Code snippets

cached_property.py

"""
Is analogous to @reify
Borrowed from here: https://github.com/aio-libs/yarl/blob/master/yarl/__init__.py

Topic references:
  https://www.reddit.com/r/learnpython/comments/2oqusj/pyramids_reify_descriptor_could_it_be_simplified/
  http://www.gghh.name/dibtp/2013/06/18/caching-in-python-with-a-descriptor-and-a-decorator.html
"""


class cached_property:
    """Use as a class method decorator.  It operates almost exactly like
    the Python `@property` decorator, but it puts the result of the
    method it decorates into the instance dict after the first call,
    effectively replacing the function it decorates with an instance
    variable.  It is, in Python parlance, a data descriptor.
    """

    def __init__(self, wrapped):
        self.wrapped = wrapped
        try:
            self.__doc__ = wrapped.__doc__
        except:  # pragma: no cover
            self.__doc__ = ""
        self.name = wrapped.__name__

    def __get__(self, inst, owner, _sentinel=sentinel):
        if inst is None:
            return self
        val = inst._cache.get(self.name, _sentinel)
        if val is not _sentinel:
            return val
        val = self.wrapped(inst)
        inst._cache[self.name] = val
        return val

    def __set__(self, inst, value):
        raise AttributeError("cached property is read-only")

gzip_file.py

import gzip


def gzip_file(file_name):
    """GZIP compress an existing file."""
    file_gz = file_name + '.gz'
    with open(file_name, 'rb') as f_in, gzip.open(file_gz, 'wb') as f_out:
        shutil.copyfileobj(f_in, f_out)
    return file_gz

handle_keyboard_interrupt.py

# -*- coding: utf-8 -*-

"""
Handle keyboard interrupt (Ctrl+C) without tracebacks.
"""

import signal
import sys
import time


def main():
    for i in range(10**6):
        print(i)
        time.sleep(1)


if __name__ == '__main__':
    signal.signal(signal.SIGINT, lambda signal, frame: sys.exit(0))
    main()

singleton_class_decorator.py

# -*- coding: utf-8 -*-

"""
Singleton implementation via a class decorator.
Source: http://intermediatepythonista.com/metaclasses-abc-class-decorators
"""


def singleton(cls):
    instances = {}

    def get_instance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]

    return get_instance


@singleton
class Foo(object):
    pass


if __name__ == '__main__':
    x = Foo()
    print(id(x))           # e.g. 4310648144

    y = Foo()
    print(id(y))           # 4310648144

    print(id(x) == id(y))  # True