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()
>>> 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 "<stdin>", line 1, in <module>
StopIteration
>>> is_just_started(a)
False

How can I implement this function?

I looked at the .gi_running attribute but it appears to be used for something else.

If I know the first value that needs to be sent into the generator, I can do something like this:

def safe_send(gen, a):
    try:
        return gen.send(a)
    except TypeError as e:
        if "just-started" in e.args[0]:
            gen.send(None)
            return gen.send(a)
        else:
            raise

However, this seems abhorrent.

Answer 1

This only works in Python 3.2+:

>>> def gen(): yield 0; yield 1
... 
>>> a = gen()
>>> import inspect
>>> inspect.getgeneratorstate(a)
'GEN_CREATED'
>>> next(a)
0
>>> inspect.getgeneratorstate(a)
'GEN_SUSPENDED'
>>> next(a)
1
>>> inspect.getgeneratorstate(a)
'GEN_SUSPENDED'
>>> next(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> inspect.getgeneratorstate(a)
'GEN_CLOSED'

So, the requested function is:

import inspect

def is_just_started(gen):
    return inspect.getgeneratorstate(gen) == inspect.GEN_CREATED:

Out of curiosity, I looked into CPython to figure out how it was determining this... Apparently it looks at generator.gi_frame.f_lasti which is the "index of last attempted instruction in bytecode". If it's -1 then it hasn't started yet.

Here's a py2 version:

def is_just_started(gen):
    return gen.gi_frame is not None and gen.gi_frame.f_lasti == -1

Answer 2

Make a new generator which simply yields from your generator of interest. It sets a flag once the first value has been consumed. Afterwards, it can simply use yield from for the rest of the items.

Use the substitute generator as a drop in replacement for the generator you're interested in monitoring the "is_just_started" state.

This technique is non-intrusive, and can be used even on generators for which you have no control over the source code.

Answer 3

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 "<stdin>", line 1, in <module>
  File "<stdin>", line 19, in next
StopIteration

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