# \[JLD2\] How to save an obj on file, replacing that object but not the whole file
**URL:** https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460
**Category:** General Usage
**Created:** [August 28, 2022, 1:48pm UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460 "2022-08-28T13:48:00Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@sylvaticus](https://discourse.julialang.org/u/sylvaticus)
#### Post date: [August 28, 2022, 1:48pm UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460/1 "2022-08-28T13:48:00Z")
In JLD2 I would like to save an object to a sort of “library file”, replacing the same object name if it exists, but keeping the other object if they exists.
The problem is that even with append JLD2 returns an error if the object already exists in the file instead of replacing it:
```julia
using JLD2
model1 = 1
model2 = 2
jldopen("modelslib.jld2", "a+") do file
write(file, "model1", model1)
end
jldopen("modelslib.jld2", "a+") do file
write(file, "model2", model2)
end
model1j = load("modelslib.jld2", "model1")
model2j = load("modelslib.jld2", "model2")
model1 = 10
jldopen("modelslib.jld2", "a+") do file
write(file, "model1", model1)
end # error, foo1 exists (I want instead it replaced)
```
---
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [August 28, 2022, 10:05pm UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460/2 "2022-08-28T22:05:20Z")
Have you tried `jldsave` or `JLD2.save_object`?
[https://juliaio.github.io/JLD2.jl/dev/#JLD2.save\_object](https://juliaio.github.io/JLD2.jl/dev/#JLD2.save_object)
Is the data that you are trying to write exactly the same size? I’m wondering if we can just write new data into an existing dataset or if we need to delete the dataset and replace it.
A JLD2 file just a HDF5 file, so you should be able to use HDF5.jl to modify it if necessary. Depending on what you do JLD2.jl may not be able to read it though.
---
### Author:  [@sylvaticus](https://discourse.julialang.org/u/sylvaticus)
#### Post date: [August 29, 2022, 7:00am UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460/3 "2022-08-29T07:00:21Z")
Yes, I have tried them but they overwrite the file.
It seems there is no way to “update” only one object (aside of loading all the objects and resaving the desired ones).
---
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [August 29, 2022, 2:48pm UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460/4 "2022-08-29T14:48:43Z")
Ah, perhaps this will help:
```julia
julia> using JLD2
julia> A = rand(16); B = rand(16);
julia> jldsave("example.jld2"; A)
julia> jldopen("example.jld2", "r") do f
println(keys(f))
end
["A"]
julia> jldopen("example.jld2", "r+") do f
f["B"] = B
end
16-element Vector{Float64}:
0.5608959073449324
0.3638963529780811
0.07184337169387223
0.18897128618321324
0.66329261122493
0.5220036617458721
0.4285742412772173
0.6509469598307095
0.7187628817194093
0.5203396168881611
0.4418571079136371
0.9003617232080112
0.2346391079104828
0.9180306350902839
0.5155522362415368
0.6388304659727359
julia> jldopen("example.jld2", "r") do f
println(keys(f))
end
["A", "B"]
julia> fill!(A, 0)
16-element Vector{Float64}:
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
julia> C = zeros(256, 256);
julia> jldopen("example.jld2", "a+") do f
f["C"] = C
end;
julia> jldopen("example.jld2", "a+") do f
f["A"] .= A
end;
julia> jldopen("example.jld2", "a+") do f
println(keys(f))
end
["A", "B", "C"]
```
---
### Author:  [@sylvaticus](https://discourse.julialang.org/u/sylvaticus)
#### Post date: [August 29, 2022, 2:59pm UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460/5 "2022-08-29T14:59:39Z")
You haven’t “updated” the object on the file:
```julia
julia> Aj = load("example.jld2", "A")
16-element Vector{Float64}:
0.5740161069259119
0.23376757546050675
0.13016224315460012
0.5986203452744622
0.0014843901260015446
0.08085386257112082
⋮
0.6432398769774624
0.3902720389879373
0.2968135277921722
0.199786865259862
0.6854068247070402
```
---
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [August 29, 2022, 3:15pm UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460/6 "2022-08-29T15:15:16Z")
You’re right. I guess we’ll have to go with my first idea for now.
```julia
using HDF5
julia> h5open("example.jld2", "r+") do f
A = f["A"][] .= 0
f["A"][:] = A
end
16-element Vector{Float64}:
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
julia> jldopen("example.jld2", "r+") do f
f["A"]
end
16-element Vector{Float64}:
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
```
---
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [August 29, 2022, 3:23pm UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460/7 "2022-08-29T15:23:34Z")
The other idea is to `delete!` the prior “variable”.
```julia
julia> using JLD2
julia> A = rand(16); B = rand(16);
julia> jldsave("example.jld2"; A, B)
julia> jldopen("example.jld2", "a+") do f
delete!(f, "A")
f["A"] = zeros(16)
end
16-element Vector{Float64}:
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
julia> jldopen("example.jld2", "a+") do f
f["A"]
end
16-element Vector{Float64}:
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
```
---
### Author:  [@sylvaticus](https://discourse.julialang.org/u/sylvaticus)
#### Post date: [August 29, 2022, 3:42pm UTC](https://discourse.julialang.org/t/jld2-how-to-save-an-obj-on-file-replacing-that-object-but-not-the-whole-file/86460/8 "2022-08-29T15:42:23Z")
thanks. Indeed it works even for complex different objects:
```julia
julia> using JLD2
julia> struct Foo
x::Int64
end
julia> struct Goo
y::String
z::Function
end
julia> function model_save(filename;kargs...)
jldopen(filename, "a+") do f
for (k,v) in kargs
ks = string(k)
if ks in keys(f)
delete!(f, ks)
end
f[ks] = v
end
end
end
model_save (generic function with 1 method)
julia> a = Foo(1)
Foo(1)
julia> b = Foo(2)
Foo(2)
julia> model_save("example.jld2";a)
Foo(1)
julia> model_save("example.jld2";b)
Foo(2)
julia> a = Goo("foo",maximum)
Goo("foo", maximum)
julia> model_save("example.jld2";a)
Goo("foo", maximum)
julia> aj = load("example.jld2", "a")
Goo("foo", maximum)
```