Public API

Instance of Value

class vo.value.Value(**kwargs)

Basic implementation of DDD immutable Value Object.

Features:

• Two objects with exact values are considered the same
• Objects are immutable (raise ImmutableInstanceError)

Example:

>>> val = Value(name='Primary', price=10.35, currency='USD')
>>> val.price
10.35

>>> val['currency']
'USD'

Parameters:**kwargs –

Any key=value pairs.

to_dict() → dict

Dump values to dict.

Returns:dict with values Return type:dict

to_json() → str

Dump values to JSON.

Returns:JSON string. Return type:str

to_bytes() → bytes

Convert values to bytes.

Returns:byte string Return type:bytes

Create new instance of Value

Value accept any key=value pairs. These pairs will be attached to object as attributes. Once created values are immutable. Can’t be changed or deleted.

>>> from vo import Value
>>> book = Value(title='Learning Python',
...              authors=['Mark Lutz', 'David Ascher'],
...              publisher="O'REILLY")
>>> book
<vo.value.Value object at 0x7f38862b3860>

Values access

Values can be accessed like object attributes or like dict keys.

>>> from vo import Value
>>> book = Value(title='Learning Python',
...              authors=['Mark Lutz', 'David Ascher'],
...              publisher="O'REILLY")
>>> book.title == book['title']
True

>>> book.authors == book['authors']
True

Objects comparison

Let’s take the same book example.

>>> from vo import Value
>>> book1 = Value(title='Learning Python',
...               authors=['Mark Lutz', 'David Ascher'],
...               publisher="O'REILLY")
>>> book2 = Value(title='Learning Python',
...               authors=['Mark Lutz', 'David Ascher'],
...               publisher="O'REILLY")
>>> book1 == book2
True

>>> book1 is book2
False

Value lookup

Check if value exists.

>>> from vo import Value
>>> book = Value(title='Learning Python',
...              authors=['Mark Lutz', 'David Ascher'],
...              publisher="O'REILLY")
>>> 'title' in book
True

>>> 'price' in book
False

>>> book.title
'Learning Python'

>>> book.price
Traceback (most recent call last):
  File "<input>", line 1, in <module>
AttributeError: 'Value' object has no attribute 'price'

Modification forbidden error

exception vo.value.ImmutableInstanceError(message: str = None)

Raised on any attempt to modify values in Value object.

Parameters:message (str) – Optional message.

message = 'Modification of Value instance is forbidden.'

Any attempt of value modification or delete will raise ImmutableInstanceError

>>> from vo import Value
>>> book = Value(title='Learning Python',
...              authors=['Mark Lutz', 'David Ascher'],
...              publisher="O'REILLY")
>>> book.title = 'Spam'
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    raise ImmutableInstanceError()
  vo.value.ImmutableInstanceError: Modification of Value frozen instance is forbidden.