Test if a package (or package option) is loaded

Question

[french]{babel} (aka frenchb) is incompatible with some other packages, such as tikz, since it tries to insert \thinspace before semi-colons.

In order to make these two play nice, you have to call \shorthandoff{;} in the document.

Now I have a package that uses tikz and I would like to locally disable the semi-colon for babel, instead of disabling it for the whole document. I could hack the .sty of the package to do that, knowing that I'm using frenchb, but I would rather make something clean and portable.

Is there a way in my package that I can test if frenchb is loaded and disable semi-colon management with babel locally, so the package doesn't have to be hacked?

Edit:

Martin's solution gives me something like this in general:

\RequirePackage{ifthen}
\newcommand{\@babel@loaded}{0}
\AtBeginDocument{\@ifpackageloaded{babel}                                                 
  {\renewcommand{\@babel@loaded}{1}}                                                      
  {\renewcommand{\@babel@loaded}{0}}                                                      
}
\newcommand{\somecommandusingsemicolon}{
  \ifthenelse{\@babel@loaded=1}{\shorthandoff{;}}{}  
  % code with semi-colon here
}

and in the specific case of tikz:

\AtBeginDocument{\@ifpackageloaded{babel}
  {%
    \g@addto@macro\tikz@installcommands{%
      \shorthandoff{;}%
    }
    \g@addto@macro\tikz@uninstallcommands{%
      \shorthandon{;}%
    }
  }
  {\relax}
}

Answer 1

You can use the internal LaTeX macro \@ifpackageloaded to test if a package was loaded:

\@ifpackageloaded{<name>}{<true>}{<false>}

There are also:

\@ifpackagewith{<name>}{<option-list>}{<true>}{<false>}
\@ifpackagelater{<name>}{<date YYYY/MM/DD>}{<true>}{<false>}

For further macro see the document linked in Documentation reference for LaTeX internal commands?.

---

In the case of tikz it would be best to add your code to the macros \tikz@installcommands and \tikz@uninstallcommands. The first is executed at the begin of a tikzpicture and enables all the tikz commands only defined there and the latter is executed at the begin of node content and changes the definitions back to there normal state. This way you can have French text inside the nodes.

You can add code to the macros using:

\AtBeginDocument{\@ifpackageloaded{babel}{%
    \g@addto@macro\tikz@installcommands{%
      \shorthandoff{;}%
    }%
    \g@addto@macro\tikz@uninstallcommands{%
      \shorthandon{;}%
    }%
}{}

I'm just guessing here that \shorthandon is the opposite of \shorthandoff.

Answer 2

In newer LaTeX versions, \@ifpackageloaded can also be \IfPackageLoadedTF.

Usage is identical.

---

Another option is \ltx@ifpackageloaded from ltxcmds package (the main advantage of ltxcmds package is it being usable from plain TeX, but there isn't really a standardized mechanism to load a package in plain TeX apart from \input which doesn't really keep track of this):