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