生成器的行为不符合预期

Question

I am new to generators. Why does the first correct function with a print statement doesn't work when I replace the print with yield (Python 2.7)

First the correct function with print:

def make_all_pairs(list_):
    pivot = list_.pop(0)
    for el in list_:
        pair = (pivot, el)
        print pair
    if len(list_) > 1:
        make_all_pairs(list_)      

make_all_pairs(["a","b","c","d","e"])

('a', 'b')  
('a', 'c')  
('a', 'd')  
('a', 'e')  
('b', 'c')  
('b', 'd')  
('b', 'e')  
('c', 'd')  
('c', 'e')  
('d', 'e')

Then the generator which doesn't give all the combinations

def make_all_pairs(list_):
    pivot = list_.pop(0)
    for el in list_:
        pair = (pivot, el)
        yield pair
    if len(list_) > 1:
        make_all_pairs(list_)

x = make_all_pairs(["a","b","c","d","e"])
for el in x:
    print el

('a', 'b')
('a', 'c')
('a', 'd')
('a', 'e')

Answer 1

While print prints from whatever nesting of calls you are at, generators yield only from the first level. If you need to "call generator" to get more values to yield, you have to consume the values from the inner generator and yield them out as follows:

def make_all_pairs(list_):
    pivot = list_.pop(0)
    for el in list_:
        pair = (pivot, el)
        yield pair
    if len(list_) > 1:
        for itm in make_all_pairs(list_):
            yield pair

x = make_all_pairs(["a","b","c","d","e"])
for el in x:
    print el

Warning: While the code yields more values now, I do not guarantee, the result is correct. The code only shows, how to deal with values generated by inner generator.

Note: In python 3 you may use alternative construct yield from

def make_all_pairs(list_):
    pivot = list_.pop(0)
    for el in list_:
        pair = (pivot, el)
        yield pair
    if len(list_) > 1:
        yield from make_all_pairs(list_)

Answer 2

When using recursion, you need to re-yield the result.

def make_all_pairs(list_):
    pivot = list_.pop(0)
    for el in list_:
        pair = (pivot, el)
        yield pair
    if len(list_) > 1:
        for pair in make_all_pairs(list_):
            yield pair

This is made easier in Python 3.X where you can yield from iterables rather than having to loop.

Answer 3

Change the last two lines of your generator function

if len(list_) > 1:
    make_all_pairs(list_)

to

if len(list_) > 1:
    for pair in make_all_pairs(list_):
        yield pair

While your printing function does not return or yield anything, you can simply call it recursively. However for a recursive function that returns or yields, those values have to be passed back up out of the recursion, thus returned or, here, yielded.

Answer 4

The answer in python 2.7 isn't great:

def make_all_pairs(list_):
    pivot = list_.pop(0)
    for el in list_:
        pair = (pivot, el)
        yield pair
    if len(list_) > 1:
        for pair in make_all_pairs(list_):
            yield pair


x = make_all_pairs(["a","b","c","d","e"])
for el in x:
    print el

In python 3.3 and greater you can use yield from instead