python_version_full := $(wordlist 2,4,$(subst ., ,$(shell python --version 2>&1)))
python_version_major := $(word 1,${python_version_full})
my_cmd := ${my_cmd.python.${python_version_major}}

py = python3
ifeq ($(python_version_major),2)
    py = python2
endif

all :
	@echo ${py}