, so Bucket and Key have to be supplied here even though
    # they are not meaningful values in this particular test scenario.
    response = s3_client.get_object(Bucket='foo', Key='bar')

    expected = b'The contents of my S3 file.'
    assert response['Body'].read() == expected, """
        Expect the mocked response body data to be returned as a
        StreamingBody object with blob/bytes contents. The lobotomy
        library introspects boto to properly convert the string body
        value in the configuration file into the expected return format
        for the particular call.
        """
    
    expected = datetime.datetime(2020, 12, 1, 1, 2, 3, 0, datetime.timezone.utc)
    assert response['LastModified'] == expected, """
        Expect the mocked response last modified value to be a timezeon-aware
        datetime value generated from the string timestamp value in the
        configuration file to match how it would be returned by boto in
        an actual response.
        """

Usage

Configuration Files

Test scenarios can be written in YAML, TOML, or JSON formats. YAML or TOML are recommended unless copying output responses from JSON calls is easier for a given use-case. In the following example, we'll define the calls using YAML:

clients:
  sts:
    get_caller_identity:
      Account: '987654321'
  s3:
    get_object:
      Body: 'The contents of my S3 file.'
      LastModified: '2020-11-01T12:23:34Z'

All call responses are stored within the root clients attribute with service names as sub-attributes and then the service method call responses defined beneath the service name attributes. Multiple services and multiple methods per service are specified in this way by the hierarchical key lists.

If the contents of a method response definition are not a list, the same response will be returned for each call to that client method during the test. Specifying a list of responses will alter the behavior such that each successive call will iterate through that list of configured responses.

clients:
  s3:
    get_object:
    - Body: 'The contents of my S3 file.'
      LastModified: '2020-11-01T12:23:34Z'
    - Body: 'Another S3 file.'
      LastModified: '2020-11-03T12:23:34Z'

The lobotomy library dynamically inspects boto for its response structure and data types, such that they match the types for the normal boto response for each client method call. In the case of the session.client('s3').get_object() calls above, the Body would return a StreamingBody object with the string converted to bytes to match the normal output. Similarly, the LastModified would be converted to a timezone-aware datetime object by lobotomy as well. This makes it easy to specify primitive data types in the configuration file that are transformed into their more complex counterparts when returned within the execution of the actual code.

Test Patching

Once the configuration file has been specified, it is used within a test via a lobotomy.Patch as shown below.

import lobotomy
import pathlib
import boto3
import datetime

my_directory = pathlib.Path(__file__).parent

@lobotomy.Patch(my_directory.joinpath('test_lobotomy.yaml'))
def test_lobotomy(lobotomized: lobotomy.Lobotomy):
    """
    Should return the mocked get_object response generated from the
    configuration data specified in the lobotomy patch above. By default
    the Patch(...) applies to the boto3.Session object, so calling 
    boto3.Session() will create a lobotomy.Session instead of a normal
    boto3.Session. From there the low-level client interface is designed
    to match normal usage.
    """
    s3_client = boto3.Session().client('s3')
    
    # Lobotomy will validate that you have specified the required keys
    # in your request, so Bucket and Key have to be supplied here even though
    # they are not meaningful values in this particular test scenario.
    response = s3_client.get_object(Bucket='foo', Key='bar')

    expected = b'The contents of my S3 file.'
    assert response['Body'].read() == expected, """
        Expect the mocked response body data to be returned as a
        StreamingBody object with blob/bytes contents. The lobotomy
        library introspects boto to properly convert the string body
        value in the configuration file into the expected return format
        for the particular call.
        """
    
    expected = datetime.datetime(2020, 12, 1, 1, 2, 3, 0, datetime.timezone.utc)
    assert response['LastModified'] == expected, """
        Expect the mocked response last modified value to be a timezeon-aware
        datetime value generated from the string timestamp value in the
        configuration file to match how it would be returned by boto in
        an actual response.
        """

The patching process replaces the boto3.Session class with a lobotomy.Lobotomy object that contains the loaded configuration data. When patched in this fashion, boto3.Session() calls will actually be lobotomy.Lobotomy() calls that return lobotomy.Session objects. These sessions have the interface of the boto3.Session object, but behave in a way such that client responses are returned from the configuration data instead of through interactivity with AWS.

For simple cases with little configuration, it is also possible to patch data stored directly within the Python code. The above test could be rewritten in this way as:

import lobotomy

configuration = {
    'clients': {
        's3': {
            'get_object': [
                { 
                    'Body': 'The contents of my S3 file.',
                    'LastModified': '2020-11-01T12:23:34Z',
                },
                {
                    'Body': 'Another S3 file.',
                    'LastModified': '2020-11-03T12:23:34Z',
                },
            ],
        },
    },
}


@lobotomy.Patch(data=configuration)
def test_lobotomy(lobotomized: lobotomy.Lobotomy):
    """..."""

Although one of the benefits of lobotomy is the ability to streamline the tests files by reducing the response configuration, which can be a bit verbose inside Python files and that is the highly recommended approach.

Command Line Interface

The lobotomy library also has a command line interface to help streamline the process of creating configuration files. The CLI has an add command that can be used to auto-generate method call response configurations to a new or existing configuration file. The values are meant to be replaced and unused keys to be removed to streamline for testing, but it helps a lot to get the full structure of the response in place and work from there instead of having to look it up yourself.

For example, creating the file:

clients:
  sts:
    get_caller_identity:
      Account: '987654321'
  s3:
    get_object:
    - Body: 'The contents of my S3 file.'
      LastModified: '2020-11-01T12:23:34Z'
    - Body: 'Another S3 file.'
      LastModified: '2020-11-03T12:23:34Z'

could be done first through the CLI commands:

$ lobotomy add sts.get_caller_identity example.yaml

After that command is executed, the example.yaml file will be created and populated initially with:

clients:
  sts:
    get_caller_identity:
      Account: '...'
      Arn: '...'
      UserId: '...'

Notice the values are placeholders. We can adjust the values to what we want and remove the unnecessary keys for our particular case such that the file contents are then:

clients:
  sts:
    get_caller_identity:
      Account: '987654321'

Next add the first s3.get_object call:

$ lobotomy add s3.get_object example.yaml

The configuration file now looks like:

clients:
  s3:
    get_object:
      AcceptRanges: '...'
      Body: '...'
      CacheControl: '...'
      ContentDisposition: '...'
      ContentEncoding: '...'
      ContentLanguage: '...'
      ContentLength: 1
      ContentRange: '...'
      ContentType: '...'
      DeleteMarker: null
      ETag: '...'
      Expiration: '...'
      Expires: '2020-11-04T14:37:18.042821Z'
      LastModified: '2020-11-04T14:37:18.042821Z'
      Metadata: null
      MissingMeta: 1
      ObjectLockLegalHoldStatus: '...'
      ObjectLockMode: '...'
      ObjectLockRetainUntilDate: '2020-11-04T14:37:18.042821Z'
      PartsCount: 1
      ReplicationStatus: '...'
      RequestCharged: '...'
      Restore: '...'
      SSECustomerAlgorithm: '...'
      SSECustomerKeyMD5: '...'
      SSEKMSKeyId: '...'
      ServerSideEncryption: '...'
      StorageClass: '...'
      TagCount: 1
      VersionId: '...'
      WebsiteRedirectLocation: '...'
  sts:
    get_caller_identity:
      Account: '987654321'

Of course, this is a simple case because we don't need much of the response structure in our simplified use-case, but hopefully you can see the value of being able to add the response structure so easily for more complex cases. Once again, the new call is adjusted to fit our particular needs:

clients:
  s3:
    get_object:
      Body: 'The contents of my S3 file.'
      LastModified: '2020-11-01T12:23:34Z'
  sts:
    get_caller_identity:
      Account: '987654321'

Adding the second s3.get_object call is identical:

$ lobotomy add s3.get_object example.yaml

However, lobotomy notices the existing call there and so converts the get_object response configuration to a list of responses for you:

clients:
  s3:
    Body: 'The contents of my S3 file.'
    LastModified: '2020-11-01T12:23:34Z'
    get_object:
      AcceptRanges: '...'
      Body: '...'
      CacheControl: '...'
      ContentDisposition: '...'
      ContentEncoding: '...'
      ContentLanguage: '...'
      ContentLength: 1
      ContentRange: '...'
      ContentType: '...'
      DeleteMarker: null
      ETag: '...'
      Expiration: '...'
      Expires: '2020-11-04T14:42:51.077364Z'
      LastModified: '2020-11-04T14:42:51.077364Z'
      Metadata: null
      MissingMeta: 1
      ObjectLockLegalHoldStatus: '...'
      ObjectLockMode: '...'
      ObjectLockRetainUntilDate: '2020-11-04T14:42:51.077364Z'
      PartsCount: 1
      ReplicationStatus: '...'
      RequestCharged: '...'
      Restore: '...'
      SSECustomerAlgorithm: '...'
      SSECustomerKeyMD5: '...'
      SSEKMSKeyId: '...'
      ServerSideEncryption: '...'
      StorageClass: '...'
      TagCount: 1
      VersionId: '...'
      WebsiteRedirectLocation: '...'
  sts:
    get_caller_identity:
      Account: '987654321'

Finally, edit the new call response configuration and we end up with the configuration we were looking for:

clients:
  sts:
    get_caller_identity:
      Account: '987654321'
  s3:
    get_object:
    - Body: 'The contents of my S3 file.'
      LastModified: '2020-11-01T12:23:34Z'
    - Body: 'Another S3 file.'
      LastModified: '2020-11-03T12:23:34Z'

Advanced Usage

Key Prefixes

By default configuration files are rooted at the clients key within the file. However, it is possible to specify a different root key prefix, which is useful when co-locating lobotomy test configuration with other test configuration data in the same file, or when co-locating multiple lobotomy test configurations within the same file. To achieve that a prefix must be specified during patching.

As an example, consider the configuration file:

lobotomy:
  test_a:
    clients:
      sts:
        get_caller_identity:
          Account: '987654321'
  test_b:
    clients:
      sts:
        get_caller_identity:
          Account: '123456678'
          UserId: 'AIFASDJWISJAVHXME'

In this case the prefixes are lobotomy.test_a and lobotomy.test_b. To use these in a test the prefix must be specified in the Patch:

import pathlib
import lobotomy

config_path = pathlib.Path(__file__).parent.joinpath('validation.yaml')


@lobotomy.Patch(config_path, prefix='lobotomy.test_a')
def test_a(lobotomized: lobotomy.Lobotomy):
    """..."""


@lobotomy.Patch(config_path, prefix='lobotomy.test_b')
def test_b(lobotomized: lobotomy.Lobotomy):
    """..."""

The prefix can be specified as a . delimited string, or as a list/tuple. The list/tuple is needed if the keys themselves contains ..

Patching Targets

By default, lobotomy will patch boto3.Session. There are scenarios where a different patch would be desired either to limit the scope of the patch or to patch another library wrapping the boto3.Session call. In those cases, specify the patch path argument:

import pathlib
import lobotomy

config_path = pathlib.Path(__file__).parent.joinpath('test.yaml')


@lobotomy.Patch(config_path, patch_path='something.else.Session')
def test_another_patch_path(lobotomized: lobotomy.Lobotomy):
    """..."""