How to make a named tuple from a dictionary?

General Usage
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24

I don’t think it is a good idea, because, as I remarked above, Dicts are not ordered, so it can happen that two <:AbstractDict values are == but the resulting NamedTuples aren’t.

25

It may be the version – I do not have v1.0.3 available …
you have a way that works, so I am not looking further

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26 
julia> adict = Dict("astr" => 5, "bstr" => 'b');

julia> (; (Symbol(k) => v for (k,v) in adict)...)
(bstr = 'b', astr = 5)
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27

There already is a much more general constructor which takes any iterator yielding Symbol => value pairs. People just don’t know about the syntax (hence add an example of programatically creating a named tuple by KristofferC · Pull Request #32330 · JuliaLang/julia · GitHub).

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28

NamedTupleTools looks awesome! Is it possible to recursively turn all the dictionaries in the dictionary into named tuples?

Also what is your opinion on doing this in the first place? I am reading some MATLAB .mat files (which is read by MAT.jl to a dictionary) which have a nested matlab structs e.g EEG.event.epoch. And in order to keep my code more readable by the MATLAB savvy ones, I was thinking of using namedtuples since they resemble MATLAB structs. Are there any pros/cons do doing this? Order isn’t really neccessary.

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29

If you plan to go deep you may want to check whether Performance of nested named tuples · Issue #25296 · JuliaLang/julia · GitHub is still a problem.

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31

That problem has been resolved (not an issue with v1.3.1).

32

NamedTuples are a good choice anywhere they are a natural fit.
It sounds like NamedTuples would be appropriate for your use.

33

Okay great. So in this use case when I convert deeply nested MATLAB structs (eg. EEG.event.epoch) using MAT.jl the output will be an Dict named EEG containing a Dict called event. So if you need to access epoch it would be accessed by:

EEG["event"]["epoch"]

And so (finally) using NameTupleTools when I make a named tuple from EEG:

namedtuple(EEG)

EEG.event will contain event as a dict; requiring me to access epoch in the following way:

EEG.event["epoch"]

Is there way for the Dict constructor to convert all nested dictionaries into named tuples too? So that I can simply use:

EEG.event.epoch
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34 
using NamedTupleTools
recursive_namedtuple(x::Any) = x
recursive_namedtuple(d::Dict) = namedtuple(Dict(k => recursive_namedtuple(v) for (k, v) in d))

You may want to restrict this to only apply to Dicts with the right type of keys.

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35

Awesome, thanks a lot! I didn’t even know keys and values could be accessed like that.

36

The shortest universal solution seems to be this:

merge(NamedTuple(), Dict(:a => 5, :b => 6, :c => 7))
37

What…? @mauro3’s solution, which is also marked as the solution, seems shorter:

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38

Perhaps you missed this one?

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39

Many of the answers here do not work if Dict is created using string keys, just for Symbol keys, probably we should have some easier functions for conversions, i got myself sometimes struggling with type conversions too…

40

If the keys are symbols as in

d = Dict(:a => 5, :b => 6, :c => 7)

the best solution in Julia 1.6 is probably

NamedTuple(d)

Now if the keys are strings you must convert them to symbols. Here are some solutions:

d = Dict("a" => 5, "b" => 6, "c" => 7)


NamedTuple(Symbol(k) => v for (k,v) in d)

# or
NamedTuple(Symbol.(keys(d)) .=> values(d))

# or
(; (Symbol(k) => v for (k,v) in d)...)

# or
(; (Symbol.(keys(d)) .=> values(d))...)
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41

If you have this a lot of times you can, of course, define

import Base.NamedTuple
NamedTuple(d::Dict{String, T} where T) = NamedTuple(Symbol(k) => v for (k,v) in d)

which allows you to do

julia> NamedTuple(d)
(c = 7, b = 6, a = 5)
42

Strictly speaking this is type piracy, although I guess it’s unlikely to break things in practice.

3 Likes
43

I agree. So the clean way would be

nt(d::Dict{AbstractString, T} where T) = NamedTuple(Symbol(k) => v for (k,v) in d)

with

julia> nt(d)
(c = 7, b = 6, a = 5)

But I think many people would like to see the above implementation as standard, perhaps with String replaced by AbstractString as here.

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45

I might as well post my solution, which is also recursive and doesn’t use any external library:

namedtuple(d::Dict) = (; map(k -> Symbol(first(k)) => namedtuple(last(k)), collect(d))...)
namedtuple(x) = x
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