Data bags combine the flexibility of dictionaries to store data and the `obj.key` syntax to mimic dynamic structures.
Author emmt
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1 Year Ago
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October 2019

Flexible data containers for Julia

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DataBags is a small Julia package providing data-bags which are a quick way to store structured data. Data-bags combine properties and dictionaries to associate keys (preferably symbols or strings) with values (of any types) in a flexible way. From the user viewpoint, data-bags behave like dynamic structures whose fields can be modified or created with the syntax of structured objects, i.e. obj.key. They can also be deleted by calling delete!(obj,key). As an example:

using DataBags, Dates
A = DataBag(date = now(), Δx = 0.1, x = -3:0.1:5)
A.Δx              # get value of key `Δx`
A.y = sin.(A.x)   # creates new key `y`

which shows how easy it is to create a data-bag and access its fields. Data-bags can also be indexed by their keys like dictionaries:

A[:Δx]              # is the same as `A.Δx`
A[:y] = cos.(A[:x]) # is the same as `A.y = cos.(A.x)`

but this is, to my opinion, less readable and boring to type especially in an interactive session. More generally, data-bag types are sub-types of AbstractDict so you can expect that data-bags can be used like dictionaries. For instance, you can apply pop, merge, merge!, delete!, etc. on a data-bag.

Admittedly, data-bags are less efficient than true Julia structures (there is some overhead for retrieving a field of a data-bag) but they can be very handful in interactive sessions or when designing new code: when the exact contents of your data structures is not yet determined, data-bags let you extend their contents without the pain of redefining your structures, re-including your code and recreating your objects, etc. Tools such as Revise can help but cannot automatically determine what to do with new members of existing objects if their type definition has changed.

Creation of data-bags

Data-bags are created by calling the DataBag(...) constructor. The initial contents of data-bags can be specified by keywords, by key-value pairs, or as a dictionary (AbstractDict). To avoid ambiguities, these different styles cannot be mixed. Below are a few examples:

using DataBags
A = DataBag( units  =  "µm",  Δx  =  0.1,  Δy  =  0.2)
B = DataBag(:units  => "µm", :Δx  => 0.1, :Δy  => 0.2)
C = DataBag("units" => "µm", "Δx" => 0.1, "Δy" => 0.2)
D = DataBag(1 => 0.9, 2 => sqrt(2), 3 => 4)

These statements yield two data-bags, A and B, with symbolic keys (of type Symbol), a data-bag, C, with textual keys (of type String) and a data-bag, D, with integer keys (of type Int). All these data-bags can store values of Any type.

Accessing a value is possible via the syntax obj[key] or, for symbolic and textual keys, via the syntax obj.key. Accessing values via the syntax obj.key is faster for symbolic keys than for textual keys (because it involves converting a symbol into a string).

Data-bag constructors attempt to favor symbolic or string keys (to exploit the obj.key syntax) and enforce unspecific values of Any type (for flexibility). In order to override these rules, the parametric versions DataBag{K} or DataBag{K,V} of the constructor, with K the key type and V the value type, can be called instead. For example:

E = DataBag{Integer}(1 => 0.9, 2 => sqrt(2), 3 => 4)
F = DataBag{Integer,Real}(1 => 0.9, 2 => sqrt(2), 3 => 4)

yield two data-bags, E and F, both with integer keys (of any Integer type), the values of E are unspecific while the values of F are restricted to be Real.

The same rules apply if the data-bag is built out of an existing dictionary (remember that data-bags are themselves abstract dictionaries). So DataBag(F) yields a data-bag with keys of the same type as those of F (that is Integer in that case) but values of Any type.

When a data-bag is built out of an existing dictionary, the data-bag creates a new dictionary to store its values and initializes it with the contents of the dictionary passed in argument. After the creation of the data-bag, the data-bag and the original dictionary are independent. Their values, which may be references to other objects, may not be independent though. If you want to make a data-bag that stores its contents in a given dictionary, say dict, call:

wrap(DataBag, dict)

instead of:


If no arguments nor keywords are specified, the data-bag created by DataBag() is initially empty and has symbolic keys with any type of values, i.e. an instance of Dict{Symbol,Any} is used for storing the key-value pairs.

Unless iterate is overridden, iterating on an AbstractDataBag is iterating on its key-value pairs.

Calling the contents method on an AbstractDataBag yields the internal object, an AbstractDict, used to store the data of the data-bag.

Defining custom data-bag types

The DataBags package provides simple means to facilitate creating new sub-types of DataBags.AbstractDataBag so as to benefit from the common interface implemented for data-bags. The following steps are needed:

  1. Make your type inherit from DataBags.AbstractDataBag{K,V,D} with K the key type, V the value type and D<:AbstractDict{K,V} the type of the dictionary storing the key-value pairs.

  2. Extend the DataBags.contents(A::T) method for your custom type T so that it returns the dictionary storing the key-value pairs in an instance A.

  3. Optionally provide some constructor(s) to facilitate creation of objects of type T. You may also consider extending the DataBags.wrap method if.

Here is a first example:

using DataBags

# Define a concrete sub-type of `DataBags.AbstractDataBag`.
struct BagEx1{K,V,D<:AbstractDict{K,V}} <: DataBags.AbstractDataBag{K,V,D}
    data::D # object used to store key-value pairs
    ...     # another member
    ...     # yet another member
    ...     # etc.

# Override `DataBags.contents` to yield the dictionary that stores the data.
DataBags.contents(A::BagEx1) = Base.getfield(A, :data)

Note that Base.getfield has to be used to retrieve a member of objects whose type is derived from DataBags.AbstractDataBag as for the member data of the object A in the above example. This is because the getproperty and setproperty! methods are overridden to implement the obj.key syntax for sub-types of DataBags.AbstractDataBag.

In the above example, it is only possible to create a data-bag of type BagEx1 out of a dictionary which is shared by the data-bag. The only advantage over a simple dictionary is the obj.key syntax provided keys have type Symbol or String.

To improve over this first example, we want to implement the same kind of creation rules as DataBag. This leads to the following code:

using DataBags

# Define a concrete sub-type of `DataBags.AbstractDataBag`.
struct BagEx2{K,V,D<:AbstractDict{K,V}} <: DataBags.AbstractDataBag{K,V,D}
    data::D # object used to store key-value pairs
    ...     # another member
    ...     # yet another member
    ...     # etc.
    # Explicitely define inner constructor to avoid outer constructor
    # automatically created by Julia.
    BagEx2{K,V,D}(data::D) where {K,V,D<:AbstractDict{K,V}} = new{K,V,D}(data)

# Outer constructor.
BagEx2(args...; kdws...) =
    wrap(BagEx2, contents(Dict{Any,Any}, args...; kdws...))

# Override `DataBags.contents` to yield the dictionary that stores the data.
DataBags.contents(A::BagEx2) = Base.getfield(A, :data)

# Override `DataBags.wrap` to create an instance of `BagEx2` that stores
# its data in a given dictionary.
DataBags.wrap(::Type{BagEx2}, data::D) where {K,V,D<:AbstractDict{K,V}} =

In this second example, we have:

  • Explictely defined an inner constructor so as to forbid creating a data-bag that shares an existing dictionary, say dict, by calling the constructor BagEx2. This is however possible by calling wrap(BagEx2,dict).

  • Defined an outer constructor that calls the wrap method over the dictionary created by the DataBags.contents method called with Dict{K,V} as a first argument, followed by all arguments and keywords passed to your constructor:

  • Overridden methods DataBags.contents (as in the first example) and DataBags.wrap. The latter is to wrap a dictionary in a new BagEx2 instance taking care of supplying the correct type parameters {K,V,D}.

To add constructors with constraints on the type of keys and values, you may have a look at the complete implementation of the DataBag type which is summarized below:

struct DataBag{K,V,D<:AbstractDict{K,V}} <: AbstractDataBag{K,V,D}
    data::D # data data-bag
    # Provide inner constructor to let outer constructors deal with type
    # parameters.
    DataBag{K,V,D}(data::D) where {K,V,D<:AbstractDict{K,V}} =

# Outer constructors.
DataBag(args...; kwds...) =
    wrap(DataBag, contents(Dict{Any,Any}, args...; kwds...))
DataBag{K}(args...; kwds...) where {K} =
    wrap(DataBag, contents(Dict{K,Any}, args...; kwds...))
DataBag{K,V}(args...; kwds...) where {K,V} =
    wrap(DataBag, contents(Dict{K,V}, args...; kwds...))

# Extends the `contents` method to benefit from the API of `AbstractDataBag`.
@inline contents(A::DataBag) = Base.getfield(A, :data)

# Extend the `wrap` method to create instances of `DataBag`.
wrap(::Type{DataBag}, data::D) where {K,V,D<:AbstractDict{K,V}} =
wrap(::Type{DataBag{K}}, data::D) where {K,V,D<:AbstractDict{K,V}} =
    wrap(DataBag, data)
wrap(::Type{DataBag{K,V}}, data::D) where {K,V,D<:AbstractDict{K,V}} =
    wrap(DataBag, data)
wrap(::Type{DataBag{K,V,D}}, data::D) where {K,V,D<:AbstractDict{K,V}} =
    wrap(DataBag, data)

A useful minimalist example

The DataBag type provided by DataBags may be sufficient for your needs but you may want to specialize it a bit to exploit the power of type dispatching in Julia and to implement some specific behavior. The most simple example of creating such a sub-type takes about half a dozen of lines of code:

using DataBags
struct BagEx3 <: DataBags.AbstractDataBag{Symbol,Any,Dict{Symbol,Any}}
    BagEx3(args...; kwds...) =
        new(DataBags.contents(Dict{Symbol,Any}, args...; kwds...))
DataBags.contents(A::BagEx3) = Base.getfield(A, :data)

Et voilà! That is all you need to create a new type, BagEx3, whose instances behave like a dictionary with symbolic keys and any type of values, implement the obj.key syntax to get/set the value of key (as a shortcut of obj[:key]) and which can be constructed using keywords, e.g. obj = BagEx3(id=1, x=-3.14:0.1:3.14, units="µm").

This usage is so common that a macro is provided by the DataBags package and the above statements can be reduced to:

using DataBags
DataBags.@newtype BagEx3

using the macro not only saves typing (to encourage creating such data-bag types) but also warrants that the implementation is correct and follows further evolutions of the DataBags package.