DataLens: The Data Tool That Refused to pip install Anything

Somewhere in the DataLens build, my teammate and I hit the wall every "zero-dependency" project eventually hits: the anomaly detector needed a neural net, and the rulebook said no third-party packages. No NumPy. No pandas. No scikit-learn. Just Python 3.14's standard library. Our first reaction was denial. You cannot build an ANN without a matrix library — everyone knows that. numpy.dot() is basically load-bearing infrastructure for machine learning in Python. We spent an embarrassing amount of time trying to convince ourselves some obscure math submodule secretly did vectorized linear algebra. It doesn't. There is no shortcut. If you want matrix multiplication in pure stdlib Python, you write nested for loops and you like it. What we normally would have installed In any other project, this is a two-second decision: pip install numpy , import it, move on with your life. Matrix ops, broadcasting, vectorized activation functions — all free. Neither of us had ever really had to think about how A @ B works under the hood, because neither of us had ever had to write it ourselves. What it actually took to replace it An autoencoder needs: matrix multiplication, transpose, element-wise activation functions (sigmoid, ReLU), and gradient computation for backprop. Without NumPy, every one of those is a hand-rolled function operating on nested Python lists. Matrix multiply becomes three nested loops instead of one line. A forward pass that would be a single .dot() call turns into a small file of helper functions: matmul() , transpose() , add_bias() , sigmoid() , sigmoid_derivative() . We split it — one of us built the forward pass and activation functions, the other took backprop and the training loop — and then spent a good while debugging the seam where the two met. The genuinely hard part wasn't the math — it was performance. Pure Python loops over lists of lists are slow, and profiling a dataset with a few thousand rows through even a small autoencoder made that obvious fas