lazy-string

lazy-string

Python library for defining strings with delayed evaluation.

|pypi_package| |python_versions| |license|

The package provides a LazyString class. Its constructor accepts a callable (say, a function) which will be called when string's value is needed. The constructor also allows to specify positional and keyword arguments for that callable:

.. code-block:: python

def init(self, func: Callable[..., str], *args: Tuple, **kwargs: Mapping) -> None: ...

The value is re-evaluated on every access.

Installation

Available as a PyPI <https://pypi.python.org/pypi/lazy-string>_ package:

.. code-block:: bash

pip install lazy-string

Usage

Using with a function having no parameters:

.. code-block:: python

from lazy_string import LazyString

def make_foo() -> str: return "foo"

s = LazyString(make_foo)

The value is evaluated on demand:

.. code-block:: python

s + " bar" 'foo bar'

str(s) 'foo'

Representation explicitly tells it's a LazyString:

.. code-block:: python

s LazyString('foo')

It's safe to pass standard strings, as they will be returned as-is:

.. code-block:: python

LazyString("foo bar") 'foo bar'

Supports methods of standard strings:

.. code-block:: python

s.upper() 'FOO'

"f" in s True

dir(s) ['add', 'class', 'contains', 'delattr', 'dir', 'doc', 'eq', 'format', 'ge', 'getattribute', 'getitem', 'getnewargs', 'gt', 'hash', 'init', 'init_subclass', 'iter', 'le', 'len', 'lt', 'mod', 'mul', 'ne', 'new', 'reduce', 'reduce_ex', 'repr', 'rmod', 'rmul', 'setattr', 'sizeof', 'str', 'subclasshook', 'capitalize', 'casefold', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'format_map', 'index', 'isalnum', 'isalpha', 'isascii', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

Supplying parameters for the callable:

.. code-block:: python

def make_foo(arg1, arg2): return f"foo {arg1} {arg2}"

s = LazyString(make_foo, 123, arg2=456)

.. code-block:: python

str(s) 'foo 123 456'

Implementation Details

LazyString is inherited from collections.UserString <https://docs.python.org/3/library/collections.html#collections.UserString>_.

.. code-block:: python

LazyString.mro (<class 'lazy_string.LazyString'>, <class 'collections.UserString'>, <class 'collections.abc.Sequence'>, <class 'collections.abc.Reversible'>, <class 'collections.abc.Collection'>, <class 'collections.abc.Sized'>, <class 'collections.abc.Iterable'>, <class 'collections.abc.Container'>, <class 'object'>)

Serialization

Pickling ^^^^^^^^

Supported out of the box:

.. code-block:: python

import pickle s == pickle.loads(pickle.dumps(s)) True

To JSON ^^^^^^^

Supported with any encoder able to encode collections.UserString:

.. code-block:: python

import json import collections

class JSONEncoder(json.JSONEncoder):

def default(self, o):
  if isinstance(o, collections.UserString):
    return str(o)
  return super().default(o)

.. code-block:: python

data = {'s': s} json.dumps(data, cls=JSONEncoder) '{"s": "foo"}'

.. |pypi_package| image:: https://img.shields.io/pypi/v/lazy-string :target: http://badge.fury.io/py/lazy-string/ :alt: Version of PyPI package

.. |python_versions| image:: https://img.shields.io/badge/Python-3.7+-brightgreen.svg :alt: Supported versions of Python

.. |license| image:: https://img.shields.io/badge/license-MIT-blue.svg :target: https://github.com/oblalex/lazy-string/blob/main/LICENSE :alt: MIT license