[Tex/LaTex] Renew Command for a footnote

Question

I would need to change the font of all footnotes in a document; the brute force way would be to change the font in each and everyone of them, except that they are a few dozen.

I thought I could use \renewcommand in the preamble, but I do not seem to have it work. How can I have that each time I have a footnote it is in italics, without having to write

\footnote{\textit{blahblah}}

I know the question is trivial for most of you, but I seem to get something wrong
Thanks!
francesco

Here is my MWE

\documentclass[12pt,a4paper]{book}
\usepackage[utf8]{inputenc}
 \usepackage[english]{babel}
 \usepackage{amsmath}
  \usepackage{amsfonts}
   \usepackage{amssymb}
  \usepackage{graphicx}
 \usepackage{lipsum}
 \renewcommand{\footnote}{\footnote{\italics}}

Answer 1

You probably don't want the footnote mark in italics as well, so it's just a matter of finding the best way where to set \itshape and the start of the call of \@makefntext seems the place.

\documentclass[12pt,a4paper]{book}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{etoolbox}

\makeatletter
\patchcmd{\@footnotetext}{\rule}{\itshape\rule}{}{}
\makeatother

\usepackage{lipsum}

\textheight=2cm % just to make a smaller picture

\begin{document}

Some text.\footnote{This footnote is in italics}
Some other text

\end{document}