python - Return and yield in the same function

Question

What exactly happens, when yield and return are used in the same function in Python, like this?

def find_all(a_str, sub):
    start = 0
    while True:
        start = a_str.find(sub, start)
        if start == -1: return
        yield start
        start += len(sub) # use start += 1 to find overlapping matches

Is it still a generator?

Answer 1

Yes, it' still a generator. The return is (almost) equivalent to raising StopIteration.

PEP 255 spells it out:

Specification: Return

A generator function can also contain return statements of the form:

"return"

Note that an expression_list is not allowed on return statements in the body of a generator (although, of course, they may appear in the bodies of non-generator functions nested within the generator).

When a return statement is encountered, control proceeds as in any function return, executing the appropriate finally clauses (if any exist). Then a StopIteration exception is raised, signalling that the iterator is exhausted. A StopIteration exception is also raised if control flows off the end of the generator without an explict return.

Note that return means "I'm done, and have nothing interesting to return", for both generator functions and non-generator functions.

Note that return isn't always equivalent to raising StopIteration: the difference lies in how enclosing try/except constructs are treated. For example,

>>> def f1():
...     try:
...         return
...     except:
...        yield 1
>>> print list(f1())
[]

because, as in any function, return simply exits, but

>>> def f2():
...     try:
...         raise StopIteration
...     except:
...         yield 42
>>> print list(f2())
[42]

because StopIteration is captured by a bare "except", as is any exception.