“Return” in Function only Returning one Value

问题

Let's say I write a for loop that will output all the numbers 1 to x:

x=4
for number in xrange(1,x+1):
    print number,
#Output:
1
2
3
4

Now, putting that same for loop into a function:

def counter(x):
    for number in xrange(1,x+1):
        return number
print counter(4)
#Output:
1

Why do I only obtain one value when I put the for-loop into a function?

I have been evading this problem by appending all the results of the for-loop to a list, and then returning the list.

Why does the for loop append all the results, and not just one?:

def counter(x):
    output=[]
    for number in xrange(1,x+1):
        output.append(number)
    return output

What is the best method of returning all the values, appending to a list seems very inefficient.

回答1:

return does exactly like the keyword's name implies. When you hit that statement, it returns and the rest of the function is not executed.

What you might want instead is the yield keyword. This will create a generator function (a function that returns a generator). Generators are iterable. They "yield" one element each time the yield expression is executed.

def func():
    for x in range(10):
        yield x

generator = func()
for item in generator:
    print item

回答2:

What you want is is called a Generator:

def counter(x):
    for number in xrange(1,x+1):
        yield number

You would then use it like this:

c = counter(5)
next(c)  # 1
next(c)  # 2

You could also consume the entire generator by doing:

xs = list(c)  # xs = [1, 2, 3, 4, 5]

See: http://docs.python.org/2/tutorial/classes.html#generators for more information.

The return statement in Python returns form the function and does not save any state. Every time you call the function a new stack frame is created. yield on the other hand is (more or less) Python's equivilent of continutations

回答3:

Once return in a function, the function ends and the remaining code won't be excuted any more. Your second solution is good and if you want better solution, you can use generator:

def counter(x):
    for number in xrange(1,x+1):
        yield number

And then you can use it like this:

>>> for i in counter(5):
...     print i
... 
1
2
3
4
5

来源：https://stackoverflow.com/questions/20604773/return-in-function-only-returning-one-value

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python

function

for-loop

printing

return