Yasm is a complete rewrite of the NASM assembler under the "new" BSD License 
(some portions are under other licenses, see COPYING for details).  Yasm 
currently supports the x86 and AMD64 instruction sets, accepts NASM and GAS 
assembler syntaxes, outputs binary, ELF32, ELF64, 32 and 64-bit Mach-O, 
RDOFF2, COFF, Win32, and Win64 object formats, and generates source debugging 
information in STABS, DWARF 2, and CodeView? 8 formats.

If you have Cython installed, you can build the python bindings by passing
PYTHON_BINDINGS=yes to the script.