# einops

Flexible and powerful tensor operations for readable and reliable code. Supports numpy, pytorch, tensorflow, and others.

## Tweets

In case you need convincing arguments for setting aside time to learn about einsum and einops... Tim Rocktรคschel, FAIR

Writing better code with PyTorch and einops ๐ Andrej Karpathy, AI at Tesla

Slowly but surely, einops is seeping in to every nook and cranny of my code. If you find yourself shuffling around bazillion dimensional tensors, this might change your life Nasim Rahaman, MILA (Montreal)

## Contents

- Tutorial
- API micro-reference
- Installation
- Naming
- Why using einops
- Supported frameworks
- Contributing
- Github repository (for issues/questions)

## Tutorial / Documentation

Tutorials are the most convenient way to see `einops`

in action (and right now work as a documentation)

- part 1: einops fundamentals
- part 2: einops for deep learning
- part 3: real code fragments improved with einops (so far only for pytorch)

## Installation

Plain and simple:

`pip install einops`

## API

`einops`

has a minimalistic yet powerful API.

Three operations provided (einops tutorial shows those cover stacking, reshape, transposition, squeeze/unsqueeze, repeat, tile, concatenate, view and numerous reductions)

```
from einops import rearrange, reduce, repeat
# rearrange elements according to the pattern
output_tensor = rearrange(input_tensor, 't b c -> b c t')
# combine rearrangement and reduction
output_tensor = reduce(input_tensor, 'b c (h h2) (w w2) -> b h w c', 'mean', h2=2, w2=2)
# copy along a new axis
output_tensor = repeat(input_tensor, 'h w -> h w c', c=3)
```

And two corresponding layers (`einops`

keeps a separate version for each framework) with the same API.

```
from einops.layers.chainer import Rearrange, Reduce
from einops.layers.gluon import Rearrange, Reduce
from einops.layers.keras import Rearrange, Reduce
from einops.layers.torch import Rearrange, Reduce
from einops.layers.tensorflow import Rearrange, Reduce
```

Layers behave similarly to operations and have the same parameters (with the exception of the first argument, which is passed during call)

```
layer = Rearrange(pattern, **axes_lengths)
layer = Reduce(pattern, reduction, **axes_lengths)
# apply created layer to a tensor / variable
x = layer(x)
```

Example of using layers within a model:

```
# example given for pytorch, but code in other frameworks is almost identical
from torch.nn import Sequential, Conv2d, MaxPool2d, Linear, ReLU
from einops.layers.torch import Rearrange
model = Sequential(
Conv2d(3, 6, kernel_size=5),
MaxPool2d(kernel_size=2),
Conv2d(6, 16, kernel_size=5),
MaxPool2d(kernel_size=2),
# flattening
Rearrange('b c h w -> b (c h w)'),
Linear(16*5*5, 120),
ReLU(),
Linear(120, 10),
)
```

## Naming

`einops`

stands for Einstein-Inspired Notation for operations
(though "Einstein operations" is more attractive and easier to remember).

Notation was loosely inspired by Einstein summation (in particular by `numpy.einsum`

operation).

## Why use `einops`

notation?!

### Semantic information (being verbose in expectations)

```
y = x.view(x.shape[0], -1)
y = rearrange(x, 'b c h w -> b (c h w)')
```

While these two lines are doing the same job in *some* context,
the second one provides information about the input and output.
In other words, `einops`

focuses on interface: *what is the input and output*, not *how* the output is computed.

The next operation looks similar:

`y = rearrange(x, 'time c h w -> time (c h w)')`

but it gives the reader a hint: this is not an independent batch of images we are processing, but rather a sequence (video).

Semantic information makes the code easier to read and maintain.

### More checks

Reconsider the same example:

```
y = x.view(x.shape[0], -1) # x: (batch, 256, 19, 19)
y = rearrange(x, 'b c h w -> b (c h w)')
```

The second line checks that the input has four dimensions, but you can also specify particular dimensions. That's opposed to just writing comments about shapes since comments don't work and don't prevent mistakes as we know

```
y = x.view(x.shape[0], -1) # x: (batch, 256, 19, 19)
y = rearrange(x, 'b c h w -> b (c h w)', c=256, h=19, w=19)
```

### Result is strictly determined

Below we have at least two ways to define the depth-to-space operation

```
# depth-to-space
rearrange(x, 'b c (h h2) (w w2) -> b (c h2 w2) h w', h2=2, w2=2)
rearrange(x, 'b c (h h2) (w w2) -> b (h2 w2 c) h w', h2=2, w2=2)
```

There are at least four more ways to do it. Which one is used by the framework?

These details are ignored, since *usually* it makes no difference,
but it can make a big difference (e.g. if you use grouped convolutions in the next stage),
and you'd like to specify this in your code.

### Uniformity

```
reduce(x, 'b c (x dx) -> b c x', 'max', dx=2)
reduce(x, 'b c (x dx) (y dy) -> b c x y', 'max', dx=2, dy=3)
reduce(x, 'b c (x dx) (y dy) (z dz)-> b c x y z', 'max', dx=2, dy=3, dz=4)
```

These examples demonstrated that we don't use separate operations for 1d/2d/3d pooling, those are all defined in a uniform way.

Space-to-depth and depth-to space are defined in many frameworks but how about width-to-height?

`rearrange(x, 'b c h (w w2) -> b c (h w2) w', w2=2)`

### Framework independent behavior

Even simple functions are defined differently by different frameworks

`y = x.flatten() # or flatten(x)`

Suppose `x`

's shape was `(3, 4, 5)`

, then `y`

has shape ...

- numpy, cupy, chainer, pytorch:
`(60,)`

- keras, tensorflow.layers, mxnet and gluon:
`(3, 20)`

### Independence of framework terminology

Example: `tile`

vs `repeat`

causes lots of confusion. To copy image along width:

```
np.tile(image, (1, 2)) # in numpy
image.repeat(1, 2) # pytorch's repeat ~ numpy's tile
```

With einops you don't need to decipher which axis was repeated:

```
repeat(image, 'h w -> h (tile w)', tile=2) # in numpy
repeat(image, 'h w -> h (tile w)', tile=2) # in pytorch
repeat(image, 'h w -> h (tile w)', tile=2) # in tf
repeat(image, 'h w -> h (tile w)', tile=2) # in jax
repeat(image, 'h w -> h (tile w)', tile=2) # in mxnet
... (etc.)
```

## Supported frameworks

Einops works with ...

## Contributing

Best ways to contribute are

- spread the word about
`einops`

- if you like explaining things, alternative tutorials would be very helpful
- some people grasp
`einops`

ideas immediately, while many others need help-by-example

- some people grasp
- translating examples in languages other than English is also a good idea
- use
`einops`

notation in your papers to strictly define used operations!

## Supported python versions

`einops`

works with python 3.5 or later.