[Python] How to add a label to each node in Graphviz?

[EDIT] as asked here is what ps does for me :

PID TTY TIME CMD
3796 pts/0 00:00:00 bash
4811 pts/0 00:00:00 ps


I'm beginning with Graphviz and I would like to display the name of the running process. I got a script that display their number and I'm trying to add a label to each node.

The problem is that only the last write label is displayed to the root node, how can I manage to write a label to every node ?

#!/usr/bin/env python3

from subprocess import Popen, PIPE, call
import re

dot = open("psgraph.dot", "w")
dot.write("digraph G {\n")

p = Popen("ps -fe", shell=True, stdout=PIPE)
psre = re.compile(r"\w+\s+(\d+)\s+(\d+)")

p.stdout.readline() # ignore first line
for line in p.stdout:
match = psre.search(line.decode("utf-8"))
if match:
if int(match.group(2)) in (0, 2):
continue
dot.write (" {1} -> {0}\n".format(match.group(1), match.group(2)))

for line in p.stdout:
match = psre.search(line.decode("utf-8"))
if match:
if int(match.group(2)) in (0, 2):
continue
dot.write ("""1 [label="loop"]\n""")

dot.write("""1 [label="laste write"]}\n""")
dot.close()

call("dot -Tpdf -O psgraph.dot", shell=True)

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