None

In [2]:
def foo():
    print("I do nothing!")
    val = 2 + 2
In [3]:
foo()
I do nothing!
In [4]:
print(foo())
I do nothing!
None
In [6]:
def bar():
    print("I do nothing!")
    val = 2 + 2
    return  # Returns `None`
In [7]:
print(foo())
I do nothing!
None
In [10]:
def barfoo():
    print("I do nothing!")
    val = 2 + 2
    return None