How can I tell whether a generator was just-started?

Question

I\'d like a function, is_just_started, which behaves like the following:

>>> def gen(): yield 0; yield 1
>>> a = gen()
>>         


        

Answer 1

You may create a iterator and set the flag as the instance property to iterator class as:

class gen(object):
    def __init__(self, n):
        self.n = n
        self.num, self.nums = 0, []
        self.is_just_started = True  # Your flag

    def __iter__(self):
        return self

    # Python 3 compatibility
    def __next__(self):
        return self.next()

    def next(self):
        self.is_just_started = False  # Reset flag with next
        if self.num < self.n:
            cur, self.num = self.num, self.num+1
            return cur
        else:
            raise StopIteration()

And your value check function would be like:

def is_just_started(my_generator):
    return my_generator.is_just_started

Sample run:

>>> a = gen(2)

>>> is_just_started(a)
True

>>> next(a)
0
>>> is_just_started(a)
False

>>> next(a)
1
>>> is_just_started(a)
False

>>> next(a)
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 19, in next
StopIteration

To know the difference between iterator and generator, check Difference between Python's Generators and Iterators