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

You can create a "preprimed" iterator fairly easily by using itertools.chain:

from itertools import chain

def primed(iterable):
    """Preprimes an iterator so the first value is calculated immediately
       but not returned until the first iteration
    """
    itr = iter(iterable)
    try:
        first = next(itr)  # itr.next() in Python 2
    except StopIteration:
        return itr
    return chain([first], itr)

>>> def g():
...     for i in range(5):
...         print("Next called")
...         yield i
...
>>> x = primed(g())
Next called
>>> for i in x: print(i)
...
0
Next called
1
Next called
2
Next called
3
Next called
4