# Use of getattr in forward function

**URL:** <https://discuss.pytorch.org/t/use-of-getattr-in-forward-function/46755>\
**Category:** Uncategorized\
**Created:** [May 31, 2019, 4:12pm UTC](https://discuss.pytorch.org/t/use-of-getattr-in-forward-function/46755 "2019-05-31T16:12:04Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![nullgeppetto](https://discuss.pytorch.org/user_avatar/discuss.pytorch.org/nullgeppetto/32/40331_2.png) [@nullgeppetto](https://discuss.pytorch.org/u/nullgeppetto)\
**Post date:** [May 31, 2019, 4:12pm UTC](https://discuss.pytorch.org/t/use-of-getattr-in-forward-function/46755/1 "2019-05-31T16:12:04Z")

</div>

I am trying to use `getattr` in the `forward` method an `nn.Module` class. A minimalistic indicative example of what I’m trying to do is shown below:

```auto
class Net(nn.Module):
    def __init__ (self, ):
        super(). __init__ ()
        self.layer = nn.Linear(8, 16)
    def forward(self, x):
        return getattr(self, 'layer')(x)

```

I get a `NotImplementedError` when I use `forward`:

```auto
File "/usr/lib/python3.7/site-packages/torch/nn/modules/module.py", line 85, in forward
    raise NotImplementedError
NotImplementedError

```

Any ideas on how to solve it? Or even an alternative way to “create” the name of an attribute of `Net` inside `forward`? Thank you!

---

<div class="post-metadata">

**Author:** ![ptrblck](https://discuss.pytorch.org/user_avatar/discuss.pytorch.org/ptrblck/32/1823_2.png) [@ptrblck](https://discuss.pytorch.org/u/ptrblck)\
**Post date:** [May 31, 2019, 4:34pm UTC](https://discuss.pytorch.org/t/use-of-getattr-in-forward-function/46755/2 "2019-05-31T16:34:03Z")

</div>

Your code works on my machine. Make sure do don’t have any typos in the definition of `forward` and also check the indentation.

---

<div class="post-metadata">

**Author:** ![nullgeppetto](https://discuss.pytorch.org/user_avatar/discuss.pytorch.org/nullgeppetto/32/40331_2.png) [@nullgeppetto](https://discuss.pytorch.org/u/nullgeppetto)\
**Post date:** [May 31, 2019, 5:48pm UTC](https://discuss.pytorch.org/t/use-of-getattr-in-forward-function/46755/3 "2019-05-31T17:48:39Z")

</div>

Hi @ptrblck, thanks for your prompt response. The truth is that I tried to make it too simple without testing it. The above indeed works, but what I truly need is something like this:

```auto
class Net(nn.Module): 
     def __init__ (self, ): 
         super(). __init__ ()
         setattr(self, 'layer', nn.ModuleDict({'a': nn.Linear(8, 16), 'b': nn.Linear(16, 4)})) 
     def forward(self, x): 
         return getattr(self, 'layer')(x)

```

I just tried it and indeed it raises a `NotImplementedError`. It seems that the use of `nn.ModuleDict` causes the problem, since simple nn modules work fine.

Any ideas on how to fix it? Many thanks 🙂

---

<div class="post-metadata">

**Author:** ![ptrblck](https://discuss.pytorch.org/user_avatar/discuss.pytorch.org/ptrblck/32/1823_2.png) [@ptrblck](https://discuss.pytorch.org/u/ptrblck)\
**Post date:** [May 31, 2019, 6:21pm UTC](https://discuss.pytorch.org/t/use-of-getattr-in-forward-function/46755/4 "2019-05-31T18:21:56Z")

</div>

You are currently trying to call the `nn.ModuleDict`, which won’t work, as you would have to address one of the layers:

```python
class Net(nn.Module): 
     def __init__ (self, ): 
         super(). __init__ ()
         setattr(self, 'layer', nn.ModuleDict({'a': nn.Linear(8, 16), 'b': nn.Linear(16, 4)})) 
     def forward(self, x): 
         x = getattr(getattr(self, 'layer'), 'a')(x)
         x = getattr(getattr(self, 'layer'), 'b')(x)
         return x

```

What is your use case, if I may ask?

---

<div class="post-metadata">

**Author:** ![nullgeppetto](https://discuss.pytorch.org/user_avatar/discuss.pytorch.org/nullgeppetto/32/40331_2.png) [@nullgeppetto](https://discuss.pytorch.org/u/nullgeppetto)\
**Post date:** [June 3, 2019, 3:16pm UTC](https://discuss.pytorch.org/t/use-of-getattr-in-forward-function/46755/5 "2019-06-03T15:16:24Z")

</div>

Hi @ptrblck, many thanks for your response. It should be obvious and I should have figured it out…

The reason for doing so is that I want my network to have a basic backbone sub-network, and a number of branches (heads). Each head is implemented as an `nn.ModuleDict`, or even `nn.ModuleList` for convenience. During forward, an `id` for which head should be used is given (as a string). If you think that’s not a good practice, please let me know.