Get the argument names of an function

For an arbitrary function

function f(x,y,z=3)
    p = x+y*z
    return p 
end

I want to be able take the argument names of f

julia> argument_names(f)
3-element Array{Symbol,1}:
 :x
 :y
 :z

Is this possible?

julia> collect(methods(f))
[1] f(x, y) in Main at REPL[1]:2
[2] f(x, y, z) in Main at REPL[1]:2 

julia> x = collect(methods(f))[2]
f(x, y, z) in Main at REPL[1]:2

julia> x.slot_syms
"#self#\0x\0y\0z\0p\0"

This is the closest I can get.

1 Like

A hint is that Julia itself can print the arguments. So step into show with the debugger:

julia> using Debugger

julia> function f(x,y::Integer,z=3)   # just for fun, add a type parameter
           p = x+y*z
           return p 
       end
f (generic function with 4 methods)

julia> m = first(methods(f))
f(x, y::Integer) in Main at REPL[5]:2

julia> @enter show(m)
In show(x) at show.jl:325
>325  show(x) = show(stdout::IO, x)

About to run: (typeassert)(Base.TTY(RawFD(0x0000000d) open, 0 bytes waiting), IO)
1|debug> se
In show(x) at show.jl:325
>325  show(x) = show(stdout::IO, x)

About to run: (show)(Base.TTY(RawFD(0x0000000d) open, 0 bytes waiting), f(x, y::Integer) in Main at REPL[5]:2)
1|debug> s
In #show#393(kwtype, , io, m) at methodshow.jl:159
 158  function show(io::IO, m::Method; kwtype::Union{DataType, Nothing}=nothing)
>159      tv, decls, file, line = arg_decl_parts(m)
 160      sig = unwrap_unionall(m.sig)
 161      ft0 = sig.parameters[1]
 162      ft = unwrap_unionall(ft0)
 163      d1 = decls[1]

About to run: Core.NewvarNode(:(_15))
1|debug> n
In #show#393(kwtype, , io, m) at methodshow.jl:159
 158  function show(io::IO, m::Method; kwtype::Union{DataType, Nothing}=nothing)
 159      tv, decls, file, line = arg_decl_parts(m)
>160      sig = unwrap_unionall(m.sig)
 161      ft0 = sig.parameters[1]
 162      ft = unwrap_unionall(ft0)
 163      d1 = decls[1]
 164      if sig === Tuple

About to run: (getproperty)(f(x, y::Integer) in Main at REPL[5]:2, :sig)
1|julia> decls
3-element Array{Tuple{String,String},1}:
 ("", "typeof(f)")
 ("x", "")        
 ("y", "Integer") 
6 Likes

using the following code from https://github.com/JuliaLang/julia/blob/master/base/methodshow.jl (Vector{Any} changed to Vector{Symbol})

function method_argnames(m::Method)
    argnames = ccall(:jl_uncompress_argnames, Vector{Symbol}, (Any,), m.slot_syms)
    isempty(argnames) && return argnames
    return argnames[1:m.nargs]
end

and then

ms = collect(methods(f))
method_argnames(last(ms))[2:end]

you’ll get

3-element Array{Symbol,1}:
 :x
 :y
 :z
5 Likes

This solution doesn’t seem to work when using keyword arguments. Any suggestions?

The procedure in Get the argument names of an function - #3 by tim.holy still works; just stepping through to see how Julia does something is a useful skill for many questions. You’ll discover Base.kwarg_decl that way.

julia> m = @which sum(rand(3))
sum(a::AbstractArray; dims, kw...) in Base at reducedim.jl:994

julia> Base.kwarg_decl(m)
2-element Vector{Symbol}:
 :dims
 Symbol("kw...")
4 Likes