I am learning how to use named pipes to pass data between processes. To test it out, I wrote two Python scripts, which I called pipewriter.py and pipereader.py, and a named pipe called my_pipe (using mkfifo my_pipe), all in the same directory. The Python scripts are included below.

pipewriter.py

#!/usr/bin/env python3
for i in range(1, 5):
  with open('my_pipe', 'a') as outfile:
    outfile.write(f'Hello {i}\n')
    if i==4:
      outfile.write('\n')

print('Done')

pipereader.py

#!/usr/bin/env python3
while True:
  with open('my_pipe', 'r') as infile:
    input = infile.read()
    print(input,end="")

The problem is that pipereader.py sometimes misses some of the lines output by pipewriter.py. When I run ./pipereader.py in one terminal and ./pipewriter.py in another, then the lines written are

Hello 1
Hello 2
Hello 3
Hello 4

But if I run ./pipewriter.py; ./pipewriter.py; ./pipewriter.py (with pipereader.py running in a different terminal), then the output of pipereader.py sometimes looks like this:

Hello 1
Hello 2
Hello 3
Hello 1    <- Missing "Hello 4"
Hello 2
Hello 3
Hello 4

Hello 1
Hello 2
Hello 3
Hello 4

How can I figure out why the lines are disappearing and ensure it doesn’t happen?

By rewriting ./pipewriter.py to open my_pipe only once for each execution, I can seemingly prevent missing lines. Still, I’m not sure if this is just a matter of reducing the frequency of missing lines, so I’ve just “gotten lucky” the several times I’ve tested it.

pipewriter.py (Modified)

#!/usr/bin/env python3 
with open('my_pipe', 'a') as outfile:
  for i in range(1, 5):
    outfile.write(f'Hello {i}\n')
  outfile.write('\n')

print('Done')

Edit: Output When using an Input Argument per this comment

Modified pipewriter.

#!/usr/bin/env python3
import sys

for i in range(1, 5):
  with open('my_pipe', 'a') as outfile:
    outfile.write(f'Hello {sys.argv[1]} {i}\n')
    if i==4:
      outfile.write('\n')

print('Done')

Output 1 of ./pipewriter.py A; ./pipewriter.py B; ./pipewriter.py C

Hello A 1
Hello A 2
Hello A 3
Hello A 4

Hello B 1
Hello B 2
Hello B 3
Hello B 4

Hello C 1
Hello C 2
Hello C 3

Output 2 of ./pipewriter.py A; ./pipewriter.py B; ./pipewriter.py C

Hello A 1
Hello A 2
Hello A 3
Hello A 4

Hello B 1
Hello B 2
Hello B 3
Hello B 4

Hello C 1
Hello C 2