Why is stdout PIPE readline() not waiting for a newline character?

Why is print(line) in read_stdout printing every character instead of the entire line? I am expecting it to add to the queue once the newline character is reached, but it's just putting every character in the queue instead.

plugin_test.py:

from subprocess import PIPE, Popen
from threading import Thread
from queue import Queue, Empty
import re
import os
import sys

def read_stdout(stdout, data_q):
    for line in stdout.readline():
        print(line)
        data_q.put(line)
    stdout.close()

class Plugin():
    name = ''

    def __init__(self, **kwargs) -> None:
        self.process = None

    def run(self) -> bool:
        try:
            self.process = Popen(
                [sys.executable, 'test.py'],
                stdin=PIPE,
                stdout=PIPE,
                text=True)
        except Exception as exc:
            print("Could not create plugin process {}. {}".format(self.name, exc))
            return False

        self.data_q = Queue()
        self.read_t = Thread(target=read_stdout, args=(self.process.stdout, self.data_q))
        self.read_t.daemon = True
        self.read_t.start()
        
        try:
            startup = self.data_q.get(timeout=10)
        except Empty:
            print("Plugin took to long to load {}.".format(self.name))
            self.stop()
            return False
        else:
            if 'Error' in startup:
                print("Could not load plugin {}. {}".format(self.name, startup))
                self.stop()
                return False
            elif '100%' in startup:
                print("Plugin \"{}\" started.".format(self.name))

        return True

    def write(self, data : str) -> None:
        self.process.stdin.write(data)
        self.process.stdin.flush()

    def read(self) -> str:
        try:
            data = self.data_q.get()
            print("Got data ::{}::".format(data))
        except Exception as exc:
            print("Error reading data from plugin '{}'".format(exc))
        
    def stop(self):
        self.process.terminate()
        self.process.wait()


if __name__ == '__main__':
    plugin = Plugin()
    plugin.run()

test.py:

print("this is a test")
print("this is a test2")
print("100%")

root@osboxes# python plugin_test.py

t
h
i
s

i
s

a

t
e
s
t

If you want to read lines in for-loop then you have to use readlines() with s at the end.

It gives list of all lines in file - and for-loop gets full line in variable.

Using readline() it reads only one line
and for-loop treats it as list of chars and it gets one char in variable.

for line in stdout.readlines():

You can also reduce it to

for line in stdout:

which will iterate file and read by one line in every loop instead of reading all lines at once.
It allows to use next(stdout) inside loop to get (or skip) next line (if you need two lines at once).