Trigger except clause in generator from exception raised in user code

Question

I have a series of nested generators, and I would like to know from the first generator if an exception ocurred in the user code, for the sake of an example, consider the code below:

#############################################################################
def generator():
    try:
        for i in (1, 2, 3, 4, 5, 6):
            print(f"Generator: {i}.")
            yield i
    except:
        print("Exception handled in generator")
        raise

#############################################################################
def intermediary_generator():
    try:
        gen = generator()
        while i := gen.send(None):
            print(f"Intermediary generator: {i}.")
            yield i
    except StopIteration:
        pass
    except:
        print ("Exception handled in intermediary generator")
        raise

############################################################################
user_code_generator = intermediary_generator()
try:
    while i := user_code_generator.send(None):
        print(f"User code generator: {i}.")
        if i == 4:
            raise Exception("The exception in the user code")
except StopIteration:
    pass
except:
    print("Exception handled in user code generator")
    raise

I need the exception in the user code to propagate down to the intermediary and main generator, I was expecting the following sequence as per the print statements:

Exception handled in generator
Exception handled in intermediary generator
Exception handled in user code generator

But if I execute the code above I do not see the exception handled in the generator or intermediary generator.

In the code you are sharing, only `"The exception in the user code"` is being raised, no errors in `generator` or `intermediary_generator`. If you force an exception there, you would see the full print stack — ALai, Apr 27 '22 at 08:40
Yes, that is correct. For clarity, I need to execute teardown functionality on the except clause of "generator" when the exception is raised in the user code. I need that to trigger somehow or perhaps some pointers on how to handle this scenario. — Mite, Apr 27 '22 at 08:41

score 2 · Accepted Answer · answered Apr 27 '22 at 08:56

I think I will answer my own question, the throw method of a generator does exactly what I need, which is propagating the exception in the user code down to the main generator.

#############################################################################
def generator():
    try:
        for i in (1, 2, 3, 4, 5, 6):
            print(f"Generator: {i}.")
            yield i
    except:
        print("Exception handled in generator")
        raise

#############################################################################
def intermediary_generator():
    try:
        gen = generator()
        while i := gen.send(None):
            print(f"Intermediary generator: {i}.")
            yield i
    except StopIteration:
        print("Stop iteration in intermediary generator")
    except Exception as exc:
        print ("Exception handled in intermediary generator")
        gen.throw(exc)
        raise

############################################################################
user_code_generator = intermediary_generator()
try:
    while i := user_code_generator.send(None):
        print(f"User code generator: {i}.")
        if i == 4:
            raise Exception("The exception in the user code")
except StopIteration:
    pass
except Exception as exc:
    print("Exception handled in user code generator")
    user_code_generator.throw(exc)
    raise

And the output:

Generator: 1.
Intermediary generator: 1.
User code generator: 1.
Generator: 2.
Intermediary generator: 2.
User code generator: 2.
Generator: 3.
Intermediary generator: 3.
User code generator: 3.
Generator: 4.
Intermediary generator: 4.
User code generator: 4.
Exception handled in user code generator
Exception handled in intermediary generator
Exception handled in generator
Traceback (most recent call last):
  File "<string>", line 39, in <module>
  File "<string>", line 25, in intermediary_generator
  File "<string>", line 9, in generator
  File "<string>", line 20, in intermediary_generator
  File "<string>", line 34, in <module>
Exception: The exception in the user code

Trigger except clause in generator from exception raised in user code

1 Answers1