Model exported in python loaded in c++ gives different results

I have a PyTorch model that looks like that:

class ResBlock(nn.Module, ABC):
    def __init__(self, in_channels: int, filters: int, conv_num: int):
        super(ResBlock, self).__init__()
        self.filters = filters
        self.conv_num = conv_num

        self.input_conv = torch.nn.Conv1d(in_channels=in_channels, out_channels=filters, kernel_size=1)
        self.inner_conv = torch.nn.Conv1d(in_channels=in_channels, out_channels=filters, kernel_size=3, padding=1)
        self.outer_conv = torch.nn.Conv1d(in_channels=filters, out_channels=filters, kernel_size=3, padding=1)
        self.max_pool = torch.nn.MaxPool1d(kernel_size=2, stride=2)

    def forward(self, x):
        y = x

        for i in range(self.conv_num - 1):
            if i == 0:
                y = self.inner_conv(y)
                y = self.outer_conv(y)

            y = torch.relu(y)

        y = self.outer_conv(y)

        s = self.input_conv(x)
        y = s + y
        y = torch.relu(y)

        return self.max_pool(y)

class Net(nn.Module, ABC):
    def __init__(self, num_of_classes: int):
        super(Net, self).__init__()
        self.block_1 = ResBlock(1, 16, 2)
        self.block_2 = ResBlock(16, 32, 2)
        self.block_3 = ResBlock(32, 64, 3)
        self.block_4 = ResBlock(64, 128, 3)
        self.block_5 = ResBlock(128, 128, 3)

        self.avg_pool = torch.nn.AvgPool1d(kernel_size=3, stride=3)
        self.flatten = torch.nn.Flatten()
        self.dense_1 = torch.nn.Linear(
            in_features=self.block_5.filters * (249 // self.avg_pool.kernel_size[0]),
        self.dense_2 = torch.nn.Linear(in_features=256, out_features=128)
        self.classifier = torch.nn.Linear(in_features=128, out_features=num_of_classes)

    def forward(self, x):
        x = self.block_1(x)
        x = self.block_2(x)
        x = self.block_3(x)
        x = self.block_4(x)
        x = self.block_5(x)

        x = self.avg_pool(x)
        x = self.flatten(x)

        x = self.dense_1(x)
        x = self.dense_2(x)
        x = self.classifier(x)

        return x

I have trained that model in python, acquired ~65% of accuracy on the test set and wanted to transfer it to C++. So here it goes. Exported it like so (of course after training):

# Training code...
jit_model = torch.jit.script(model)'')

So, the next logical step was to import it into the C++:

#include <torch/torch.h>
#include <torch/script.h>

#include "constants.hh"

int main () {
  torch::Device device(torch::cuda::is_available() ? torch::kCUDA : torch::kCPU);

  auto model = torch::jit::load("./");;
  auto test_raw_dataset = CsvDataset(constants::kTestCsv);
  auto test_dataset =<>());
  auto test_data_loader = torch::data::make_data_loader<torch::data::samplers::SequentialSampler>(std::move(test_dataset), torch::data::DataLoaderOptions(constants::kBatchSize));
  size_t correct_count = 0;
  for (const auto& batch : *test_data_loader) {
    auto inputs =;
    auto labels =;

    inputs = inputs.unsqueeze(1);
    labels = labels.squeeze(1);

    auto outputs = model.forward(inputs).toTensor();

    auto prediction = outputs.argmax(1);
    correct_count += prediction.eq(labels).sum().item<int64_t>();

  // Printing metrics...

Running it showed some weird problem. Accuracy on the same test set is now equal to ~12%.
Dataset is the same. It goes into the network in the same order as in python and I’m getting totally different resulsts.

Scripts are running on CPU only.

So I tried to debug it and this is what I’ve found:

  1. All of the predictions when model is loaded into C++ are the same. (Equal to 6)
  2. When tried to load exported model into Python (in the same way as in C++) results are good. Accuracy is equal to the same ~65%.
  3. Summing, calculating predictions are results are the same on each step in C++ and python. Only outputs from the net differs.

And that’s all. Maybe you can see If I’m doing something wrong, or is it error within the libtorch.

Thanks in advance.

Could you explain point 1 and 3 a bit more, please?
What kind of predictions are the same in C++, if the outputs differ?

Sorry it can be a bit misleading:

  1. auto prediction = outputs.argmax(1); in C++ always returns 6 for every sample fed into the network - so it looks like model was not trained, or weight were not updated. But since model was loaded from file, everything should be ok but it is not.
  2. I have just synthetised the outputs tensor, and checked if maybe my code for summing correct count or something is invalid in the C++

Output differs from Python to C++.
But even when I import model in python (which was exported from python) everything works correctly. Problem shows up only if I import model in the C++ application.

Thanks for the explanation.
Could you check the model outputs using a static input, e.g. torch.ones, and see if Python and C++ would still yield different results?

I have checked model loaded in python and in C++ and this the results are different for the static input. I have used torch.ones

Python results:

-0.0281, -0.0559,  0.0320,  0.0885, -0.0396, -0.1131, -0.0559,  0.0181, -0.0240

C++ results:

0.3452  0.7106 -2.0337  1.1069 -1.6179 -2.0724  1.2417 -0.0166  0.2315
[ CPUFloatType{1,9} ]

Skipped metrics, because output of network is different.

I assume you are already calling model.eval()?
If so, you could compare intermediate outputs by e.g. removing layers and try to isolate which layer creates the different results.

Yes, in both cases models are in the eval mode.

Additionally, problem looks even weirder when I export model without training it, then the results in the Python and in the C++ are the same.
So it seems that it has problem with exporting trained weights?
I’ll try to debug it layer by layer then.

//create you net structure
auto net = std::make_shared<Net0>();
//load saved model
torch::jit::script::Module module = torch::jit::load("./");

//You need to transfer the weights from the saved module to the net you are using
for (const auto& pair :module.named_parameters()) {
		if(net->named_parameters().find( != nullptr){
			net->named_parameters().find(>data() = pair.value;
//with out the above for loop the weight of net are not your save weights but the random
//weights pytorch initlizes with you first construct your net