OS X Mach-O universal executable files often contain multiple architectures including a combination of 32-bit and 64-bit archs, as with the newer OS X installer variants provided on python.org.  In such cases, the platform.architecture() function always returns '64bit' as the bit architecture regardless of whether the interpreter is running in 32-bit or 64-bit mode.  Thus, there is no documented way to reliably tell whether an interpreter is running in 32- or 64-bit in OS X.  Instead of the platform module, one must resort to hacks like examining sys.maxsize (or sys.maxint) or checking type sizes from the struct module.

$ arch -x86_64 /usr/local/bin/python3.2 -c 'import sys,platform; 
print(sys.maxsize,platform.architecture())'
9223372036854775807 ('64bit', '')
$ arch -i386 /usr/local/bin/python3.2 -c 'import sys,platform; 
print(sys.maxsize,platform.architecture())'
2147483647 ('64bit', '')