What happens when a Python yield statement has no expression?

Question

I'm a C# programmer trying to understand some Python code. The code in question is a generator function, and looks like this:

def func():
    oldValue = curValue
    yield
    curValue = oldValue

If I understand this correctly, this will generate a iterable sequence with one member. However, there is no expression after the yield statement. What is such an expression-less statement supposed to yield? Are there any Python idioms that make use of this way of coding?

Answer 1

It'll yield None; just like an empty return expression would:

>>> def func():
...     yield
... 
>>> f = func()
>>> next(f) is None
True

You'd use it to pause code. Everything before the yield is run when you first call next() on the generator, everything after the yield is only run when you call next() on it again:

>>> def func():
...     print("Run this first for a while")
...     yield
...     print("Run this last, but only when we want it to")
... 
>>> f = func()
>>> next(f, None)
Run this first for a while
>>> next(f, None)
Run this last, but only when we want it to

I used the two-argument form of next() to ignore the StopIteration exception thrown. The above doesn't care what is yielded, only that the function is paused at that point.

For a practical example, the @contextlib.contextmanager decorator fully expects you to use yield in this manner; you can optionally yield an object to use in the with ... as target. The point is that everything before the yield is run when the context is entered, everything after is run when the context is exited.