editabletuple

editabletuple

• Overview
• Examples
• API
  • Notes

Overview

This module provides the editabletuple() and editableobject() functions.

The editabletuple() function is used tor creating classes with a fixed sequence of fields, similar to a namedtuple, except editable.

Each instance of a class created by the editabletuple() function's fields can be accessed by index et[i] (or by slice), or by fieldname et.name. Although fields can be read and written, they cannot be added or deleted. Since instances are mutable they can't be used in sets or as dict keys.

If you provide a validator, it will be used when new instances are created and updated.

The editableobject() function creates classes very similar to those created by editabletuple(). The essential difference is that editableobject()'s class's instances don't support indexing or iteration, so support only fieldname access. They also have an addtional totuple property (not needed for editabletuple()s since tuple(et) is sufficient due to their iteration support).

See the function docstrings for examples and more about the editabletuple and editableobject APIs.

To install just use python3 -m pip install editabletuple. (See PyPI.)

Or just copy the editabletuple.py file which is self-contained and depends only on the standard library.

Examples

Example #1: no defaults; no validator

>>> Options = editabletuple('Options', 'maxcolors shape zoom restore')
>>> options = Options(5, 'square', 0.9, True)
>>> options
Options(maxcolors=5, shape='square', zoom=0.9, restore=True)
>>> options.maxcolors = 7
>>> options[-1] = False
>>> options[2] -= 0.1
>>> options
Options(maxcolors=7, shape='square', zoom=0.8, restore=False)

Example #2: with defaults but no validator

>>> Rgb = editabletuple('Rgb', 'red green blue', defaults=(0, 0, 0))
>>> black = Rgb()
>>> black
Rgb(red=0, green=0, blue=0)
>>> navy = Rgb(blue=128)
>>> navy
Rgb(red=0, green=0, blue=128)
>>> violet = Rgb(238, 130, 238)
>>> violet
Rgb(red=238, green=130, blue=238)

Example #3: with defaults and a validator

If you provide a validator function, it will be called whenever an attempt is made to set a value, whether at construction time or later by et[i] = value or et.fieldname = value. It is passed an attribute name and an attribute value. It should check the value and either return the value (or an acceptable alternative value) which will be the one actually set, or raise a ValueError.

>>> def validate_rgba(name, value):
...     if name == 'alpha':
...         if not (0.0 <= value <= 1.0):
...             return 1.0 # silently default to opaque
...     elif not (0 <= value <= 255):
...         raise ValueError(f'color value must be 0-255, got {value}')
...     return value # must return a valid value or raise ValueError
>>>
>>> Rgba = editabletuple('Rgba', 'red', 'green', 'blue', 'alpha',
...                      defaults=(0, 0, 0, 1.0), validator=validate_rgba)
>>> black = Rgba()
>>> black
Rgba(red=0, green=0, blue=0, alpha=1.0)
>>> seminavy = Rgba(blue=128, alpha=0.5)
>>> seminavy
Rgba(red=0, green=0, blue=128, alpha=0.5)
>>> violet = Rgba(238, 130, 238, alpha=2.5) # alpha too big
>>> violet
Rgba(red=238, green=130, blue=238, alpha=1.0)
>>>
>>> color = Rgba(green=99)
>>> color
Rgba(red=0, green=99, blue=0, alpha=1.0)
>>> assert color.green == 99
>>> color.red = 128
>>> assert color[2] == 0
>>> color[2] = 240
>>> assert color[2] == 240
>>> color[-1] = 0.5
>>> color
Rgba(red=128, green=99, blue=240, alpha=0.5)
>>> color[1] = 299
Traceback (most recent call last):
    ...
ValueError: color value must be 0-255, got 299
>>> color.blue = -65
Traceback (most recent call last):
    ...
ValueError: color value must be 0-255, got -65

These examples—and several others—are in the module's function's docstrings.

API

def editabletuple(classname, *fieldnames, defaults=None, validator=None, doc=None):

Creates a new class called classname with the given fieldnames, optional defaults, optional validator, and optional doc docstring.

Instances of the class behave almost exactly like collections.namedtuple's except that fields may be set as well as get using their index position or fieldname. They support len(), in, the comparison operators, and are iterable—which means they can be converted to a list or tuple by passing to either's eponymous factory function. They also provide an .asdict property, and also an update() method that accepts name=value arguments.

def editableobject(classname, *fieldnames, defaults=None, validator=None, doc=None):

Creates a new class called classname with the given fieldnames, optional defaults, optional validator, and optional doc docstring.

Instances of the class have fields which can get and set by fieldname. They support the comparison operators and .astuple and .asdict properties, the former returning a tuple of the instance's values, the latter a dict of fielname-value items. It also has an update() method that accepts name=value arguments.

Notes

I can't work out how to make editabletuple and editableobject instances picklable. Patches or suggestions on how to do this would be welcome.

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License: GPLv3