Mutable records

## mutablerecords.Record

This is similar to collections.namedtuple, except it supports optional attributes and mutability. A class definition is generated (with __slots__, __str__ and other niceties), and can be used to instantiate new records of that type. The record can also be subclassed to add new attributes or to add methods to that data.

Sometimes, a Record definition is used to replace a simple __init__ method that only takes N arguments and sets them as instance variables. These __init__ methods are tedious to write and, even if you do, you still have to write str, hash, eq functions, and set __slots__ to be fully correct, but who has the time for that? With records, you get all of that in a single declaration, which you can even inline as your base class.

# This acts like a mutable namedtuple, taking the same arguments.
Simple = mutablerecords.Record('Simple', ['foo'])

# Now let's use a default argument.
SecondRecord = mutablerecords.Record('SecondRecord', ['attr1', 'attr2'], {'attr3': 0})
foo = SecondRecord(1, 2, attr3=3)
# str(foo) --> 'SecondRecord(attr1=1, attr2=2, attr3=3)'
bar = SecondRecord(attr1=1, attr2=2, attr3=5)
# str(bar) --> 'SecondRecord(attr1=1, attr2=2, attr3=5)'

class Third(SecondRecord):
required_attributes = ['third1']
optional_attributes = {'third2': 5}

# Third requires attr1, attr2, and third1.
baz = Third(1, 2, 3, third2=4)
# Here, second1 is required, so it goes before attr3:
# str(baz) --> 'Third(attr1=1, attr2=2, third1=3, attr3=0, third2=5)'

class OptionalMaker(mutablerecords.Record('Required', ['required'])):
required = None
required_attributes = ['other']

opt = OptionalMaker(1)
# OptionalMaker has a class attribute that matches the name of a
#   required_attribute (required), so it becomes an optional_attribute with a
#   default equal to the attribute value (None). It also defines a new
#   required attribute 'other', which is set in opt as 1:
# str(opt) --> 'OptionalMaker(other=1, required=None)'
opt2 = OptionalMaker(2, required=3)
# This time, opt2 has required set, too, which is still an attribute.
# str(opt2) --> 'OptionalMaker(other=2, required=3)'

## mutablerecords.HashableRecord

All this does is add a __hash__ implementation for when the record will be hashed, such as when a key in a dict or in a set.

## mutablerecords.CopyRecord

This will copy a record and all of its fields. If a field is set to a Record instance, then it is also copied using CopyRecord. A future feature may also check for collections of Records, such as tuples or lists of instances, and recurse into those as well.

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