\ifblank does not work correctly with \pgfkeys

Question

The macro \test below is used to test if its argument is blank, and it gives unexpected result.

What's wrong with my code?

Example:

\documentclass{article}
\usepackage{tikz,etoolbox}
\begin{document}
\pgfkeys{aaa/.initial={}}
\newcommand{\test}[1]{%
  \expandafter\ifblank\expandafter{#1}{blank}{#1}%
}
<\pgfkeysvalueof{/aaa}>% Typeset "<>"
|\test{\pgfkeysvalueof{/aaa}}|%I think it should be "|blank|", but actually it typesets "||". Why?
\end{document}

Answer 1

\pgfkeysvalueof doesn't expand in one step, so a single \expandafter can't work here. You need to force expansion: probably easiest using the 'roman numeral trick':

\newcommand{\test}[1]{%
  \expandafter\ifblank\expandafter{\romannumeral-`Q#1 }{blank}{#1}%
}

Answer 2

Rommannumeral trick does full expansion until there is something unexpandable. But if the argument is in the format bla\macro and the \macro expands to nk then rommannumeral trick fails. So, I suggest to use \expanded primitive:

\def\test#1{\expandafter\ifblank\expandafter{\expanded{#1}}{blank}{#1}}

Answer 3

It's a one-liner with expl3:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\pgfkeys{aaa/.initial={}}
\ExplSyntaxOn
\NewExpandableDocumentCommand{\test}{m}
  {
    \tl_if_blank:eTF { #1 } {blank} {#1}
  }
\ExplSyntaxOff
\begin{document}
<\pgfkeysvalueof{/aaa}>% Typeset "<>"
|\test{\pgfkeysvalueof{/aaa}}|
\pgfkeys{aaa=xyz}
<\pgfkeysvalueof{/aaa}>% Typeset "<>"
|\test{\pgfkeysvalueof{/aaa}}|
\end{document}

Answer 4

\pgfkeysvalueof{<full key>} need three expansions

According to its definition, it takes three expansions for \pgfkeysvalueof{<full key>} to expand to the value held in <full key>.

% run `latexdef -p pgfkeys -s \pgfkeysvalueof` and you'll get
% pgfkeys.code.tex, line 172:
\def\pgfkeysvalueof#1{\csname pgfk@#1\endcsname}

• step one, \pgfkeysvalueof expands to its replacement text, which is in the form of \csname ... \endcsname.
• step two, \csname ... \endcsname expands to a control sequence that holds the value of <full key>. For /aaa, it's \pgfk@/aaa.
• step three, that control sequence expands to its replacement text. For /aaa, the expansion result is empty.

The exact definition of \pgfkeysvalueof will change in the next release, see https://github.com/pgf-tikz/pgf/pull/1132, but the number of steps will remain the same.

Therefore

\expandafter\expandafter\expandafter\test
\expandafter\expandafter\expandafter{\pgfkeysvalueof{/aaa}}

works.

\pgfkeysValueOf that takes only two steps

In general,

• case 1: if the string to be tested is the replacement text of a macro, then OP's \test works for this macro, like \test{\cmd};
• case 2: if the string to be tested is the (finite steps of) expansion of replace text of a macro, then in theory we can define a command \cmdExpanded, based on \cmd, that only need two expansions. That means, \expandafter\test\expandafter{\cmdExpanded}}} works.

The \pgfkeysValueOf provided in example below belongs to case 2.

In addition, if expandability is not the key point, then pgfkeys's \pgfkeysgetvalue{<full key>}{<macro>} is another choice, for example

\pgfkeysgetvalue{/aaa}{\temp} % here \temp belongs to case 1
|\test{\temp}|

Example, with test cases to distinguish "blank value" from "non-blank value that expands to blank":

\documentclass{article}
\usepackage{tikz,etoolbox}
% OP's definiton
\newcommand{\test}[1]{%
  \expandafter\ifblank\expandafter{#1}{blank}{#1}%
}
% egreg's answer
\ExplSyntaxOn
\NewExpandableDocumentCommand{\testExpl}{m}
  {
    \tl_if_blank:eTF { #1 } {blank} {#1}
  }
\ExplSyntaxOff
% wipet's answer
\def\testExpanded#1{%
  \expandafter\ifblank\expandafter{\expanded{#1}}{blank}{#1}}
% joseph's answer
\newcommand{\testRomannumeral}[1]{%
  \expandafter\ifblank\expandafter{\romannumeral-`Q#1 }{blank}{#1}%
}
% my solution 1, expand three steps 
\newcommand{\testThreeExps}[1]{%
  \expandafter\expandafter\expandafter\test
  \expandafter\expandafter\expandafter{#1}%
}
\begin{document}
\ttfamily
% three tests
\pgfkeys{
  aaa/.initial={},       % blank: empty
  bbb/.initial={{ }},    % blank: a space
  ccc/.initial={\empty}, % non-blank: expand to empty
  ddd/.initial={\space}, % non-blank: expand to a space
  eee/.initial={eee},    % non-blank: expand to non-blank
}
\leavevmode\llap{Expected}%
\foreach \j in {aaa, bbb, ccc, ddd, eee} {%
  % use \pgfkeysgetvalue, then one step expansion
  \pgfkeysgetvalue{/\j}{\temp}%
  \makebox[2cm]{|\test{\temp}|}%
}\bigskip
\foreach \i in {\test, \testExpl, \testExpanded, \testRomannumeral, \testThreeExps} {%
  \leavevmode\llap{\detokenize\expandafter{\i}}%
  \foreach \j in {aaa, bbb, ccc, ddd, eee} {%
    \makebox[2cm]{|\i{\pgfkeysvalueof{/\j}}|}%
  }
  \par
}
% \pgfkeysValueOf{<full key>} takes one step to expansion to <full key>'s value
\newcommand{\pgfkeysValueOf}[1]{%
  \expanded{\unexpanded
    \expandafter\expandafter\expandafter\expandafter
    \expandafter\expandafter\expandafter{\pgfkeysvalueof{#1}}}%
}
\leavevmode\hspace*{-4cm}%
\verb|\expandafter\test\expandafter{\pgfkeysValueOf{<full key>}}|\par%
\foreach \j in {aaa, bbb, ccc, ddd, eee} {%
  \makebox[2cm]{|\expandafter\test\expandafter{\pgfkeysValueOf{/\j}}|}%
}
\end{document}

Note the test cases distinguish "blank key value" from "non-blank key value that expands to blank". Values of /aaa and /bbb are both blank, but values of /ccc and /ddd are neither blank, though they both expand to blank.

Answer 5

Another thing you can do is to test if the key typeset to a size-0 box (this is a mixed answer with the XY question here).

But notice that an explicit space is not the same thing as a blank content...

\documentclass{article}
\usepackage[T1]{fontenc}%<- you forgot this!
\usepackage{tikz,etoolbox}
\newsavebox{\measbox}
\newcommand{\test}[1]{%
    \savebox{\measbox}{#1}%
    \ifdim\wd\measbox>0pt \usebox\measbox \else blank\fi
}
\begin{document}
\pgfkeys{aaa/.initial={}}
<\pgfkeysvalueof{/aaa}>% Typesets "<>" (with the T1 fontenc, otherwise ¡?)
|\test{\pgfkeysvalueof{/aaa}}|% typesets |blank|
\pgfkeys{aaa=\bfseries}
|\test{\pgfkeysvalueof{/aaa}}|% typesets |blank|
\pgfkeys{aaa=~}
|\test{\pgfkeysvalueof{/aaa}}|% typesets | |
\pgfkeys{aaa=\bfseries ~}
|\test{\pgfkeysvalueof{/aaa}}|% typesets | | but slightly wider
\end{document}