versionedobj 0.2.4

Easy object serialization & versioning framework

versionedobj is a framework for creating complex python objects that can be serialized/deserialized to and from strings, or dicts, or JSON files.

versionedobj also provides a versioning mechanism, to track changes in object structure across time, and to migrate between different object versions.

See API documentation

Table of Contents

• Example– VersionedObject as a configuration file

• Performance/stress test visualization

• Accessing versioned object instance attributes

• Iterating over versioned object instance attributes

• Filtering serialization/deserialization output

  • Whitelisting by field name

  • Blacklisting by field name

• Migrations – making use of the version number

  • Example scenario, part 1: you have created a beautiful versioned object

  • Example scenario, part 2: you update your software, modifying the versioned object

  • Uh-oh, you have a problem…

  • Solution– migrations!

• Validating input data without deserializing

• Contributions

Example– VersionedObject as a configuration file

from versionedobj import VersionedObject

class DisplayConfig(VersionedObject):
    display_mode = "windowed"
    resolution = "1920x1080"
    volume = 0.66

# Populate class attributes to build your object
class UserConfig(VersionedObject):
    version = "v1.0.0"
    username = "john smith"
    friend_list = ["user1", "user2", "user3"]
    display_config = DisplayConfig() # VersionedObjects can be nested

# Create an instance of your object (instance attributes will match class attributes,
# and the initial values will be whatever values you set on the class attributes)
cfg = UserConfig()

# Change some values on the object instance
cfg.display_config.volume = 1.0
cfg.username = "jane doe"

# Save object instance to JSON file
cfg.to_file('user_config.json', indent=4)

# Load object instance from JSON file
cfg.from_file('user_config.json')

You can also save/load object data as a JSON string:

>>> obj_as_json = cfg.to_json(indent=4) # Serialize to JSON string
>>> obj_as_json                         # Print JSON string

{
    "version": "v1.0.0",
    "username": "jane doe",
    "friend_list": [
            "user1",
            "user2",
            "user3"
    ],
    "display_config": {
        "display_mode": "windowed",
        "resolution": "1920x1080",
        "volume": 1.0
    }
}

>>> cfg.from_json(obj_as_json)          # Load from JSON string

Or, as a dict:

>>> obj_as_dict = cfg.to_dict()   # Serialize to dict
>>> obj_as_dict                   # Print dict

{'version': '1.0.0', 'username': 'jane doe', 'friend_list': ['user1', 'user2', 'user3'], 'display_config': {'display_mode': 'windowed', 'resolution': '1920x1080', 'volume': 1.0}}

>>> cfg.from_dict(obj_as_dict)    # Load from dict

Performance/stress test visualization

Accessing versioned object instance attributes

When you create an instance of your VersionedObject class, the instance attributes will be automatically populated to match the class attributes you have created:

from versionedobj import VersionedObject

class AccountInfo(VersionedObject):
    user_name = "john"
    user_id = 11223344

class Session(VersionedObject):
    ip_addr = "255.255.255.255"
    port = 22
    account_info = AccountInfo()

session = Session()

print(session.ip_addr)
# "255.255.255.255"

print(session.account_info.user_name)
# "john"

session.account_info.user_name = "jane"

print(session.account_info.user_name)
# "jane"

Alternatively, you can treat a VersionedObject instance as a dict, and access attributes by passing their full name as the key:

print(session['account_info.user_name'])
# "jane"

session['account_info.user_name'] = "jack"

print(session['account_info.user_name'])
# "jack"

Iterating over versioned object instance attributes

If you want to enumerate all attribute names & values on a versioned object instance, you can use the object_attributes() method, which returns a generator for all instance attributes:

from versionedobj import VersionedObject

class AccountInfo(VersionedObject):
    user_name = "john"
    user_id = 11223344

class Session(VersionedObject):
    ip_addr = "255.255.255.255"
    port = 22
    account_info = AccountInfo()

session = Session()

for attr_name, attr_value in session.object_attributes():
    print(f"{attr_name}: {attr_value}")

# Output looks like this:
#
# ip_addr: 255.255.255.255
# port: 22
# account_info.user_name: john
# account_info.user_id: 11223344

Filtering serialization/deserialization output

Whitelisting by field name

When serializing, if you only want to output certain fields, you can use the ‘only’ parameter to specify which fields should be output (effectively a whitelist by field name):

cfg.to_file('user_config.json', only=['version', 'username', 'display_config.resolution'])

# Output looks like this:
#
# {
#     "version": "v1.0.0",
#     "username": "jane doe",
#     "display_config": {
#         "resolution": "1920x1080",
#     }
# }

The same parameter can be used for de-serializing:

cfg.from_file('user_config.json', only=['display_config.display_mode'])

# Only the 'display_config.display_mode' field is loaded from the file

Blacklisting by field name

When serializing, if you don’t want to output certain fields, you can use the ‘ignore’ parameter to specify which fields should be excluded from output (effectively a blacklist by field name):

cfg.to_file('user_config.json', ignore=['friend_list', 'display_config.volume'])

# Output looks like this:
#
# {
#     "version": "v1.0.0",
#     "username": "jane doe",
#     "display_config": {
#         "display_mode": "windowed",
#         "resolution": "1920x1080"
#     }
# }

The same parameter can be used for de-serializing:

cfg.from_file('user_config.json', ignore=['friend_list'])

# Every field except for the 'friend_list' field is loaded from the file

Migrations – making use of the version number

Any VersionedObject object can have a version attribute, which can be any object, although it is typically a string (e.g. "v1.2.3"). This version attribute can be used to support migrations for older objects, in the event that you need to change the format of your object.

Example scenario, part 1: you have created a beautiful versioned object

Let’s take the same config file definition from the previous example:

from versionedobj import VersionedObject

# Nested config object
class DisplayConfig(VersionedObject):
    display_mode = "windowed"
    resolution = "1920x1080"
    volume = 0.66

# Top-level config object with another nested config object
class UserConfig(VersionedObject):
    version = "v1.0.0"
    username = "john smith"
    friend_list = ["user1", "user2", "user3"]
    display_config = DisplayConfig()

Imagine you’ve already released this code out into the world. People are already using it, and they have JSON files generated by your UserConfig class sitting on their computers.

Example scenario, part 2: you update your software, modifying the versioned object

Now, imagine you are making a new release of your software, and some new features require you to make the following changes to your versioned object:

• remove the the DisplayConfig.resolution field entirely

• change the name of DisplayConfig.volume to DisplayConfig.volumes

• change the value of DisplayConfig.volumes from a float to a list

from versionedobj import VersionedObject

# Nested config object
class DisplayConfig(VersionedObject):
    display_mode = "windowed"
    # 'resolution' field is deleted
    volumes = [0.66, 0.1] # 'volume' is now called 'volumes', and is a list

# Top-level config object with another nested config object
class UserConfig(VersionedObject):
    version = "v1.0.0"
    username = "john smith"
    friend_list = ["user1", "user2", "user3"]
    display_config = DisplayConfig()

Uh-oh, you have a problem…

Right now, if you send this updated UserConfig class to your existing users, it will fail to load their existing JSON files with version v1.0.0, since those files will contain the DisplayConfig.resolution field that we deleted in v1.0.1, and DisplayConfig.volume will similarly be gone, having been replaced with DisplayConfig.volumes. This situation is what migrations are for.

Solution– migrations!

The solution is to:

1. Change the version number to something new, e.g. v1.0.0 becomes v1.0.1

2. Write a migration function to transform v1.0.0 object data into v1.0.1 object data

from versionedobj import VersionedObject

# Nested config object
class DisplayConfig(VersionedObject):
    display_mode = "windowed"
    # 'resolution' field is deleted
    volumes = [0.66, 0.1] # 'volume' is now called 'volumes', and is a list

# Top-level config object with another nested config object
class UserConfig(VersionedObject):
    version = "v1.0.1" # Version has been updated to 1.0.1
    username = "john smith"
    friend_list = ["user1", "user2", "user3"]
    display_config = DisplayConfig()

# Create the migration function for v1.0.0 to v1.0.1
def migrate_100_to_101(attrs):
    del attrs['display_config']['resolution']        # Delete resolution field
    del attrs['display_config']['volume']            # Delete volume field
    attrs['display_config']['volumes'] = [0.66, 0.1] # Add defaults for new volume values
    return attrs                                     # Return modified data (important!)

# Add the migration function for v1.0.0 to v1.0.1
UserConfig.add_migration("v1.0.0", "v1.0.1", migrate_100_to_101)

after you add the migration function and update the version to v1.0.1, JSON files that are loaded and contain the version v1.0.0 will be automatically migrated to version v1.0.1 using the migration function you added.

The downside to this approach, is that you have to manually udpate the version number, and write a new migration function, anytime the structure of your config data changes.

The upside, of course, is that you can relatively easily support migrating any older version of your config file to the current version.

If you don’t need the versioning/migration functionality, just never change your version number, or don’t create a version attribute on your VersionedObject classes.

Validating input data without deserializing

You may want to validate some serialized object data without actually deserializing and loading the object values. You can use the validate_dict method for this.

from versionedobj import VersionedObject

class Recipe(VersionedObject):
    ingredient_1 = "onions"
    ingredient_2 = "tomatoes"
    ingredient_3 = "garlic"

rcp = Recipe()

rcp.validate_dict({"ingredient_1": "celery", "ingredient_2": "carrots"})
# Raises versionedobj.exceptions.InputValidationError because 'ingredient_3' is missing

rcp.validate_dict({"ingredient_1": "celery", "ingredient_2": "carrots", "ingredient_12": "cumin"})
# Raises versionedobj.exceptions.InputValidationError because 'ingredient_12' is not a valid attribute

Contributions

Contributions are welcome, please open a pull request at https://github.com/eriknyquist/versionedobj and ensure that:

1. All existing unit tests pass (run tests via python setup.py test)

2. New unit tests are added to cover any modified/new functionality

If you have any questions about / need help with contributions or unit tests, please contact Erik at [e-mail].