# @unpack with Parameters.jl when key is not found
**URL:** https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716
**Category:** General Usage
**Tags:** question, package
**Created:** [May 7, 2021, 1:42pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716 "2021-05-07T13:42:19Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@gregory](https://discourse.julialang.org/u/gregory)
#### Post date: [May 7, 2021, 1:42pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/1 "2021-05-07T13:42:19Z")
Hi, is there an easy way to have `Parameters.jl` skip missing keys or fields?
```
using Parameters
mydict = Dict(a=>3,b=>2.3,c=>"mystr")
@unpack (a,b,c,d) = mydict
ERROR: KeyError: key "d" not found
```
I would like it to just skip over the missing key. The reason I want this behavior is I am carrying around two lists which hold fixed parameters and ones to be found via optimization. It would be nice if I could just unpack them, without having to check which ones were in which list like:
```
@unpack (fixed1,fixed2,...) = fixedParams
@unpack (fitted1,fitted2,...) fittedParams
```
and instead being able to just say
```
@unpack (p1,p2,...) = fixedParams
@unpack (p1,p2,...) = fittedParams
```
---
### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [May 7, 2021, 1:48pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/2 "2021-05-07T13:48:12Z")
I don’t think so. Part of the reason `@unpack` is fast is because it doesn’t have these kinds of checks. A macro doesn’t know what’s inside a `Dict`.
---
### Author:  [@gregory](https://discourse.julialang.org/u/gregory)
#### Post date: [May 7, 2021, 2:05pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/3 "2021-05-07T14:05:50Z")
Is there a way to write my own method then that does this check? It doesn’t need to be fast.
---
### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [May 7, 2021, 2:10pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/4 "2021-05-07T14:10:00Z")
UnPack.jl is actually super simple once you understand metaprogramming.
[https://github.com/mauro3/UnPack.jl/blob/master/src/UnPack.jl](https://github.com/mauro3/UnPack.jl/blob/master/src/UnPack.jl)
In particular, lines 32-36 are what you would want to change. Instead of `getindex` you would want `get(d, x, nothing)` or similar.
---
### Author:  [@gregory](https://discourse.julialang.org/u/gregory)
#### Post date: [May 7, 2021, 4:08pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/5 "2021-05-07T16:08:34Z")
I tried adding both of the following methods:
```julia
@inline unpack(x::AbstractDict{<:AbstractString}, k) = get(x,string(k),nothing)
@inline unpack(x::AbstractDict{<:AbstractString}, ::Val{k}) where {k} = get(x,string(k),nothing)
```
but still got the same error:
```julia
test=Dict("a"=>1,"b"=>2,"c"=>"mystr")
@unpack (a,b,c,d) = test
ERROR: KeyError: key "d" not found
```
---
### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [May 7, 2021, 4:10pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/6 "2021-05-07T16:10:34Z")
Could you please tell us more about what steps you did to add the methods? Did you add the package for development using `] dev`?
---
### Author:  [@gregory](https://discourse.julialang.org/u/gregory)
#### Post date: [May 7, 2021, 4:12pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/7 "2021-05-07T16:12:01Z")
Ah sorry. I thought I was able to add methods similar to how I can add methods in `Base.jl`. So I was just adding them inline in my Julia script.
---
### Author:  [@Skoffer](https://discourse.julialang.org/u/Skoffer)
#### Post date: [May 7, 2021, 4:49pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/8 "2021-05-07T16:49:57Z")
You can, but you should `import` methods first.
But this is type piracy and shouldn’t be used if you are writing a library (but more or less fine in your local script)
---
### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [May 7, 2021, 5:29pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/9 "2021-05-07T17:29:45Z")
Here is an MWE
```julia
julia> using UnPack
julia> import UnPack: unpack
help?> unpack
search: unpack UnPack @unpack
This function is invoked to unpack one field/entry of some DataType dt and has signature:
unpack(dt::Any, ::Val{sym}) -> value of sym
The sym is the symbol of the assigned variable.
Three definitions are included in the package to unpack a composite type or a dictionary with Symbol
or string keys:
@inline unpack(x, ::Val{f}) where {f} = getproperty(x, f)
@inline unpack(x::AbstractDict{Symbol}, ::Val{k}) where {k} = x[k]
@inline unpack(x::AbstractDict{S}, ::Val{k}) where {S<:AbstractString,k} = x[string(k)]
More methods can be added to allow for specialized unpacking of other datatypes.
See also pack!.
julia> unpack(x::AbstractDict{S}, ::Val{k}) where {S<:AbstractString,k} = get(x, string(k), nothing)
unpack (generic function with 3 methods)
julia> d = Dict(["a" => 1])
Dict{String, Int64} with 1 entry:
"a" => 1
julia> @unpack a, c = d
Dict{String, Int64} with 1 entry:
"a" => 1
julia> c
```
That said, don’t do this… Maybe you can define your own macro, use `@unpack`’s parsing, but call your own function instead of `unpack`.
---
### Author:  [@roi.holtzman](https://discourse.julialang.org/u/roi.holtzman)
#### Post date: [September 9, 2022, 7:49pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/10 "2022-09-09T19:49:07Z")
So how should one do this properly?
---
### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [September 9, 2022, 9:03pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/11 "2022-09-09T21:03:09Z")
Fork `@unpack` and change the name of the macro?
---
### Author:  [@roi.holtzman](https://discourse.julialang.org/u/roi.holtzman)
#### Post date: [September 9, 2022, 9:58pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/12 "2022-09-09T21:58:25Z")
I would like to use it as a convenience function in my own code. I mean, I still want to use the default `Parameters.jl`.
---
### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [September 9, 2022, 10:28pm UTC](https://discourse.julialang.org/t/unpack-with-parameters-jl-when-key-is-not-found/60716/13 "2022-09-09T22:28:43Z")
When I say “improper” it means that if you want
```julia
@unpack a = d
```
to _not_ fail when `a` is not in `d`, you need to commit [type piracy](https://docs.julialang.org/en/v1/manual/style-guide/#Avoid-type-piracy), which means overwriting a method you don’t own for a type you don’t own.
You can do that in your own code (see above), but the consequence is
1. When people read your code, the behavior of `@unpack` will be different than they expect
2. If other packages use your code in any way, existing uses of `@unpack` will behave differently than the original authors expected.
So you can’t be “correct” and change the behavior of `@unpack`.