subprocess interactive bash behavior

I'm using python 3.5 on Mac OS X Sierra

The following code works as expected, printing 'hello world' 5 times:

for _ in range(5):
    print(subprocess.check_output(['/bin/bash', '-i', '-c', 'echo hell world']))

while this one causes the script to stop, after printing the directory listing only once.

for _ in range(5):
    print(subprocess.check_output(['/bin/bash', '-i', '-c', 'ls']))

displays

... my directory listing ...
[2]+ Stopped    python demo.py

I have aliased my ls in ~/.bashrc as ls -alf. If I remove the alias, the program seems to work fine. Is there something wrong with starting bash in interactive mode?

EDIT:

Interestingly, I can hack the command to force the correct behavior:

for _ in range(5):
    print(subprocess.check_output(['/bin/bash', '-i', '-c', 'echo; ls']))

Now ls doesn't kill the script anymore, regardless of whether it's aliased or not. Any idea?