[Maya-Python] StopIteration, where art thou?

[Marcus Ottosson]

I remember having this a while back, but this time it really flabbergasted me.

def iterator():
    yield

it = iterator()
next(it)
next(it)

Or take this one.

next(i for i in range(0))

What should happen is this.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

But instead, nothing. It’s problem when this happens somewhere in your code.

def create_transform():
  transform = cmds.createNode("transform", name="MyTransform")
  next(i for i in range(0))
  return transform

transform = create_transform()
print(transform)
# Error: name 'transform' is not defined
# Traceback (most recent call last):
#   File "<maya console>", line 1, in <module>
# NameError: name 'transform' is not defined #

It’s not defined, because that line never runs. An exception is thrown, silently, stopping the script.

Other exceptions run fine.

1/0
# Error: integer division or modulo by zero
# Traceback (most recent call last):
#   File "<maya console>", line 1, in <module>
# ZeroDivisionError: integer division or modulo by zero #

Normally, you can instantiate an exception yourself, like this.

ZeroDivisionError
# Result: <type 'exceptions.ZeroDivisionError'> #

But guess what happens for StopIteration?

StopIteration
# Error: name 'StopIterationError' is not defined
# Traceback (most recent call last):
#   File "<maya console>", line 1, in <module>
# NameError: name 'StopIterationError' is not defined #

This is in Maya 2015 and 2018, and I bet it happens in other versions too. A native exception is undefined. What gives?

​

[Michael Boon]

Wow! Happens in 2016 too.

However, StopIteration is defined, and I can catch it.

This:

a = iter([1,2])
for i in range(4):
    print i, next(a)

gives

0 1
1 2
2

While this:

a = iter([1,2])
for i in range(4):
    try:
        print i, next(a)
    except StopIteration:
        print 'StopIteration'

gives

0 1
1 2
2 StopIteration
3 StopIteration

[Marcus Ottosson]

Can confirm that it is defined in 2018 as well. The above was in 2015. How peculiar.

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[Justin Israel]

Is this only in Windows? I just tested 2016, 2017, and 2018. They all have StopIteration available.

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[Robert White]

I can see StopIteration just fine in 2018 on windows.

It should live in both __builtins__ and the exceptions module. 

Maybe some other script is messing those up?

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[Michael Boon]

I'm on Windows. To clarify though, it's not about StopIteration being available (except in 2015, apparently). It's about StopIteration being silently caught. This appears to only happen in the Listener. In 2018, at least, if I try it in an imported Python module, it works as expected. I don't know what would happen in external editors.

To add to the confusion, if you catch and raise the exception, it works.

This

i = iter([0,1])
for n in range(3):
    print n, next(i)

fails silently:

0 0
1 1
2

While this

i = iter([0,1])
for n in range(3):
    try:
        print n, next(i)
    except Exception as e:
        raise

shows the unhandled exception:

0 0
1 1
2# Error:

# Traceback (most recent call last):

#   File "<maya console>", line 4, in <module>
# StopIteration #

[Robert White]

Huh, that is super weird.

I'm getting the same behavior. It probably has something to do with how maya is exec-ing the listener. Clearly it is throwing the exception, it just getting swallowed somewhere along the way. Maybe the for loop is grabbing it? execution does stop though.

Really weird.

[Marcus Ottosson]

This appears to only happen in the Listener. In 2018, at least, if I try it in an imported Python module, it works as expected

Not for me, this is what initially bummed me out. My script got imported right, but one of the functions silently threw this exception and refused to continue. This was in 2018 on Windows.

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