Macros with 0, 1 or 2 arguments

Question

Is it possible to build a macro accepting 0, 1 or 2 arguments ? Why ? I want to have the following functionalities.

1. \exercice will print Exercice alone.
2. \exercice{2} will print Exercice (2 pts).
3. \exercice{2}{Source:...} will print Exercice (2 pts) [Source: ...].

Answer 1

Better: use a key-value syntax.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\exercice}{O{}}
 {
  \group_begin:
  \keys_set:nn { projetmbc/ex } { #1 }
  \par\vspace{\topsep}\noindent Exercice
  \tl_if_empty:NF \l_projetembc_ex_pts_tl
   { ~(\tl_use:N \l_projetembc_ex_pts_tl\nobreakspace pts) }
  \tl_if_empty:NF \l_projetembc_ex_note_tl
   { ~[\tl_use:N \l_projetembc_ex_note_tl] }
  \group_end:
  \quad\ignorespaces
 }
\keys_define:nn { projetmbc/ex }
 {
  points .tl_set:N = \l_projetembc_ex_pts_tl ,
  note   .tl_set:N = \l_projetembc_ex_note_tl ,
 }
\ExplSyntaxOff

\begin{document}

\exercice Text

\exercice[points=2] Text

\exercice[note=Source: folklore] Text

\exercice[points=4,note=Source: folklore] Text

\end{document}