How can I run the initialization code for a generator function immediately, rather than at the first call?

Question

I have a generator function that goes something like this:

def mygenerator():
    next_value = compute_first_value() # Costly operation
    while next_value          


        

Answer 1

For my use case I used a modified version of @ncoghlan answer but wrapped in a factory function to decorate the generating function:

import collections, functools, itertools

def primed_generator(generating_function):
    @functools.wraps(generating_function)
    def get_first_right_away_wrapper(*args,**kw):
        "call the generator function, prime before returning"
        gen = generating_function(*args,**kw)
        assert isinstance(gen,collections.Iterator)
        first_value = next(gen)
        return itertools.chain((first_value,),gen)
    return get_first_right_away_wrapper

Then just decorate the function:

@primed_generator
def mygenerator():
    next_value = compute_first_value() # Costly operation
    while next_value != terminating_value:
        yield next_value
        next_value = compute_next_value()

and the first value will be calculated immediately, and the result is transparent.