Use letters as item labels by default in enumerate and with Beamer

Question

I know how to use letters for individual lists:

\documentclass{beamer}
\begin{document}
\begin{frame}{Self-test 2.4:}
What are the following limits?

\begin{enumerate}[(a)]
\item First item
\item Second item
\item Third item
\end{enumerate}
\end{frame}
\end{document}

But how can I tell Beamer that I want to use letters by default? Note that solutions for ordinary enumerations likely won't work since Beamer has its own implementation of lists.

Answer 1

To make my comment a bit more precise, the following works for me:

\documentclass{beamer}
\begin{document}
\renewcommand{\theenumi}{\alph{enumi}}
\begin{frame}{Self-test 2.4:}
What are the following limits?
\begin{enumerate}
\item First item
\item Second item
\item Third item
\end{enumerate}
\end{frame}
\end{document}

Notice that the \renewcommand is placed after \begin{document}. I guess that beginning the document has the effect of overwriting \theenumi, but I don't exactly understand how/why. (related? Difference between renewing commands before and after \begin{document})

Answer 2

An alternative solution to jchd's, but perhaps slightly more idiomatic, is to include the following in the header:

\setbeamertemplate{enumerate item}{(\alph{enumi})}

The complete example is then:

\documentclass{beamer}
\setbeamertemplate{enumerate item}{(\alph{enumi})}
\begin{document}
\begin{frame}{Self-test 2.4:}
What are the following limits?

\begin{enumerate}[(a)]
\item First item
\item Second item
\item Third item
\end{enumerate}
\end{frame}
\end{document}