Default value for a key defined with pgfkeys

Question

Hereafter is a simple macro with keys nom and prenom defined with `pgfkeys.

\documentclass[a4paper]{article}

\usepackage{tikz}

\pgfkeys{
    /test/pgf/.cd,
    nom/.store in = \nom,
    nom/.default = toto,
    prenom/.store in = \prenom,
    prenom/.default = titi,
}

\newcommand{\qui}[1][]{
    \pgfkeys{/test/pgf/.cd,#1}
    I am \prenom{} \nom{} !
}

\begin{document}
\qui[prenom = toto, nom = titi]
\end{document}

The issue is that if you use macro \qui without the key \nom or \prenom you get an error message saying ! Undefined control sequence. I think that when I execute the macro with \qui[nom = titi] LaTeX does not know the macro \prenom and thus gives me an error message.

I do not understand why I got this error since I give a default value to all keys with the .default handler. I tried also the .initial handler but if you want the macro to work you have to replace \prenom and \nom by \pgfkeysvalueof{prenom} and \pgfkeysvalueof{nom}.

PGF Keys differences between .initial and .default

But the following code works. You can use macro \qui without any keys. But again I do not understand why the .default handler does not do this job ?

\documentclass[a4paper]{article}

\usepackage{tikz}

\pgfkeys{
    /test/pgf/.cd,
    nom/.store in = \nom,
    nom = toto,
    prenom/.store in = \prenom,
    prenom = titi,
}

\newcommand{\qui}[1][]{
    \pgfkeys{/test/pgf/.cd,#1}
    I am \prenom{} \nom{} !
}

\begin{document}
\qui[prenom = toto, nom = titi]
\end{document}

Answer 1

The problem with your first example is that, despite the fact that key nom actually has a value, that value is never stored in macro \nom, so that macro remains undefined. pgfkeys only will create \nom and store in it the value of the key nom when the key nom is executed. Only then the value of nom is retrieved (and if none was assigned the default one is used) and stored in \nom.

So your problem is that you have to "execute" nom and prenom keys. You can do it a three different places:

• When calling command \qui, as for example: \qui[nom, prenom]. Since you are executing those keys but not assigning them a value, defaults are used.

• Inside \qui macro definition:

  \newcommand{\qui}[1][]{
      \pgfkeys{/test/pgf/.cd, nom, prenom, #1}
      I am \prenom{} \nom{} !
  }
  

  This way you can use simply \qui{} in main program. If you use instead \qui[nom=foo], this value will override the default one.

• Inside the initial \pgfkeys definition:

  \pgfkeys{
      /test/pgf/.cd,
      nom/.default = toto,
      nom/.store in = \nom,
      prenom/.store in = \prenom,
      prenom/.default = titi,
      nom, prenom
  }
  

  This has the same effect than the second option, and it is more general, so it should be the preferred one.