why can subprocess run print but not my own function code example

Example: how to print stdout while processing the execution in python

from subprocess import Popen, PIPE, CalledProcessError

with Popen(cmd, stdout=PIPE, bufsize=1, universal_newlines=True) as p:
    for b in p.stdout:
        print(b, end='') # b is the byte from stdout

if p.returncode != 0:
    raise CalledProcessError(p.returncode, p.args)