Why does returning in Interactive Python print to sys.stdout?

Question

I ran into something different today. Consider this simple function:

def hi():
    return \'hi\'

If I call it in a Python shell,

Answer 1

The interactive interpreter will print whatever is returned by the expression you type and execute, as a way to make testing and debugging convenient.

>>> 5
5
>>> 42
42
>>> 'hello'
'hello'
>>> (lambda : 'hello')()
'hello'
>>> def f():
...     print 'this is printed'
...     return 'this is returned, and printed by the interpreter'
...
>>> f()
this is printed
'this is returned, and printed by the interpreter'
>>> None
>>>

See Read–eval–print loop on Wikipedia for more information about this.