prefab-classes 0.9.1

Boilerplate Generator for Classes

PrefabClasses - Python Class Boilerplate Generator

Writes the class boilerplate code so you don't have to. Yet another variation on attrs/dataclasses.

Either written lazily when you first access the methods or eagerly when the class is compiled into a .pyc. Can optionally be made to rewrite .py source code with plain classes.

The dynamic method of evaluating lazily is more flexible, while the compiled method is faster (once the .pyc file has been generated).

For more detail look at the documentation.

Usage

Define the class using plain assignment and attribute function calls:

from prefab_classes import prefab, attribute

@prefab
class Settings:
    hostname = attribute(default="localhost")
    template_folder = attribute(default='base/path')
    template_name = attribute(default='index')

Or with type hinting:

from prefab_classes import prefab

@prefab
class Settings:
    hostname: str = "localhost"
    template_folder: str = 'base/path'
    template_name: str = 'index'

In either case the result behaves the same.

>>> from prefab_classes.funcs import to_json
>>> s = Settings()
>>> print(s)
Settings(hostname='localhost', template_folder='base/path', template_name='index')
>>> to_json(s)
'{"hostname": "localhost", "template_folder": "base/path", "template_name": "index"}'

For further details see the usage pages in the documentation.

Why not just use attrs/dataclasses?

If attrs or dataclasses solves your problem then you should use them. They are thoroughly tested, well supported packages. This is a new project and has not had the rigorous real world testing of either of those.

Initially this was just an investigation into how these modules worked, but evolved into a more performance oriented version of the same idea. I don't think that writing classes like this should impose a significant performance penalty over writing the classes by hand.

For the short answer, here are the start and import times of various modules on my development computer, with python -c "pass" as a baseline.

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
python -c "pass"	26.6 ± 1.2	25.0	30.6	1.00
python -c "import prefab_classes"	28.4 ± 0.4	27.6	29.5	1.07 ± 0.05
python -c "import dataclasses"	48.4 ± 1.0	46.5	51.5	1.82 ± 0.09
python -c "import attrs"	67.3 ± 0.7	65.9	71.1	2.53 ± 0.11
python -c "import pydantic"	105.4 ± 3.4	100.3	115.7	3.96 ± 0.22

This difference in import time means prefab_classes can generate approximately 75 simple classes with all of their methods before dataclasses has finished importing.

dataclasses_example.py

import dataclasses

prefab_example.py

from prefab_classes import build_prefab, attribute


for i in range(75):
    p = build_prefab(
        f"Prefab{i}",
        [
            ('a', attribute()),
            ('b', attribute()),
            ('c', attribute()),
            ('d', attribute()),
            ('e', attribute()),
        ],
    )
    # Force method generation
    p.__init__, p.__repr__, p.__eq__

hyperfine -w 3 -r 100 --export-markdown result.md 'python dataclasses_example.py' 'python prefab_example.py'

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
python dataclasses_example.py	47.0 ± 2.0	43.5	53.0	1.01 ± 0.06
python prefab_example.py	46.4 ± 1.8	43.4	50.5	1.00

For more detailed tests you can look at the performance section of the docs.

How does it work

The @prefab decorator either rewrites the class dynamically, putting methods in place that will be generated as they are first accessed OR it acts as a marker to indicate the class should be transformed for the compiled classes.

Compiled classes can both be imported directly or converted back to new .py files. Direct import will perform the conversion before creating the .pyc file.

example.py

# COMPILE_PREFABS
from prefab_classes import prefab, attribute
from pathlib import Path


@prefab(compile_prefab=True)
class SettingsPath:
    hostname = attribute(default="localhost")
    template_folder = attribute(default='base/path')
    template_name = attribute(default='index')
    file_types = attribute(default_factory=list)

    def __prefab_post_init__(self, template_folder, file_types):
        self.template_folder = Path(template_folder)
        file_types.extend(['.md', '.html'])
        self.file_types = file_types

Direct import using prefab_compiler

from prefab_classes_hook import prefab_compiler

with prefab_compiler():
    from example import SettingsPath

# Use normally from here

Compile to a new .py file using rewrite_to_py:

>>> from prefab_classes.compiled import rewrite_to_py
>>> rewrite_to_py('example.py', 'example_compiled.py', use_black=True)

Using black to format for ease of reading.

example_compiled.py

# DO NOT MANUALLY EDIT THIS FILE
# MODULE: example_compiled.py
# GENERATED FROM: example.py
# USING prefab_classes VERSION: v0.9.1

from pathlib import Path


class SettingsPath:
    COMPILED = True
    PREFAB_FIELDS = ["hostname", "template_folder", "template_name", "file_types"]
    __match_args__ = ("hostname", "template_folder", "template_name", "file_types")

    def __init__(
        self,
        hostname="localhost",
        template_folder="base/path",
        template_name="index",
        file_types=None,
    ):
        self.hostname = hostname
        self.template_name = template_name
        file_types = file_types if file_types is not None else list()
        self.__prefab_post_init__(
            template_folder=template_folder, file_types=file_types
        )

    def __repr__(self):
        return f"{type(self).__qualname__}(hostname={self.hostname!r}, template_folder={self.template_folder!r}, template_name={self.template_name!r}, file_types={self.file_types!r})"

    def __eq__(self, other):
        return (
            (self.hostname, self.template_folder, self.template_name, self.file_types)
            == (
                other.hostname,
                other.template_folder,
                other.template_name,
                other.file_types,
            )
            if self.__class__ == other.__class__
            else NotImplemented
        )

    def __prefab_post_init__(self, template_folder, file_types):
        self.template_folder = Path(template_folder)
        file_types.extend([".md", ".html"])
        self.file_types = file_types

If compile_plain=True is provided as an argument to @prefab the COMPILED and PREFAB_FIELD variables will not be set on the class.

Credit

autogen function and some magic method definitions taken from David Beazley's Cluegen

General design based on previous experience using dataclasses and attrs and trying to match the requirements for PEP 681.