Python method which computes the result only on the first call, without extra parameters

Question

Is there an idiomatic way to implement an "once method", that is: a method whose return value gets evaluated on the first call only? Something like what the following simplified code does for the return value of x:

class X:
    def __init__(self):
        self._x = None

    def x(self):
        if not self._x:
            self._x = some_expensive_call()
        return self._x

Answer 1

As of Python 3.2, you can use the functools.lru_cache decorator (but it may be overkill for what you need):

import functools

@functools.lru_cache(maxsize=1)
def once():
    print("calculating expensive result")
    return "expensive result"

once()
once()

output:

calculating expensive result         # <- only prints on first call
'expensive result'                   # returned value on first call
'expensive result'                   # <- just return value on second call

Alternatively, you can write your own decorator:

def cache_result(func):
    def wrapper(*args, **kwds):
        if not wrapper.cached:
            wrapper.value = func(*args, **kwds)
            wrapper.cached = True

        return wrapper.value

    wrapper.cached = False

    return functools.update_wrapper(wrapper, func)

And use it on any function you want to run only once and cache the result

@cache_result
def do_once():
    print('doing it once')
    return 'expensive result'