yield2

Aladdin

Nov 1, 2019 11:00

def mygen():
    print("one")
    yield 10
    print("two")
    yield 20
    print("three")
    yield 30
    print("four")

f = mygen()

for i in f:
    print(i)
D:\rhs\Python>python test.py
one
10
two
20
three
30
four

so here:

when u run the for loop

John Rennie

Suppose you have an iterator f. Don't worry about how the iterator got created.

Aladdin

it runs the function

John Rennie

There are two ways to step through an iterator:

1. use a for loop. The loop steps through the iterator each time through the loop.

OK so far?

Aladdin

ok

John Rennie

2. use the next(f) function.

Every time you call next(f) it returns the next value from the iterator.

Aladdin

Nov 1, 2019 11:03

ok

John Rennie

Let's take a concrete example. Suppose you have an iterator f that returns 1 then 2 then 3 then 4.

So if we do:

for i in f:
  print(i)

This will print:

OK so far?

Aladdin

iterator is same as iterable?

is list iterator?

John Rennie

when you iterate through a list Python quietly defines an iterator that steps through the list. You never actually get to see this.

Lists are not themselves iterators.

Aladdin

ok

@JohnRennie Suppose you have an iterator f that returns 1 then 2 then 3 then 4.

Whar do you mean by this

the iterator contains(1,2,3) right

so when u call ,it returns them?

one by one

John Rennie

We need to be clear about the terminology. An iterator is an object that steps through some data.

The iterator doesn't necessarily contain the data though it can do.

Suppose you had a list l=[1,2,3] and you define a function:

def foo(l):
    for i in len(l):
        yield l[i]

Aladdin

Nov 1, 2019 11:12

ok

John Rennie

This defines a generator called foo and when you call foo(x) it returns an iterator that steps through the list x.

So we have two different objects:
1. we have the list `x`
2. we have the iterator that steps through the list

Aladdin

ok

John Rennie

The iterator is not the same as the iterable object.

Aladdin

ok

John Rennie

This would normally be the case. The mygen generator used as an example is unusual in that the data is actually contained inside the generator.

Aladdin

Nov 1, 2019 11:15

ok

how would replacing for with next work here

John Rennie

Suppose we have an iterator f, then next(f) returns the first value that the iterator produces.

Aladdin

so 1

John Rennie

Then when we call next(f) again it returns the second value that the iterator produces.

Aladdin

are generators and iterators same

last doubt

John Rennie

When you call a generator it retruns an iterator.

In your original code we have a generator called mygen, then did:

f = mygen()

This makes f the iterator.

Aladdin

Nov 1, 2019 11:22

ok

John Rennie

Then we can do for i in f or i = next(f) to get the data from the iterator f.

Aladdin

ok

John Rennie

def foo(l):
    for i in range(len(l)):
        yield l[i]

x = ["rats", "cats", "bats"]
f = foo(x)

for i in f:
    print(i)

Aladdin

foo(l) is generator

John Rennie

x is the data i.e. a list. foo is the generator and f is the iterator.

Aladdin

Nov 1, 2019 11:30

@JohnRennie okay got t

it

John Rennie

:-)

Aladdin

so next(f) is same as for loop

@JohnRennie One final doubt

John Rennie

Yes?

Aladdin

def mygen():
    	print("one")
    	yield 10
    	print("two")
    	yield 20
    	print("three")
    	yield 30
    	print("four")

f=mygen()

res=next(f)
print(res)

res=next(f)
print(res)

res=next(f)
print(res

so mygen() is generator

John Rennie

Yes

Aladdin

Nov 1, 2019 11:32

f is iterator

John Rennie

Yes.

Aladdin

so when u do res=next(f)

why it also shows one

print doesn't return anything

John Rennie

Because behind the scenes the iterator f is calling the generator function mygen, and that contains print statements.

The print statements don't return anything they just print. It's the yield commands that return the values in the iterator.

Aladdin

yeah but u are printing (res)

John Rennie

A real generator would not have print statements in it. I guess they have been put in to show that the iterator is calling the function mygen.

Aladdin

Nov 1, 2019 11:37

@JohnRennie So is res a variable

John Rennie

When you call next(f) that get the next value from the iterator f

Aladdin

@JohnRennie Aha got it

John Rennie

The iterator f gets that value from the generator mygen

And the generator mygen has print statements in it.

Aladdin

next(f) doesn't contain print

it contains 10,20,30

John Rennie

Yes

Aladdin

Nov 1, 2019 11:41

@JohnRennie So iterator is stepping though what data?yield(10),yield(20)?

print(one)..these are not what iterator steps through

John Rennie

Yes, the iterator gets each yield value in turn.

Aladdin

aha got it

John Rennie

The print statements don't affect the data returned by the iterator at all.

Aladdin

yeah

so i guess iterator teps through both print and yield

but it returns yield

am i right

John Rennie

Yes

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