What is the correct way to grab weights for before and after comparison after backpropagation?

I am trying to grab an entire model’s weight before training and also after training to check their differences but for some reason pytorch is telling me the old stored weight is the same as the newly stored weights after my optimizer.step()?

My code is below, one thing to notice is I only trained the input with layer1, layer2 should not update it’s weight because it is not used. After 1 forward/backward propagation I should have different weights for layer1 and the activation, layer2’s weight should be untouched.

class mymodel(nn.Module):
    def __init__(self):
        self.layer1 = nn.Linear(10, 5)
        self.layer2 = nn.Conv2d(1, 5, 4, 2, 1)
        self.act = nn.Sigmoid()
    def forward(self, x):
        x = self.layer1(x) #only layer1 and act are used layer 2 is ignored so only layer1 and act's weight should be updated
        x = self.act(x)
        return x
model = mymodel()

weights = []

for param in model.parameters(): # loop the weights in the model before updating and store them

critertion = nn.BCELoss() #criterion and optimizer setup
optimizer = optim.Adam(model.parameters(), lr = 0.001)

foo = torch.randn(3, 10) #fake input
target = torch.randn(3, 5) #fake target

result = model(foo) #predictions and comparison and backprop
loss = criterion(result, target)

weights_after_backprop = [] # weights after backprop
for param in model.parameters():
    weights_after_backprop.append(param) # only layer1's weight should update, layer2 is not used

for i in zip(weights, weights_after_backprop):
    print(torch.equal(i[0], i[1]))

# **prints all Trues when "layer1" and "act" should be different, I have also tried to call param.detach in the loop but I got the same result.


The thing is to reduce memory usage, all optimizer modify the weights inplace.
Which means that the content of your list called weights contains the same tensors that are modified inplace by the optimizer.
You can add a weights.append(param.clone()) to make sure your store a different copy of the weights.