# DataFrames.jl: metadata

**URL:** https://discourse.julialang.org/t/dataframes-jl-metadata/84544
**Category:** Data
**Tags:** package, dataframes, metadata
**Created:** [July 20, 2022, 3:14pm UTC](https://discourse.julialang.org/t/dataframes-jl-metadata/84544 "2022-07-20T15:14:25Z")
**Posts on this page:** 1
**Showing post:** 94

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### Author: ![gcalderone](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gcalderone/32/1539_2.png) [@gcalderone](https://discourse.julialang.org/u/gcalderone)
#### Post date: [July 26, 2022, 5:00pm UTC](https://discourse.julialang.org/t/dataframes-jl-metadata/84544/94 "2022-07-26T17:00:56Z")

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My two cents concerning the “metadata” functionality (without worrying about propagation):

- the way to use and propagate “metadata” is definitely application-specific, and is in no way related to the DataFrames.jl package (quoting @bkamins: _“metadata is not used for any logic of processing data in DataFrames.jl”_), hence the functionality should be provided by a separate package;
- however, such external package would hardly integrate transparently with DataFrames.jl since the latter expose a complex interface based on several methods:

```julia
julia> length(methodswith(DataFrame, supertypes=true))
308

```

The consequence is that in order to exploit the “metadata” functionality we would likely be forced to use an alternative interface for the `DataFrame` object. Taking as an example the above-mentioned [Metadata.jl](https://github.com/Tokazama/Metadata.jl) package:

```julia
julia> using Metadata, DataFrames
julia> df = DataFrame(id=[1,2]);
julia> mdf = attach_metadata(df, (x = 1, y = 2));
julia> names(mdf) # <-- can't use DataFrame methods on the wrapped object!
ERROR: MethodError: no method matching names(::Metadata.MetaStruct{DataFrame, NamedTuple{(:x, :y), Tuple{Int64, Int64}}})
Closest candidates are:
   ...

```

(the purpose here is of course not to blame Metadata.jl, but just to provide an example…).

In summary: “metadata” should be implemented in an external package, but implementing such package would be unreasonably hard. This issue has been discussed in several posts on discourse, e.g. [here](https://discourse.julialang.org/t/how-to-add-metadata-info-to-a-dataframe/11168/95).

I’m afraid this whole discussion highlight some kind of limitation in the Julia type system 😭, or at least some difficulty in implementing a facility which is conceptually very simple.

Concerning propagation: I believe having something working in 90% of cases (or even 99%) is a very dangerous option which I really hope will be avoided. Hence my choice is #2: Add metadata to DataFrame [(because it is very useful and would hardly be used if implemented in a separate package)], but never propagate metadata.

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