To facilitate deep learning project development, some popular platforms provide model (sub)packages for developers to import and instantiate a deep learning model with few lines of code. For example, PyTorch provides \texttt{torchvision.models} for developers to instantiate models such as VGG and ResNet. Although those model packages are easy to install and use, their integrity may not be well-protected locally. In this paper, we show that an adversary can manipulate the \texttt{.py} files in the developers' locally installed model packages, if the developers install the adversary's PyPI package for using its claimed features. When installing the adversary's package, the system does not report any warning or error related to the manipulation. Leveraging this integrity vulnerability, we design an attack to manipulate the model forward function in the local \texttt{.py} files, such as \texttt{resnet.py} in the local \texttt{torchvision.models} subpackage. With our attack, the adversary can implant a backdoor into the developers' trained model weights, even supposing that the developers use seemingly clean training data and seemingly normal training code.