Biblatex: Citation delimiter between works of the same author with postnotes

Question

I have a problem which is similar to this: Change citation delimiter between works of the same author (biblatex)

I want to change the delimiter between multiple citations by the same author depending on if there has been a postnote. If there is a post-note, citations should be separated with a semicolon, if there isn't one, with a comma. So what I have is:

Lennon 1971: 282; 1973; McCartney 1979; 1980

what I want is:

Lennon 1971: 282; 1973; McCartney 1979, 1980

MWE:

\documentclass{article}
\usepackage[style=authoryear, citestyle=authoryear-comp]{biblatex}
\DeclareFieldFormat{postnote}{#1}
\renewcommand{\postnotedelim}{\addcolon\addspace}
\renewcommand{\multicitedelim}{\addsemicolon\space}
\renewcommand{\compcitedelim}{\multicitedelim}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@BOOK{lennon1971,
   AUTHOR = "John Lennon",
   TITLE = "My really long book on my life",
   YEAR = "1971",
   LOCATION = "Liverpool",
   PUBLISHER = "Penny Lane Press"}
@BOOK{lennon1973,
   AUTHOR = "John Lennon",
   TITLE = "Music -- why I make it",
   YEAR = "1973",
   LOCATION = "London",
   PUBLISHER = "Johnny Smith"}
@BOOK{mccartney1979,
   AUTHOR = "Paul McCartney",
   TITLE = "Penny Lane is still in my ears",
   YEAR = "1979",
   LOCATION = "New York",
   PUBLISHER = "Peter Alden"}
@BOOK{mccartney1980,
   AUTHOR = "Paul McCartney",
   TITLE = "Penny Lane is not anymore my ears",
   YEAR = "1980",
   LOCATION = "New York",
   PUBLISHER = "Peter Alden"}       
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\cites[282]{lennon1971}{lennon1973}{mccartney1979}{mccartney1980}
\printbibliography
\end{document}

Answer 1

Set

\renewcommand{\multicitedelim}{\addsemicolon\space}
\renewcommand{\compcitedelim}{\addcomma\space}

And then define

\renewbibmacro*{postnote}{%
  \iffieldundef{postnote}
    {}
    {\setunit{\postnotedelim}%
     \printfield{postnote}%
     \printunit{\multicitedelim}}}

So we effectively turn a \compcitedelim into a \multicitedelim after a postnote. The \printunit (in contrast to \setunit) makes sure that the punctuation is not overridden by another \setunit command following it.

MWE

\documentclass{article}
\usepackage[style=authoryear, citestyle=authoryear-comp]{biblatex}
\DeclareFieldFormat{postnote}{#1}
\renewcommand{\postnotedelim}{\addcolon\addspace}
\renewcommand{\multicitedelim}{\addsemicolon\space}
\renewcommand{\compcitedelim}{\addcomma\space}

\renewbibmacro*{postnote}{%
  \iffieldundef{postnote}
    {}
    {\setunit{\postnotedelim}%
     \printfield{postnote}%
     \printunit{\multicitedelim}}}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@BOOK{lennon1971,
   AUTHOR = "John Lennon",
   TITLE = "My really long book on my life",
   YEAR = "1971",
   LOCATION = "Liverpool",
   PUBLISHER = "Penny Lane Press"}
@BOOK{lennon1973,
   AUTHOR = "John Lennon",
   TITLE = "Music -- why I make it",
   YEAR = "1973",
   LOCATION = "London",
   PUBLISHER = "Johnny Smith"}
@BOOK{mccartney1979,
   AUTHOR = "Paul McCartney",
   TITLE = "Penny Lane is still in my ears",
   YEAR = "1979",
   LOCATION = "New York",
   PUBLISHER = "Peter Alden"}
@BOOK{mccartney1980,
   AUTHOR = "Paul McCartney",
   TITLE = "Penny Lane is not anymore my ears",
   YEAR = "1980",
   LOCATION = "New York",
   PUBLISHER = "Peter Alden"}       
\end{filecontents}
\addbibresource{\jobname.bib}

\begin{document}
\cites[282]{lennon1971}{lennon1973}{mccartney1979}{mccartney1980}
\printbibliography
\end{document}

Answer 2

A simpler solution, which avoids having to mess with the postnote macro, and has the advantage of working independently of your definition of \multicitedelim, is just:

\renewcommand{\compcitedelim}{\iffieldundef{postnote}{\addcomma\space}{\addsemicolon\space}}