fields 1.0.1

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Container class boilerplate killer.

Features:

• Human-readable __repr__

• Complete set of comparison methods

• Keyword and positional argument support. Works like a normal class - you can override just about anything in the subclass (eg: a custom __init__). In contrast, hynek/characteristic forces different call schematics and calls your __init__ with different arguments.

Installation

pip install fields

Usage

Make a class that has 2 attributes, a and b:

>>> from fields import Fields
>>> class Pair(Fields.a.b):
...     pass
...
>>> p = Pair(1, 2)
>>> p.a
1
>>> p.b
2

Make a class that has one required attribute value and two attributes (left and right) with default value None:

>>> class Node(Fields.value.left[None].right[None]):
...     pass
...
>>> p = Node(1, left=Node(2), right=Node(3, left=Node(4)))
>>> p
Node(value=1, left=Node(value=2, left=None, right=None), right=Node(value=3, left=Node(value=4, left=None, right=None), right=None))

Want tuples?

Namedtuple alternative:

>>> from fields import Tuple
>>> class Pair(Tuple.a.b):
...     pass
...
>>> p = Pair(1, 2)
>>> p.a
1
>>> p.b
2
>>> tuple(p)
(1, 2)
>>> a, b = p
>>> a
1
>>> b
2

FAQ

Why should I use this?

It’s less to type, why have quotes around when the names need to be valid symbols anyway. In fact, this is one of the shortest forms possible to specify a container with fields.

But you’re abusing a very well known syntax. You’re using attribute access instead of a list of strings. Why?

Symbols should be symbols. Why validate strings so they are valid symbols when you can avoid that? Just use symbols. Save on both typing and validation code.

The use of language constructs is not that surprising or confusing in the sense that semantics precede conventional syntax use. For example, if we have class Person(Fields.first_name.last_name.height.weight): pass then it’s going to be clear we’re talking about a Person object with first_name, last_name, height and width fields: the words have clear meaning.

Again, you should not name your varibles as f1, f2 or any other non-semantic symbols anyway.

Semantics precede syntax: it’s like looking at a cake resembling a dog, you won’t expect the cake to bark and run around.

Is this stable? Is it tested?

Yes. Mercilessly tested on Travis and AppVeyor.

Is the API stable?

Yes, ofcourse.

Why not namedtuple?

It’s ugly, repetivive and unflexible. Compare this:

>>> from collections import namedtuple
>>> class MyContainer(namedtuple("MyContainer", ["field1", "field2"])):
...     pass
>>> MyContainer(1, 2)
MyContainer(field1=1, field2=2)

To this:

>>> class MyContainer(Tuple.field1.field2):
...     pass
>>> MyContainer(1, 2)
MyContainer(field1=1, field2=2)

Why not characteristic?

Ugly, inconsistent - you don’t own the class:

Lets try this:

>>> import characteristic
>>> @characteristic.attributes(["field1", "field2"])
... class MyContainer(object):
...     def __init__(self, a, b):
...         if a > b:
...             raise ValueError("Expected %s < %s" % (a, b))
>>> MyContainer(1, 2)
Traceback (most recent call last):
    ...
ValueError: Missing keyword value for 'field1'.

WHAT !? Ok, lets write some more code:

>>> MyContainer(field1=1, field2=2)
Traceback (most recent call last):
    ...
TypeError: __init__() ... arguments...

This is bananas. You have to write your class around these quirks.

Lets try this:

>>> class MyContainer(Fields.field1.field2):
...     def __init__(self, a, b):
...         if a > b:
...             raise ValueError("Expected %s < %s" % (a, b))
...         super(MyContainer, self).__init__(a, b)

Just like a normal class, works as expected:

>>> MyContainer(1, 2)
MyContainer(field1=1, field2=2)

Documentation

https://python-fields.readthedocs.org/

Development

To run the all tests run:

tox

Changelog

1.0.0 (2014-10-05)

• Lots of internal changes, the metaclass is not created in a closure anymore. No more closures.

• Added RegexValidate container creator (should be taken as an example on using the Factory metaclass).

• Added support for using multiple containers as baseclasses.

• Added a super() sink so that super().__init__(*args, **kwargs) always works. Everything inherits from a baseclass that has an __init__ that can take any argument (unlike object.__init__). This allows for flexible usage.

• Added validation so that you can’t use conflicting field layout when using multiple containers as the baseclass.

• Changed the __init__ function in the class container so it works like a python function w.r.t. positional and keyword arguments. Example: class MyContainer(Fields.a.b.c[1].d[2]) will function the same way as def func(a, b, c=1, d=2) would when arguments are passed in. You can now use MyContainer(1, 2, 3, 4) (everything positional) or MyContainer(1, 2, 3, d=4) (mixed).

0.3.0 (2014-07-19)

• Corrected string repr

0.2.0 (2014-06-28)

• Lots of breaking changes. Switched from __call__ to __getitem__ for default value assignment.

0.1.0 (2014-06-27)

• Alpha