# How to Efficiently Index PyArrays?
**URL:** https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785
**Category:** Performance
**Tags:** question, pycall, python
**Created:** [October 14, 2021, 10:04pm UTC](https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785 "2021-10-14T22:04:28Z")
**Posts on this page:** 6
**Page:** 2
### Author:  [@cjdoris](https://discourse.julialang.org/u/cjdoris)
#### Post date: [October 16, 2021, 11:29am UTC](https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785/21 "2021-10-16T11:29:28Z")
> [@goerch](#):
>
> ```julia
> function index_test(pyarray::PyArray{UInt8, 3})
> s = 0
> for i in 1:size(array, 1)
> for j in 1:size(array, 2)
> for k in 1:size(array, 3)
> s += array[i, j, k]
> end
> end
> end
> s
> end
>
> ```
The argument is `pyarray` not `array`. So you are timing reading a global variable and not touching PyCall at all.
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [October 16, 2021, 11:56am UTC](https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785/22 "2021-10-16T11:56:07Z")
Embarrassing mistake on my side. Corrected test:
```julia
using PyCall
function index_test(pyarray::PyArray{UInt8, 3})
s = 0
for i in 1:size(pyarray, 1)
for j in 1:size(pyarray, 2)
for k in 1:size(pyarray, 3)
s += pyarray[i, j, k]
end
end
end
s
end
function from_python(pyarray::PyArray{UInt8, 3})
return convert(Array{UInt8, 3}, pyarray)
end
function index_test(array::Array{UInt8, 3})
s = 0
for i in 1:size(array, 1)
for j in 1:size(array, 2)
for k in 1:size(array, 3)
s += array[i, j, k]
end
end
end
s
end
function to_python(array::Array{UInt8, 3})
PyArray(PyObject(array))
end
array = Array{UInt8, 3}(undef, (250, 250, 3))
@btime index_test($array)
@btime to_python($array)
pyarray = PyArray(PyObject(array))
@btime index_test($pyarray)
@btime from_python($pyarray)
println()
```
with result
```julia
87.800 μs (0 allocations: 0 bytes)
1.820 μs (20 allocations: 1.16 KiB)
161.300 μs (0 allocations: 0 bytes)
469.700 μs (2 allocations: 183.20 KiB)
```
Everything looks fine!
---
### Author:  [@rewindturtle](https://discourse.julialang.org/u/rewindturtle)
#### Post date: [October 16, 2021, 7:39pm UTC](https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785/23 "2021-10-16T19:39:07Z")
My bad. Tested it again. It is 50us faster with the `Array{Float32, 2}(undef, 92, 120)`. I originally tested it with `Matrix{Float32}(undef, 92, 120)`.
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [October 16, 2021, 7:41pm UTC](https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785/24 "2021-10-16T19:41:27Z")
those are exactly the same
---
### Author:  [@rewindturtle](https://discourse.julialang.org/u/rewindturtle)
#### Post date: [October 16, 2021, 7:56pm UTC](https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785/25 "2021-10-16T19:56:42Z")
I am new to Julia so sorry if I seem so naive.
---
### Author:  [@rewindturtle](https://discourse.julialang.org/u/rewindturtle)
#### Post date: [October 16, 2021, 7:59pm UTC](https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785/26 "2021-10-16T19:59:01Z")
I tried the index test with different versions of PyCall. Using v1.90.0, only 11 allocations are made. Using any earlier version gives me the error:
```julia
ERROR: LoadError: InitError: could not load symbol "Py_CompileString":
The specified procedure could not be found.
Stacktrace:
[1] macro expansion
@ C:\Users\user\.julia\packages\PyCall\WcrLS\src\exception.jl:81 [inlined]
[2] pyeval_(s::String, globals::PyCall.PyDict{String, PyCall.PyObject, false}, locals::PyCall.PyDict{String, PyCall.PyObject, false}, input_type::Int64, fname::String)
@ PyCall C:\Users\user\.julia\packages\PyCall\WcrLS\src\pyeval.jl:20
[3] pyeval_
@ C:\Users\user\.julia\packages\PyCall\WcrLS\src\pyeval.jl:17 [inlined]
[4] __init__ ()
@ PyCall C:\Users\user\.julia\packages\PyCall\WcrLS\src\pyinit.jl:131
[5] _include_from_serialized(path::String, depmods::Vector{Any})
@ Base .\loading.jl:696
[6] _require_search_from_serialized(pkg::Base.PkgId, sourcepath::String)
@ Base .\loading.jl:782
[7] _require(pkg::Base.PkgId)
@ Base .\loading.jl:1020
[8] require(uuidkey::Base.PkgId)
@ Base .\loading.jl:936
[9] require(into::Module, mod::Symbol)
@ Base .\loading.jl:923
during initialization of module PyCall
in expression starting at C:\Users\user\PycharmProjects\juliaProject\arcade.jl:7
```
[Previous page](https://discourse.julialang.org/t/how-to-efficiently-index-pyarrays/69785.md?page=1)