Yield Keyword

[6pathsMadara]

Below is the code I have done following a tutorial, it involves creating a generator that makes the Fibonacci sequence, and then prints it out for values under 100. My problem that I have is I don't understand why I am using 'yield a' and what this does. Code is below:

def fib():
    a = 0
    b = 1
    while True:
        yield a
        a = b
        b = a + b
for f in fib():
    if f > 100:
        break
    print(f)

[Larz60+]

You capture the value (a from fib in your loop) returned by fib, but don't do anything with it except for the last iteration.
you should save it to a list, or print out results immediately:

to list:

def fib():
    a = 0
    b = 1
    while True:
        yield a
        a = b
        b = a + b
fibs = []
for f in fib():
    fibs.append(f)
    if f > 100:
        break
print(fibs)

immediate:

def fib():
    a = 0
    b = 1
    while True:
        yield a
        a = b
        b = a + b
for f in fib():
    print('{}, '.format(f), end = '')
    if f > 100:
        break
print()

[6pathsMadara]

Below is the code I have done following a tutorial, it involves creating a generator that makes the Fibonacci sequence, and then prints it out for values under 100. My problem that I have is I don't understand why I am using 'yield a' and what this does. Code is below:

def fib(): a = 0 b = 1 while True: yield a a = b b = a + b for f in fib(): if f > 100: break print(f)

You capture the value (a from fib in your loop) returned by fib, but don't do anything with it except for the last iteration. you should save it to a list, or print out results immediately: to list:

 def fib(): a = 0 b = 1 while True: yield a a = b b = a + b fibs = [] for f in fib(): fibs.append(f) if f > 100: break print(fibs) 

immediate:

 def fib(): a = 0 b = 1 while True: yield a a = b b = a + b for f in fib(): print('{}, '.format(f), end = '') if f > 100: break print() 

But I still don't understand what yield does, when i remove it, the code doesn't do anything, there's no errors but it doesn't do anything, but I still don't understand it's functionality.

[Larz60+]

yield returns one value for each iteration.
so every time fib() is called, it yields 1 more value.
It't like it has a jar of cookies, and it only gives you one, just one (each time you ask), but only when you ask for it

[heras]

return instead of yield would exit the function and forget about a and b (try it!). yield will return a value while retaining the state of the function in memory until the next time fib() is called. It will therefore remember what a and b were since the last time the function ran.

[DeaD_EyE]

A function where the yield keyword is used, is a generator.
Calling the function returns a generator object. This objects saves it's internal state.
The generator object is also an iterator.
This is where 'magic' comes in-place. Calling next() on a generator
object, executes the code until the first yield has been reached.

def gen1():
    yield 1337

g = gen1() # no code execution
next(g) # gives you 1337
next(g) # Raises StopIteration

Since Python 3.4 you can and should use the return statement inside a generator to stop the iteration. It's better then raise StopIteration, which makes trouble with newer version of Python.

The Fibonacci example has a loop, where the yield keyword is hit more than one time. This is the cause, why it behaves like a loop.

[6pathsMadara]

A function where the yield keyword is used, is a generator. Calling the function returns a generator object. This objects saves it's internal state. The generator object is also an iterator. This is where 'magic' comes in-place. Calling next() on a generator object, executes the code until the first yield has been reached.

def gen1(): yield 1337 g = gen1() # no code execution next(g) # gives you 1337 next(g) # Raises StopIteration 

Since Python 3.4 you can and should use the return statement inside a generator to stop the iteration. It's better then raise StopIteration, which makes trouble with newer version of Python. The Fibonacci example has a loop, where the yield keyword is hit more than one time. This is the cause, why it behaves like a loop.

Thank you, you have been great help and i now understand it.