Julia package for reading and writing .npy files
Also bump version in Project.toml
Bump version to 1.1.0
Update Copyright notices



You can also use your local clone with git send-email.

#Npy.jl - read and write .npy files in Julia

Npy.jl is a Julia package which implements reading and writing of a subset of the .npy file format. Files are read and written using persistent memory mapping. As a result, very large files (larger than main memory) are supported.

Files with one of the following datatypes are supported: Bool, Int8, Int16, Int32, Int64, UInt8, UInt16, UInt32, UInt64, Float32, Float64, Complex{Float32}, Complex{Float64}.

Test status: builds.sr.ht status

The NpyArray data type which represents a .npy file conforms to the conventional AbstractArray interface. This means that the files can be used and manipulated more or less like a regular Array. However, it is not recommended to use them directly in performance-critical calculations.


  • Create (or overwrite) a .npy file with data from a regular Array, then force the changes to disk immediately.
julia> using Npy: NpyArray, sync!

julia> npy_arr = NpyArray("abc.npy", Float64[23.0 π; NaN exp(-42)])
2×2 NpyArray{Float64,2}:
  23.0  3.14159    
 NaN    5.74952e-19

julia> sync!(npy_arr)
  • First create a file, fill it with ascending integers and alternating imaginary part, then close it. Afterwards, open it again, and compute the sum of the entries.
julia> using Npy: NpyArray

julia> NpyArray("def.npy", Complex{Float32}, (4, 5, 7)) do arr
         arr .= reshape(map(i -> ComplexF32(i)+(-im)^i, 1:length(arr)), size(arr))

julia> sum(NpyArray("def.npy")) == (4*5*7) * (4*5*7+1) / 2

For more information on how to create or read an NpyArray, see its Julia docstring.


Npy.jl does not and will not support the following:

  • compressed archives (file ending ".npz") - they are impossible to mmap efficiently.
  • bytestrings, unicode strings, objects - they are impossible to mmap efficiently.
  • 'host endianness' ("=" in descr) - it is a terrible idea to use this for data storage.


Npy.jl is written in Julia 1.0 and has no third-party dependencies (except for testing). Npy.jl follows semanting versioning v2.0.0. The current version is 1.1.0.

#Bugs and caveats

  • Npy.jl does not support writing .npy version 2.0 or 3.0. This is not expected to be a problem as long as the data size does not reach approximately 10^6500 bytes (or the data files have more than approximately 2900 dimensions).
  • Creating .npy files from an AbstractArray with non-standard axes is neither supported nor tested and may silently eat your data.
  • Npy.jl should work on big-endian architectures, but this has not been tested.
  • Using linear indices with C contiguous (row major) NpyArrays is inefficient due to limitations of the Julia language.
  • "Cold" performance is a bit slow: Opening an NpyArray for the first time takes around one second (on a slow VPS). Opening another NpyArray afterwards takes one or two hundred microseconds.
  • There are no performance benchmarks available. To give a rough impression: On the author's laptop, reading a 10 GB file (with Float64 entries and shape (50, 125, 200, 1000)) and summing its entries takes around 25 s when Julia was just started and the file is not cached; and 9.8 s when all relevant functions are precompiled and the file is cached. For comparison, summing an equivalent Array (completely in main memory) takes 0.7 s.


Copyright © 2019, 2020 Lukas Himbert

Npy.jl is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 3 of the License.

Npy.jl is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with Npy.jl. If not, see https://www.gnu.org/licenses/.


  • version 1.1.0 added (partial) read support for numpy version 3. This version also introduced linear indexing for the NpyArray type, with emulation of column major linearisation for arrays with row major linearisation. This change also fixed a regression caused by a change in Julia 1.5.

Npy.jl is considered feature-complete by its author. No major changes are planned.

  • numpy: Python numerical library; origin of the .npy format.
  • NPZ.jl: Another Julia library for .npy files. Supports the same data types as this library. Also supports .npz (compressed .npy files). It does not use mmap and is unsuitable for files larger than your main memory.
  • cnpy.h The same thing as this library in form of a C library (by the author of this package).
  • npy.hpp A similar C++ library (by the author of this package). Uses pread(3) and pwrite(3) instead of mmap.


  • fuzz?
  • test on big endian