Call multiple Python subprocesses with iterative input

Question

I am writing a pipeline script that incorporates different Python scripts which take the output of one script as the input to another, sequentially. The scripts are called from the command line using the subprocess module. The application must be run on Windows, so I cannot use the pexpect module.

I have a Python script, pipeline.py, which is the main entry-point for my application, a first.py script that gets called first and expects input from second.py.

Here, first.py will prompt for input for a fixed number of iterations:

# first.py
import sys

sum = 0
for i in range(0, 10):
    num = int(input('value: '))   # wait for input from pipeline.py, given by second.py
    # TODO: sleep to avoid EOFFile error?
    # while not num:
    #    time.sleep(5) until value is given?
    sum += num
sys.stdout.write(sum)   # return with answer to pipeline.py

Now, first.py will prompt for a number that is generated by second.py:

# second.py
import random
import sys

rand_num = random.randint(1, 10)
sys.stdout.write(rand_num) # or print(rand_num)

In pipeline.py I call first.py, wait until it asks for a value, call second.py to generate that value, then pass it back to first.py.

# pipeline.py
import subprocess as sp

cmd1 = "python first.py"
cmd2 = "python second.py"

prc1 = sp.Popen(cmd1, shell=True, stdin=sp.PIPE, stdout=sp.PIPE, stderr=sp.PIPE)
# TODO: put prc1 on hold until value is passed

# do this for each iteration in first.py (i.e. range(0,10))
while True:
    line = prc1.stdout.readline():  # TODO: generates EOFError
    if line == "value: ":     # input prompt
        prc2 = sp.Popen(cmd2, shell=True, stdin=sp.PIPE, stdout=sp.PIPE, stderr=sp.PIPE)
        (val, err) = prc2.communicate()
        prc1.stdin.write(val)   # pass value back to first.py
    if type(line) == int: # iterations finished, first.py returns sum
        break

It seems my issue arises because prc1 asks for input but does not wait for the input to be provided. When running first.py directly from the command line, however, it does not crash and actually waits for input. Any thoughts? Help would be greatly appreciated. Thanks in advance!