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I'm trying to implement an iterable class for the h5 dataset.

class Argh():
    def __init__(self, data):
        self.data = data
        self.c_idx = 0 

    def __getitem__(self, idx):
        return self.data[idx]

    def __len__(self):
        return len(self.data)

    def __next__(self):
        try:
            x = self.data[self.c_idx]
        except IndexError or ValueError:
            raise StopIteration
        self.c_idx += 1
        return x

    def __iter__(self):
        return self

When I try to go through it as through sequence it fails to raise StopIteration exception before ValueError or IndexError occures. This doesn't happen if I just iterate through the h5 dataset directly or if I use iteration by indexing and getitem:

with h5py.File('test.h5', 'w') as f:
     f.create_dataset(name='matrix', data=np.arange(10), dtype=np.float32)

f =  h5py.File('test.h5', 'r')
A = Argh(np.arange(0,10))
B = Argh(f['matrix'])

for x in A: pass
for x in B.data: pass
for i in range(len(B)): B[i]
for x in f['matrix']: pass
for x in B: pass

ValueError  Traceback (most recent call last)
<ipython-input-7-1dcb814e7a79> in <module>
      3 for i in range(len(B)): B[i]
      4 for x in f['matrix']: pass
----> 5 for x in B: pass
ValueError: Index (10) out of range (0-9)

I've tested this on several other objects including zarr arrays but observed this behavior only for h5py opened h5 datasets.

Sinbud
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  • `IndexError or ValueError` is an expression that evaluates to `IndexError`. The syntax for matching multiple exception types is `except IndexError, ValueError:`. – chepner May 27 '20 at 15:06
  • Thanks, indeed it worked! (although I had to add ( ) on python 3.6). Still, it would be nice to understand, why many other objects worked before this fix but h5 dataset didn't? – Sinbud May 27 '20 at 15:10
  • I find it odd that the `h5` dataset is causing a `ValueError` to be raised in order to report an indexing error; the other objects are better "behaved" and raise an `IndexError`, which is the only type of exception you are actually catching with the `or` syntax. – chepner May 27 '20 at 15:11
  • Alright, thanks! Now I will look for similar issues on their github page and maybe open one about changing `ValueError` exception to conventional (if there is no good reason to keep it as it is). – Sinbud May 27 '20 at 15:17

1 Answers1

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IndexError or ValueError is an expression that evaluates to IndexError. The syntax for matching multiple exception types is

except (IndexError, ValueError):
chepner
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