Advancing Python generator function to just before the first yield [duplicate]

Question

When you instantiate a generator function, it won't execute any code until you call next on it.

It means that if generator function contains some kind of of initialization code, it won't be executed until it's iterated on.

Consider this example:

def generator(filename):
    with open(filename) as f:
        data = f.read()

    while True:
        yield data

gen = generator('/tmp/some_file')
# at this point, no generator code is executed

# initialization code is executed only at the first iteration
for x in gen:
    pass

If the file does not exist, the exception will be raised at the for loop. I'd like the code before the first yield to execute before generator is iterated over, so any exceptions during the initialization will be raised at generator instantiation.

Is there a clean pythonic way to do so?

Answer 1

Wrap a regular function around the generator and put the initialization there:

def file_contents_repeated(filename):
    with open(filename) as f:
        data = f.read()
    return _gen(data)

def _gen(data):
    while True:
        yield data

(In this case, you could replace the inner generator with itertools.repeat, but not in general.)