Global key-value dictionary

Question

Is there any package that implements a data structure that provides a key-value mapping (akin to Python's dict)? The keys will be user input and may contain spaces and non-ASCII characters. I would like to have such a thing at global level so that I can add items inside macros.

If not, I appreciate any hints or code snippets.

I have peeked into the docs of keyval, but couldn't see how to use this for my purpose. Also, I am aware that etoolbox has list processing, perhaps this can be turned into a dictionary with some work?

Answer 1

The 'classical' way to store values with a key is to use a csname-based approach

\def\addvalue#1#2{\expandafter\gdef\csname my@data@#1\endcsname{#2}}
\def\usevalue#1{\csname my@data@#1\endcsname}

To allow arbitrary material in the key, you can use e-TeX's \detokenize, and also add a test for undefined values if you wish

\def\addvalue#1#2{\expandafter\gdef\csname my@data@\detokenize{#1}\endcsname{#2}}
\def\usevalue#1{%
  \ifcsname my@data@\detokenize{#1}\endcsname
    \csname my@data@\detokenize{#1}\expandafter\endcsname
  \else
    \expandafter\ERROR
  \fi
}

This approach needs one csname per key, which can get awkward for very large data sets. It also means you end up doing all of the work yourself every time you want to use such an approach. As has been pointed out, the etoolbox package offers some support for data storage. I would also point to the LaTeX3 prop data structure

\usepackage{expl3}
\ExplSyntaxOn
\prop_new:N \g_my_data_prop
\cs_new_protected:Npn \addvalue #1#2 { \prop_gput:Nnn \g_my_data_prop {#1} {#2} }
\cs_new:Npn \usevalue #1 { \prop_get:Nn \g_my_data_prop {#1} }
\ExplSyntaxOff

This approach offers a full range of support functions, for example tests for the presence of a key, counting the total number of keys, and so on.

Answer 2

You can also use the pgfkeys library for a slightly different approach, using a key value system, which is designed as a tree structure.

First you define a family,

 \pgfkeys{/combo/.is family}

and then the keys,

 \pgfkeys{/combo  
  left/.store in=\left@c,
  left/.default=0,
  right/.store in=\right@c
  ...}

You can set default values:

\def\setdefaults{
   \pgfkeys{/combo
      left=100,
      right=25,
      top=30,
      bottom=60,
   }
}
\setdefaults

and you incorporate the keys in a newcommand:

\newcommand{\putgraphic}[5][\setdefaults]{%
      \pgfkeys{/combo #1}
   ...
}

You can find a full implementation based on this method at Using an overpic grid to crop a resized image in Beamer and https://tex.stackexchange.com/a/45023/963. For global values see Is there a way to set a *global* key value using `pgfkeys`?.