Import symbol from other font without wasting math alphabet

Question

I wish to import a symbol from newtxmath for use with kpfonts without using up a math alphabet to do it. As seen in this question it is easy to do so if one is willing to waste a math alphabet, but in their answer to this question, egreg seems to say that it is possible to avoid this by using the pifont package.

For reference, this works but uses up a math alphabet:

\documentclass{article}
\usepackage{kpfonts}
\makeatletter
\DeclareSymbolFont{ntxletters}{OML}{ntxmi}{m}{it}
\re@DeclareMathSymbol{\newpartial}{\mathord}{ntxletters}{64}
\makeatother
\begin{document}
\[ \partial \newpartial \]
\end{document}

Following (as far as I can tell correctly) egreg's instructions would lead me to the following:

\documentclass{article}
\usepackage{kpfonts}
\usepackage{pifont}
\makeatletter
\newcommand\Pimathsymbol[3][\mathord]{%
  #1{\@Pimathsymbol{#2}{#3}}}
\def\@Pimathsymbol#1#2{\mathchoice
  {\@Pim@thsymbol{#1}{#2}\tf@size}
  {\@Pim@thsymbol{#1}{#2}\tf@size}
  {\@Pim@thsymbol{#1}{#2}\sf@size}
  {\@Pim@thsymbol{#1}{#2}\ssf@size}}
\def\@Pim@thsymbol#1#2#3{%
  \mbox{\fontsize{#3}{#3}\Pisymbol{#1}{#2}}}
\makeatother
\newcommand{\newpartial}{\Pimathsymbol[\mathord]{ntxmmi}{64}}
\begin{document}
\[ \partial \newpartial \]
\end{document}

This compiles but gives the following error messages:

Some font shapes were not available, defaults substituted.
Font shape `U/ntxmmi/m/n' undefined
(Font)  using `U/jkpexa/m/n' instead.

The output is also wrong, substituting a (subscript?) \oiintctrclockwise from kpfonts instead. I suppose this symbol fills the 64 slot of the jkpexa font.

If I understand egreg correctly, this method is not sufficient in the linked question above since the asker wishes to import a symbol from mathabx, and this does not provide .fd files. But as far as I can tell, newtxmath does provide (at least some) .fd files here.

(Note that I am using pdfLaTeX. Not sure if that is significant enough for it to be a tag.)

Answer 1

That's just for symbols in the “Pi” font.

If you don't need the symbol to respect \boldmath, it's as easy as

\newcommand{\newpartial}{\mathord{\text{\usefont{OML}{ntxmi}{m}{it}\symbol{64}}}}

Complete example.

\documentclass{article}
\usepackage{amsmath}
\usepackage{kpfonts}
\newcommand{\newpartial}{}
\DeclareRobustCommand{\newpartial}{%
  \mathord{\text{\usefont{OML}{ntxmi}{m}{it}\symbol{64}}}%
}
\begin{document}
[ \partial \newpartial ]
\end{document}

Better using \DeclareRobustCommand so if the symbol gets its way in some moving argument you don't get garbage in the .aux file.

For \boldmath

\documentclass{article}
\usepackage{amsmath}
\usepackage{kpfonts}
\usepackage{pdftexcmds}
\newcommand{\newpartial}{%
  \mathord{\text{\usefont{OML}{ntxmi}{\mediumorbold}{it}\symbol{64}}}%
}
\makeatletter
\newcommand{\mediumorbold}{%
  \ifnum\pdf@strcmp{\math@version}{bold}=0 b\else m\fi
}
\makeatother
\begin{document}
[ \partial \newpartial ]
\boldmath
[ \partial \newpartial ]
\end{document}