pgfkeys - .is family handler

Question

This might seem a trivial question, but not for someone wanting to understand pgfkeys very well. In the following definition from pgfkeys package, why can't the \edef be replaced by \def? I have asked a pgfkeys master this question before but apparently he didn't get the question correctly, since I mixed it with other issues.

\pgfkeys{%
    /handlers/.is family/.code=
    \pgfkeys{\pgfkeyscurrentpath/.ecode=
      \edef\noexpand\pgfkeysdefaultpath{\pgfkeyscurrentpath/}
    }
}

The following gives me the same result even as \edef is replaced by \def. Am I right?

\pgfkeys{%
    /handlers/.is family/.code=
    \pgfkeys{\pgfkeyscurrentpath/.ecode=
      \def\noexpand\pgfkeysdefaultpath{\pgfkeyscurrentpath/}
    }
}

\def\firstlayerfam{/fam/subfam}
\def\secondlayerfam{\firstlayerfam}
\def\family{\secondlayerfam}

\pgfkeys{
  \family/.is family,
  \family % This changes the path even without '.cd'. OK!
}

EDIT

The reason I see is in the following

\long\def\pgfkeys@unpack#1=#2=#3\pgfkeys@stop{%
  \pgfkeys@spdef\pgfkeyscurrentkey{#1}%
   % The following \edef is crucial. In fact, it should come before the last line
   % (ie, before calling the Carlisle despacer \pgfkeys@spdef). Because if #1 is
   % a macro, the content may be the one that carries spurious leading and 
   % trailing spaces. Without expansion, \pgfkeys@add@path@as@needed may even add 
   % default path when it isn't needed. See below.
   \edef\pgfkeyscurrentkey{\pgfkeyscurrentkey}%
   \ifx\pgfkeyscurrentkey\pgfkeys@empty%
    % Skip
  \else
    \pgfkeys@add@path@as@needed
    \pgfkeys@spdef\pgfkeyscurrentvalue{#2}%
    \ifx\pgfkeyscurrentvalue\pgfkeysnovalue@text% Hmm... no value
      \pgfkeysifdefined{\pgfkeyscurrentkey/.@def}%
      {\pgfkeysgetvalue{\pgfkeyscurrentkey/.@def}{\pgfkeyscurrentvalue}}%
      {}% no default, so leave it
    \fi%
    \ifx\pgfkeyscurrentvalue\pgfkeysvaluerequired%
      \pgfkeysvalueof{/errors/value required/.@cmd}%
       \pgfkeyscurrentkey\pgfkeyscurrentvalue\pgfeov%
    \else%
      \pgfkeys@case@one%
    \fi%
  \fi
}

Trace log:

{changing \pgfkeyscurrentkey=macro:->\family /.is family}
{into \pgfkeyscurrentkey=macro:->//fam/subfam/.is family}

Answer 1

It's clear (without ever having used pgfkeys) that edef and def are almost certainly the same in that context.

If

  \edef\noexpand\pgfkeysdefaultpath{\pgfkeyscurrentpath/}

works then presumably it doesn't define \noexpand but rather the \noexpand is preventing expansion of \pgfkeysdefaultpath. This tells you the whole line is being put through an expansion context (almost certainly edef) and if it is edef then the definition of the macro will already have been expanded before the definition, so \edef and \def will give the same result.