[Bug] Inconsistency between HybridBlock and Block

[sxjscience]

There are lots of operators that have inconsistent behaviors between hybridized and not-hybridized versions.

import mxnet as mx
from mxnet.gluon import HybridBlock

class Foo2(HybridBlock):
    def hybrid_forward(self, F, a):
        return a[0]


b1 = Foo2(prefix='hybridized')
b1.hybridize()
b2 = Foo2(prefix='no_hybrid')

out1 = b1(mx.nd.ones((10,)))
out2 = b2(mx.nd.ones((10,)))
print(out1)
print(out2)

[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
<NDArray 10 @cpu(0)>

[1.]
<NDArray 1 @cpu(0)>

Even if we use the numpy interface, the following code cannot run:

import mxnet as mx
from mxnet.gluon import HybridBlock
mx.npx.set_np()


class Foo2(HybridBlock):
    def hybrid_forward(self, F, a):
        return a[0]


b1 = Foo2(prefix='hybridized')
b1.hybridize()
b2 = Foo2(prefix='no_hybrid')

out1 = b1(mx.np.ones((10,)))
out2 = b2(mx.np.ones((10,)))
print(out1)
print(out2)

      6 class Foo2(HybridBlock):
      7     def hybrid_forward(self, F, a):
----> 8         return a[0]
      9 
     10 

~/mxnet/python/mxnet/symbol/numpy/_symbol.py in __getitem__(self, key)
     49         num_outputs = _num_outputs(self)
     50         if num_outputs == 1:
---> 51             raise NotImplementedError
     52         if not isinstance(key, int):
     53             raise NotImplementedError

NotImplementedError: 

[mxnet-label-bot]

Hey, this is the MXNet Label Bot.
Thank you for submitting the issue! I will try and suggest some labels so that the appropriate MXNet community members can help resolve it.
Here are my recommended label(s): Bug

[reminisce]

The problem is that ndarray indexing is not hybridizable. And more general, old symbol's __getitem__ semantics is not consistent with ndarray's. There is a PR trying to address this problem in numpy module, but needs more diving-deep review.
#15905

[sxjscience]

@reminisce Thanks, we should fix these problems in the new numpy interface (also add the checks).

[reminisce]

@sxjscience Agree. It's in the scope. 15905 tries to address the problem by differentiating indexing a list of ndarrays and a single ndarray. I'm afraid there may still be pitfalls/inconsistencies/incompatibility we have not considered. If you have time, could you review that PR, or we can discuss offline as well.

[sxjscience]

Sure, I’ll review that. Also, I think we should keep a list to track the inconsistent behaviors so the user would be aware of that. Get Outlook for iOS<https://aka.ms/o0ukef>

…

________________________________ From: reminisce <notifications@github.com> Sent: Wednesday, September 25, 2019 9:43:49 PM To: apache/incubator-mxnet <incubator-mxnet@noreply.github.com> Cc: Xingjian SHI <xshiab@connect.ust.hk>; Mention <mention@noreply.github.com> Subject: Re: [apache/incubator-mxnet] [Bug] Inconsistency between HybridBlock and Block (#16279) @sxjscience<https://github.com/sxjscience> Agree. It's in the scope. 15905 tries to address the problem by differentiating indexing a list of ndarrays and a single ndarray. I'm afraid there may still be pitfalls/inconsistencies/incompatibility we have not considered. If you have time, could you review that PR, or we can discuss offline as well. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub<#16279>, or mute the thread<https://github.com/notifications/unsubscribe-auth/ABHQH3VLV2UMBVPDVTILRI3QLQ4YLANCNFSM4I2TBU4Q>.

[samskalicky]

@zachgk assign [@sxjscience ]

[sxjscience]

This will be solved once #16621 is merged. We have the following example:

import mxnet as mx
from mxnet.gluon import HybridBlock
mx.npx.set_np()

class Foo2(HybridBlock):
    def hybrid_forward(self, F, a):
        return a[0]


b1 = Foo2(prefix='hybridized')
b1.hybridize()
b2 = Foo2(prefix='no_hybrid')

out1 = b1(mx.np.ones((10,)))
out2 = b2(mx.np.ones((10,)))
print(out1)
print(out2)

Out:

1.0
1.0

[szha]

To clarify, this inconsistency still exists in the NDArray and Symbol implementation and it's considered as "won't fix".