PyTorch `permute` vs `transpose`: What's the Difference (and the `reshape` Bug That Scrambles Your Images)

A developer explains the critical difference between PyTorch's permute, transpose, and reshape operations, warning that reshape can silently scramble image data when converting from (N, H, W, C) to (N, C, H, W) layout. The post demonstrates that while permute and transpose return views with reordered strides, reshape reinterprets flat memory and can produce the correct shape but incorrect values, leading to corrupted images.

You loaded an image, got a tensor shaped batch, height, width, channels , and your convolution wants batch, channels, height, width . Stack Overflow says permute . Someone else says transpose . And reshape 2, 3, 28, 28 gives you the right shape too — so why is everyone making this complicated? Because two of those three are the same tool, and the third one silently destroys your data. transpose dim0, dim1 swaps exactly two dimensions. permute ... reorders all of them in one call, and you must list every dimension. transpose is a special case of permute . Both return a view — no data is copied, only the strides change — which also means both leave you with a non-contiguous tensor. reshape is not in this family at all. It reinterprets the flat memory under a new shape without moving anything, so it can produce the shape you asked for while completely scrambling what the numbers mean. python import torch t = torch.arange 24 .reshape 2, 3, 4 print t.transpose 0, 1 .shape torch.Size 3, 2, 4 — swapped dims 0 and 1 print t.permute 2, 0, 1 .shape torch.Size 4, 2, 3 — full reorder transpose — swap two axes transpose dim0, dim1 takes two dimension indices and swaps them. Everything else stays put. t = torch.arange 24 .reshape 2, 3, 4 print t.shape torch.Size 2, 3, 4 print t.transpose 0, 1 .shape torch.Size 3, 2, 4 print t.transpose 1, 2 .shape torch.Size 2, 4, 3 The order of the two arguments doesn't matter — t.transpose 0, 1 and t.transpose 1, 0 are the same thing. A swap is a swap. On a 2-D tensor this is the matrix transpose you already know, and .T is the shorthand: m = torch.arange 6 .reshape 2, 3 print m.T.shape torch.Size 3, 2 print m.transpose 0, 1 .shape torch.Size 3, 2 — identical One caution on .T: on tensors with more than two dimensions, .T reverses .mT t = torch.arange 24 .reshape 2, 3, 4 print t.mT.shape torch.Size 2, 4, 3 — swaps the last two dims, batch untouched Reserve plain .T for 2-D matrices. On anything higher, say what you mean with permute or .mT . permute — state the whole new order permute doesn't swap; it rewrites the dimension order in full . You pass the old index of each dimension in the position you want it to end up. t = torch.arange 24 .reshape 2, 3, 4 print t.permute 2, 0, 1 .shape torch.Size 4, 2, 3 Read permute 2, 0, 1 as: "new dim 0 is old dim 2, new dim 1 is old dim 0, new dim 2 is old dim 1." Not "move dim 2 somewhere" — you are writing out the destination order, left to right. Two consequences worth internalising: RuntimeError: number of dims don't match in permute . permute : t = torch.arange 24 .reshape 2, 3, 4 via permute = t.permute 1, 2, 0 torch.Size 3, 4, 2 via transposes = t.transpose 0, 1 .transpose 1, 2 torch.Size 3, 4, 2 print torch.equal via permute, via transposes True Same result. The permute version says the destination shape out loud; the chained version makes the reader simulate two swaps in their head. Prefer permute for anything beyond a single swap. permute is not reshape Here is the bug this article exists for. You have a batch of images in N, H, W, C — the layout you get from OpenCV, PIL, TensorFlow, and most image files — and PyTorch convolutions need N, C, H, W . Both of these produce a tensor of the right shape: img = torch.arange 1 2 2 3 .reshape 1, 2, 2, 3 N=1, H=2, W=2, C=3 print img.permute 0, 3, 1, 2 .shape torch.Size 1, 3, 2, 2 ✅ print img.reshape 1, 3, 2, 2 .shape torch.Size 1, 3, 2, 2 ⚠️ same shape Same shape. No error. No warning. Now look at what's actually inside the first channel: print img.permute 0, 3, 1, 2 0, 0 tensor 0, 3 , 6, 9 ← the red value of each of the 4 pixels ✅ print img.reshape 1, 3, 2, 2 0, 0 tensor 0, 1 , 2, 3 ← pixel 0's R, G, B and then pixel 1's R ❌ The permute version collected the red channel: pixels are stored as R,G,B R,G,B … , so the reds are elements 0, 3, 6, 9. That's a real red channel. The reshape version just took the first four numbers in memory and called them "channel 0" — that's one whole pixel plus a third of the next one. Your "red channel" is now a mixture of red, green and blue from different pixels. The tensor is the right shape, the model trains, the loss goes down a little, and the accuracy is quietly terrible. Nothing on screen ever tells you. The rule: permute movesdimensions. reshape re-cuts thesame flat sequence of numbersinto new brackets. If you want to change what an axismeans, you need permute — always. If you want the full picture on how reshape re-cuts memory, that's covered in Reshape vs View in PyTorch https://dev.to/pytorchfromgroundup/reshape-vs-view-in-pytorch-whats-the-difference-and-when-view-breaks-4a9o . Images — channels-last to channels-first: img = torch.randn 2, 28, 28, 3 N, H, W, C from PIL / OpenCV x = img.permute 0, 3, 1, 2 N, C, H, W what nn.Conv2d wants print x.shape torch.Size 2, 3, 28, 28 Attention — splitting into heads. This is the one place where reshape and permute correctly appear back-to-back, and seeing why makes the distinction click: B, S, E, H = 2, 5, 8, 2 batch, seq len, embed dim, heads x = torch.randn B, S, E torch.Size 2, 5, 8 x = x.reshape B, S, H, E // H torch.Size 2, 5, 2, 4 — split, don't move x = x.permute 0, 2, 1, 3 torch.Size 2, 2, 5, 4 — move heads forward The reshape is legitimate here because it only splits the last axis — the 8 embedding values were already laid out contiguously, so cutting them into 2 groups of 4 doesn't reorder anything. The permute then genuinely moves the head axis in front of the sequence axis. Splitting an axis is reshape work; moving an axis is permute work. transpose and permute never touch the underlying data. They change the tensor's strides — the step size PyTorch takes through memory for each dimension: a = torch.arange 6 .reshape 2, 3 print a.stride , a.is contiguous 3, 1 True b = a.permute 1, 0 print b.shape, b.stride , b.is contiguous torch.Size 3, 2 1, 3 False That's why they're free. It's also why the very next .view you call will blow up: b.view 6 RuntimeError: view size is not compatible with input tensor's size and stride at least one dimension spans across two contiguous subspaces . Use .reshape ... instead. Two fixes, and the choice matters: b.reshape 6 works — copies quietly when it has to b.contiguous .view 6 works — reorders memory first, then views Use .reshape by default. Reach for an explicit .contiguous when you're about to do many operations on the permuted tensor and want the memory laid out well once, up front, rather than copied repeatedly. reshape to convert NHWC → NCHW. permute 0, 3, 1, 2 . permute 2, 0, 1 as instructions to move things. args i . Say it out loud before you trust it. permute needs all of them → RuntimeError: number of dims don't match in permute . .T on 3-D+ tensors. .mT for a batch of matrices, or permute to say it exactly. .view right after a permute. .reshape or .contiguous .view . print x.stride next to print x.shape when a shape bug won't make sense — the strides usually tell you the real story. transpose a, b swaps two dimensions; permute ... rewrites the whole dimension order and needs every index listed. They do the same kind of work, and permute is the one to reach for whenever more than one swap is involved. Both are free view operations that leave the tensor non-contiguous, so the next .view will error — use .reshape . And the part that costs people a weekend: reshape is not a reordering tool. It can hand you a correctly-shaped tensor full of scrambled values, with nothing on screen to warn you. When an axis needs to More in this series: Reshape vs View https://dev.to/pytorchfromgroundup/reshape-vs-view-in-pytorch-whats-the-difference-and-when-view-breaks-4a9o · Broadcasting Explained https://dev.to/pytorchfromgroundup/pytorch-broadcasting-explained-the-3-rules-and-the-silent-bug-that-bites-everyone-3606 · What unsqueeze Does https://dev.to/pytorchfromgroundup/what-does-unsqueeze-do-in-pytorch-and-why-your-model-keeps-asking-for-it-181a · keepdim Does A question for you: what's the shape bug that cost you the most time? I'm collecting the ones that hit hardest — drop it in the comments and I'll write up the fix. This is one idea from my book PyTorch From Ground Up , which builds everything from tensors upward so nothing stays vague. If it helped, you can grab the free PyTorch tensor cheat-sheet here https://payhip.com/b/7ukxh , run every example from the book in the companion notebooks on GitHub https://github.com/pytorch-from-ground-up/book code , get the digital edition on Leanpub https://leanpub.com/pytorchfromgroundup , or find the full paperback on Amazon here https://www.amazon.com/dp/B0H8WMCV33 .