python, iterate on subprocess.Popen() stdout/stderr

There are a lot of similar posts, but I didn't find answer.

On Gnu/Linux, with Python and subprocess module, I use the following code to iterate over the stdout/sdterr of a command launched with subprocess:

class Shell:
    """ 
    run a command and iterate over the stdout/stderr lines
    """

    def __init__(self):

        pass

    def __call__(self,args,cwd='./'):

        p = subprocess.Popen(args,
                cwd=cwd, 
                stdout = subprocess.PIPE,
                stderr = subprocess.STDOUT,
                )

        while True:

            line = p.stdout.readline()
            self.code = p.poll()

            if line == '':
                if self.code != None:
                    break
                else:
                    continue

            yield line

#example of use
args = ["./foo"]
shell = Shell()
for line in shell(args):
     #do something with line
     print line,

This works fine... except if the command executed is python, for example `args = ['python','foo.py'], in which case the output is not flushed but printed only when the command is finished.

Is there a solution?