The objective of this research is to analyse the ways members of open-source software communities participate in design. In particular we focus on how users of an Open Source (OS) programming language (Python) participate in adding new functionalities to the language. Indeed, in the OS communities, users are highly skilled in computer sciences; they do not correspond to the common representation of end-users and can potentially participate to the design process. Our study characterizes the Python galaxy and analyses a formal process to introduce new functionalities to the language called Python Enhancement Proposal (PEP) from the idea of language evolution to the PEP implementation. The analysis of a particular pushed-by-users PEP from one application domain community (financial), shows: that the design process is distributed and specialized between online and physical interactions spaces; and there are some cross participants between users and developers communities which may reveal boundary spanners roles.