rbco.caseclasses

Contents

• Overview
• Compatibility
• Instalation
• Usage
  • Basics
  • Mutability and __slots__
  • Structural equality
  • Copy-constructor
  • Conversion from/to dictionary and tuple
  • Custom members
  • Inheritance
  • Overriding case class behavior
  • Using CaseClassMixin for more flexibility
• Limitations
• Motivation, design decisions and other implementations
  • Comparison with MacroPy
  • Other implementations
  • Discarded implementation ideas
    • Functional syntax
    • Fields specification as parameters to the class decorator
    • Fields specification as class attributes
• Contributing

Overview

Provide a compact syntax to define simple "struct-like" (or "record-like", "bean-like") classes. The resulting classes are very similar to namedtuple, but mutable, with a nicer syntax, more flexibility and more features.

Here's a summary of the features:

• It's possible to define default values for fields.
• Useful __repr__ and __str__ implementations.
• Structural equality, i.e, useful __eq__ and __ne__ implementations.
• copy method (copy-constructor).
• Conversion from/to dictionary and tuple.
• __slots__ declaration to improve performance and prevent assignment on unknown fields.
• It's possible to define custom methods.
• Supports inheritance.

See also the motivation section for other implementations of the concept, specially MacroPy which was the inspiration for this project and uses a very different approach.

Compatibility

Currently only Python 2.7 is supported.

Instalation

The usual:

pip install rbco.caseclasses

Or:

easy_install rbco.caseclasses

Usage

Basics

Let's start by creating a simple case class:

>>> from rbco.caseclasses import case
>>>
>>> @case
... class Person(object):
...     """Represent a person."""
...     def __init__(self, name, age=None, gender=None): pass

The declared __init__ is just a stub. The parameters defines which fields the class will have and its default values. The __init__ method is replaced by a new one, which takes care of assigning the values of the fields.

The constructor works as expected, according to the provided __init__ stub:

>>> Person('John')
Person(name='John', age=None, gender=None)
>>> Person('John', 30, 'm')
Person(name='John', age=30, gender='m')
>>> Person(name='John', age=30, gender='m')
Person(name='John', age=30, gender='m')
>>> Person('John', gender='m')
Person(name='John', age=None, gender='m')

Note that in the string representation the fields are in the same order as defined in the constructor.

The docstring of the class is preserved:

>>> Person.__doc__
'Represent a person.'

The signature of the constructor is not preserved. The resulting __init__ method signature is a generic one, taking only *args and **kwargs:

>>> from inspect import getargspec
>>> getargspec(Person.__init__)
ArgSpec(args=['self'], varargs='args', keywords='kwargs', defaults=None)

However the docstring contains the original signature:

>>> Person.__init__.__doc__
'Original signature: (self, name, age=None, gender=None)'

It's not possible to create a case class without a constructor:

>>> from rbco.caseclasses import case
>>>
>>> @case
... class Foo(object): pass
Traceback (most recent call last):
...
RuntimeError: Case class must define a constructor.

Mutability and __slots__

Instances are mutable:

>>> p = Person('John')
>>> p
Person(name='John', age=None, gender=None)
>>> p.name = 'Bob'
>>> p.age = 35
>>> p
Person(name='Bob', age=35, gender=None)

However it's not possible to assign to unknown attributes:

>>> p.department = 'sales'
Traceback (most recent call last):
...
AttributeError: 'Person' object has no attribute 'department'

This is because of the __slots__ declaration:

>>> p.__slots__
['name', 'age', 'gender']

Structural equality

Structural equality is supported:

>>> p1 = Person('John', 30)
>>> p2 = Person('Bob', 25)
>>> p1 == p2
False
>>> p1 != p2
True
>>> p2.name = 'John'
>>> p2.age = 30
>>> p1 == p2
True
>>> p1 != p2
False
>>> p2.gender = 'm'
>>> p1 == p2
False

Copy-constructor

A copy-constructor is provided:

>>> p1 = Person('John', 30)
>>> copy_of_p1 = p1.copy()
>>> p1
Person(name='John', age=30, gender=None)
>>> copy_of_p1
Person(name='John', age=30, gender=None)
>>> p1 is copy_of_p1
False
>>> p2 = p1.copy(name='Bob', gender='m')
>>> p2
Person(name='Bob', age=30, gender='m')

Conversion from/to dictionary and tuple

Conversion from/to dictionary is easy. The as_dict method return an OrderedDict:

>>> p1 = Person('Mary', 33)
>>> p1
Person(name='Mary', age=33, gender=None)
>>> p1.as_dict()
OrderedDict([('name', 'Mary'), ('age', 33), ('gender', None)])
>>> Person(**p1.as_dict())
Person(name='Mary', age=33, gender=None)

Conversion from/to tuple is also possible:

>>> p1 = Person('John', 30)
>>> p1
Person(name='John', age=30, gender=None)
>>> p1.as_tuple()
('John', 30, None)
>>> Person(*p1.as_tuple())
Person(name='John', age=30, gender=None)

Custom members

Case classes are very much like regular classes. It's possible to define any kind of custom members.

The most common case should be adding a custom instance method:

>>> import math
>>> @case
... class Point(object):
...     def __init__(self, x, y): pass
...
...     def distance(self, other):
...         return math.sqrt((self.x - other.x)**2 + (self.y - other.y)**2)
>>> p1 = Point(0, 0)
>>> p2 = Point(10, 0)
>>> p1.distance(p2)
10.0

Other kinds of class members are supported as well:

>>> @case
... class Example(object):
...     class_attribute = 'some value'
...
...     def __init__(self, field1): pass
...
...     @staticmethod
...     def static_method():
...         print 'This is an static method.'
...
...     @classmethod
...     def class_method(cls):
...         print 'This is a class method of the class {}.'.format(cls.__name__)
...
>>> e = Example('example')
>>> Example.class_attribute
'some value'
>>> e.class_attribute
'some value'
>>> Example.static_method()
This is an static method.
>>> Example.class_method()
This is a class method of the class Example.

Inheritance

Let's create a base case class and a derived one:

>>> @case
... class Person(object):
...     def __init__(self, name, age=None, gender=None): pass
...
...     def present(self):
...         print "I'm {}, {} years old and my gender is '{}'.".format(
...             self.name,
...             self.age,
...             self.gender
...         )
...
>>> @case
... class Employee(Person):
...     def __init__(self, name, age=None, gender=None, department=None): pass

It's necessary to repeat the fields of the base class, but you would have to do that anyway if you were implementing the case classes manually.

Methods from the base class are inherited:

>>> p = Person('John', 30, 'm')
>>> p.present()
I'm John, 30 years old and my gender is 'm'.
>>> e = Employee('Mary', 33, 'f', 'sales')
>>> e.present()
I'm Mary, 33 years old and my gender is 'f'.

Instances of Person and Employee will always be considered different, since employees have an extra field:

>>> p = Person('John')
>>> e = Employee('John')
>>> p == e
False

Overriding a base class method works as expected:

>>> @case
... class ImprovedEmployee(Employee):
...     def present(self):
...         super(ImprovedEmployee, self).present()
...         print 'I work at the {} department.'.format(self.department)
...
>>> ie = ImprovedEmployee(name='Mary', department='marketing', age=33, gender='f')
>>> ie.present()
I'm Mary, 33 years old and my gender is 'f'.
I work at the marketing department.

Overriding case class behavior

It's possible to override the standard case class methods (__repr__, __eq__, etc). For example:

>>> @case
... class Foo(object):
...     def __init__(self, bar): pass
...
...     def __eq__(self, other):
...         return True  # All `Foo`s are equal.
...
>>> Foo('bar') == Foo('baz')
True

It's even possible to call the original version on the subclass method:

>>> @case
... class Foo(object):
...     def __init__(self, bar):
...         pass
...
...     def __repr__(self):
...         return 'This is my string representation: ' + super(Foo, self).__repr__()
...
>>> Foo('bar')
This is my string representation: Foo(bar='bar')

It's not possible to override the __init__ method, because it's replaced when the @case decorator is applied. If a custom constructor is needed using the CaseClassMixin can be a solution.

Using CaseClassMixin for more flexibility

The classes created by the @case decorator inherits from CaseClassMixin.

>>> from rbco.caseclasses import CaseClassMixin
>>> issubclass(Person, CaseClassMixin)
True

The CaseClassMixin provides all the "case class" behavior, except for the constructor. To use CaseClassMixin directly the only requirement the subclass must match is to provide a __fields__ attribute, containing a sequence of field names.

This can be useful if greater flexibility is required. In the following example we create a case class with a custom constructor:

>>> class Foo(CaseClassMixin):
...     __fields__ = ('field1', 'field2')
...
...     def __init__(self, field1, *args):
...         self.field1 = field1 + '_modified'
...         self.field2 = list(args)
...
>>> Foo('bar', 1, 2)
Foo(field1='bar_modified', field2=[1, 2])

Limitations

• The constructor of a case class cannot be customized because it's replaced when the @case decorator is applied. See the section about CaseClassMixin for an alternative.

• It's not possible to assign to unknow fields because of the __slots__ declaration.

• The constructor cannot take *args or **kwargs:

  >>> @case
  ... class Foo(object):
  ...     def __init__(self, **kwargs): pass
  Traceback (most recent call last):
  ...
  RuntimeError: Case class constructor cannot take *args or **kwargs.

  See the section about CaseClassMixin for an alternative.

Motivation, design decisions and other implementations

Comparison with MacroPy

The idea for this project came from MacroPy. It provides an implementation of case classes using syntactic macros, which results in a very elegant way to define the case classes. The motivation was to provide similar functionality without resorting to syntactic macros nor string evaluation (the approach took by namedtuple). In other words: to provide the best implementation possible without using much magic.

The comparison to MacroPy can be summarized as follows:

Advantages:

• No magic.
• Allows any kind of custom members, including instance methods.
• Since case classes are just regular classes, any kind of inheritance is allowed.

Disadvantages:

• MacroPy syntax is much nicer. The __init__ stub thing can be considered kind of ugly in comparison.
• Do not support custom initialization logic. This can be achieved by using CaseClassMixin but additional work will have to be done by the programmer.
• Do not support *args and **kwargs in the constructor. Again, this can be achieved by using CaseClassMixin at the expense of doing more work.

Other implementations

Other implementations of the "case class" concept (or similar) in Python exists:

• The constructor stub mechanism idea was stole from this implementation by hwiechers.

• A simple implementation by Brian Wickman can be found in this Gist.

• This discussion on stackoverflow has some implementation ideas.

Discarded implementation ideas

Some implementation ideas were considered but discarded afterwards. Here some of them are discussed.

Functional syntax

This means using a function to generate the class. This would be something like this:

Person = case_class('Person', 'name', age=None, gender=None)

The first problem with this idea is that there's no way to preserve the order of the fields. The case_class function would have to be defined like this:

def case_class(__name__, *args, **kwargs):
    ...

**kwargs is a unordered dictionary, so the order of the fields is lost.

To overcome this the following syntax could be used:

Person = case_class('Person', 'name', 'age', 'gender', age=None, gender=None)

I thinks this syntax is not elegant enough. I don't like the repetition of field names and to have field names represented as both strings and parameter names.

Perhaps something like this would work too:

Person = case_class('Person', ['name', 'age', 'gender'], {'age': None, 'gender': None})

But again I think the syntax is not elegant.

Also, some functionalities would be difficult to support using this syntax, namely:

• Custom members. This would mean complicate the signature of the case_class function or add the custom members after the class is created. Like this:

  Person = case_class('Person', ...)
  
  def present(self):
      print ...
  
  Person.present = present

  Not very elegant.

• Inheritance. This would require a new parameter to the case_class function, to allow to pass in a base class.

Fields specification as parameters to the class decorator

This would end the necessity to define an empty constructor. The syntax would be like this:

@case(name, age=None, gender=None)
class Person(object):
    'Represent a person.'

The same problem faced by the function syntax arises: field ordering is not preserved, since the case function would have to accept a **kwargs argument, which is an unordered dict.

Alternate syntaxes, similar to the ones presented for the functional syntax, could overcome the field ordering problem. However I think the solution using a __init__ stub to define the fields is more elegant.

Fields specification as class attributes

The syntax would be like this:

@case
class Person(object):
    name = NO_DEFAULT_VALUE
    age = None
    gender = None

Again, there's no way to preserve the order of the fields. The case function would have to retrieve the class attributes from Person.__dic__, which is unordered.

Maybe something like this would work:

@case
class Person(object):
    __fields__ = (
        ('name', NO_DEFAULT_VALUE),
        ('age', None),
        ('gender', None)
    )

However I think the solution using a __init__ stub to define the fields is more elegant.

Contributing

Please fork this project and submit a pull request if you would like to contribute. Thanks in advance !