I wrote attached execv_curdir.py to check if os.execv() tries to find the executable in the current directory if it doesn't contain a directory: yes, it does.

$ python3 execv_curdir.py 
execv() searchs in the current directory

I also wrote attached subprocess_curdir.py which confirms that subprocess runs a program from the current directory if it exists.

$ python3 subprocess_curdir.py 
defpath = :/bin:/usr/bin
subprocess searchs in the current directory

Moreover, the current directory has the priority over /bin and /usr/bin.