Biblatex - \DeclareFieldFormat

Question

I'm writing a thesis in french law and I was able to, after many, many hours of hard work, customize some entry type to fit my needs. Here is my example of my @BOOKLET that I use for legislation:

\documentclass[11pt,a4paper,twoside]{book}

\usepackage[T1]{fontenc}
\usepackage[ansinew]{inputenc}
\usepackage{lmodern}
\usepackage{csquotes}
\usepackage[frenchb]{babel}
\usepackage[language=french,sorting=nyt,hyperref=true,urldate=long,
bibstyle=authortitle,citestyle=verbose-trad1,citepages=omit,dateabbrev=false,
labelyear=true,backend=bibtex]{biblatex}

\addbibresource{These.bib}

\DeclareBibliographyDriver{booklet}{%
\usebibmacro{author}
\setunit{\labelnamepunct}\newblock
\printfield{title}
\newunit\newblock
\printlist{publisher}%
\newunit
\printfield{number}%
\newunit
\printdate%
\newunit
\printfield{pages}%
{\space}
\usebibmacro{finentry}
 }

\DeclareFieldFormat[booklet]{title}{{{#1}}}
\DeclareFieldFormat[booklet]{publisher}{{\mkbibemph{#1}}}
\DeclareFieldFormat[booklet]{number}{\bibstring{number}~#1}% 
\renewcommand*{\newunitpunct}{\addcomma\space}

\begin{document}

\nocite{*}
\printbibliography[keyword=loi,heading=subbibliography,title=Textes législatifs]

\end{document}

It's almost how I want it, I'd just like to get the field publisher in italics, which is what was intended with the command:

\DeclareFieldFormat[booklet]{publisher}{{\mkbibemph{#1}}}

But it doesn't change anything, and I don't know why, especially as if I replace publisher by date or title, it works just fine!

Any suggestions? I really need your expertise here ;-)

PS : Here an example of my .bib

@BOOKLET{AA2012,
TITLE = "The title of the legislation I need",
PUBLISHER = "Official journal",
NUMBER = "173",
DATE = "2012-11-25",
ADDRESS = "Paris",
PAGES = "582",
KEYWORDS = "loi"
}

Answer 1

As Marco Daniel has commented, "publisher isn't a field, but a [literal] list". Actually, there are three general types of "fields" in biblatex: name lists (e.g., translator), literal lists (e.g., publisher), and other fields. The format of literal lists can be defined using \DeclareListFormat, and that of other fields using \DeclareFieldFormat. (The correspondent \DeclareNameFormat macro isn't directly appplied to name lists, but to auxiliary formats like last-first.) A complete list of fields and their categories can be found in sections 2.2.1 and 2.2.2 of the biblatex manual.