Python: Redirect various output sections to separate variables

Question

I'm trying to create a mechanism to redirect print outputs to a number of variables. Following code simulates what I'm looking for :-

import sys
import io

class MultiOut(object):
    def __init__(self, stream_out):
        self.terminal = sys.stdout
        self.output = stream_out
    def write(self, message):
        self.terminal.write(message)
        self.output.write(message)
    def flush(self):
        self.terminal.flush()
        self.output.flush()

vals = {'a1': io.StringIO(), 'a2': io.StringIO(), 'a3': io.StringIO()}

for i,val in enumerate(vals):
    sys.stdout = MultiOut(vals[val])
    [print(x**i, end=' ') for x in range(11)]
    print("\n")

with open('temp.txt', 'w') as f:
    for x in vals:
        f.write(f"{x} :-\n")
        f.write(vals[x].getvalue())
        f.write(f"{'='*50}\n")

File Output (tmp.txt) :-

a1 :-
1 1 1 1 1 1 1 1 1 1 1 

0 1 2 3 4 5 6 7 8 9 10 

0 1 4 9 16 25 36 49 64 81 100 

==================================================
a2 :-
0 1 2 3 4 5 6 7 8 9 10 

0 1 4 9 16 25 36 49 64 81 100 

==================================================
a3 :-
0 1 4 9 16 25 36 49 64 81 100 

==================================================

What I'm trying to do here is to redirect various 'sections' of the outputs to different variables of vals - a1, a2, a3 and dump all the outputs to the terminal. Strangely, each successive variable contains data starting from that point till the end. Is there a way to avoid this issue and save each section in a different variable ?

Answer 1

The issue is you're replacing sys.stdout with your object:

sys.stdout = MultiOut(vals[val])

and in your object init, you're setting sys.stdout as object attribute

self.terminal = sys.stdout

On the second iteration, when executing this statement

self.terminal = sys.stdout

sys.stdout is the replacement from the first iteration, which includes the first MultiOut object.

I hope this makes sense.

---

Rather then modifying stdout, I'd use the logging module to achieve what you want.