The Yield statement

[Alex Bryan]

Okay, so i don't really understand the Yield thing and i know it is
useful. I've read a few things about it but it is all programming
jargon and so basically it is hard for me to understand. So can anyone
give me a description or link me to a site that has a good definition
and/or examples of it? If you could I would really appreciate it.

[Larry Bates]

Re: The Yield statement

Alex Bryan wrote:

Okay, so i don't really understand the Yield thing and i know it is
useful. I've read a few things about it but it is all programming jargon
and so basically it is hard for me to understand. So can anyone give me
a description or link me to a site that has a good definition and/or
examples of it? If you could I would really appreciate it.

Use yield when you want the function to act as a generator. That is each time
it is called it generates a response and returns it, but leaves its state intact
so that the next time you call it, it can pick up where it left off and continue on.

Best example I ever saw is the code that implements os.walk() function:

def walk(top, topdown=True, onerror=None):
"""
Example:

from os.path import join, getsize
for root, dirs, files in walk('python/Lib/email'):
print root, "consumes",
print sum([getsize(join(ro ot, name)) for name in files]),
print "bytes in", len(files), "non-directory files"
if 'CVS' in dirs:
dirs.remove('CV S') # don't visit CVS directories
"""

from os.path import join, isdir, islink

try:
names = listdir(top)
except error, err:
if onerror is not None:
onerror(err)
return

dirs, nondirs = [], []
for name in names:
if isdir(join(top, name)):
dirs.append(nam e)
else:
nondirs.append( name)

if topdown:
yield top, dirs, nondirs
for name in dirs:
path = join(top, name)
if not islink(path):
for x in walk(path, topdown, onerror):
yield x
if not topdown:
yield top, dirs, nondirs


Actually this code uses yield and is recursive. Pretty neat.

-Larry

[Mensanator]

Re: The Yield statement

On Jun 30, 5:31 pm, Alex Bryan <alexnbr...@gma il.comwrote:

Okay, so i don't really understand the Yield thing and i know it is  
useful. I've read a few things about it but it is all programming  
jargon and so basically it is hard for me to understand. So can anyone  
give me a description or link me to a site that has a good definition  
and/or examples of it? If you could I would really appreciate it.

You know what a function does, right? If I were to say

x = NextPrime(3)

then x should be set to 5. The NextPrime function
does this by evaluating the input argument (3) and
doing

return p

as the last statement (p having been determined to be 5).

Now, suppose you had a function Primes (i.e., a function
that returns ALL the primes). Obviously, since there are
an infinite number of them, the function would never
end since it would eventually eat up all your memory
trying to create a list with infinite elements.

However, if Primes were a generator, you could have it
return just 1 result at a time. The generator would
keep track of the last prime issued so that every .next()
call would give you the next prime in the sequence.

That's what "yield" does. It functions like a return,
only returning a single element of an iteration.
For infinite loops, the program simply stops calling
the function when it has enough (or tells the function
to stop).

Here's an example. Note that before it enters the output
loop it does a couple things. First, it checks for valid
input. If not valid, it executes a "return" statement
which kills the generator.

If valid, it issues a "yield" prior to the loop. It must
do this to issue the first partition. The loop will issue
all subsequent partitions with its "yield" statements.

This particular generator is always a finite loop. Although
the number of generated partitions can be quite large, best
to test by doing C(depth-1,width-1) before asking for
all the results (which is what "for i in p" does).

def partition_gener ator(depth,widt h):
"""creates all partions of a given depth,widtth (depth>=width)
depth objects in width bins such that each bin has at least 1 object
this function is a generator (does comb(depth-1,width-1) partitions)

partition_gener ator(depth,widt h)
depth: total inverse rule 1 count (p of 2**p)
width: total inverse rule 2 count (q of 3**q)
sv: sequence vector (the partition)
returns sequence vector [sv]
"""
def move_col(c):
sv[c-1] += 1
sv[c] -= 1
def find_c():
i = -1
while i<0:
if sv[i]>1:
return i
i -= 1
def rollover(c):
move_col(c)
sv[-1] = sv[c]
sv[c] = 1
if depth<width: # error
print 'depth',depth,' must be >= to width',width
return # kills generator
max_element = depth - width + 1
sv = [1 for i in range(width)]
sv[-1] = max_element
yield sv[:] # first partition
while sv[0]<max_element: # rest are generated by this loop
c = find_c()
if c < -1:
rollover(c)
yield sv[:] # must yield copy of sv, not sv
else:
move_col(c)
yield sv[:]

p = partition_gener ator(6,4)

for i in p: print i

## [1, 1, 1, 3]
## [1, 1, 2, 2]
## [1, 1, 3, 1]
## [1, 2, 1, 2]
## [1, 2, 2, 1]
## [1, 3, 1, 1]
## [2, 1, 1, 2]
## [2, 1, 2, 1]
## [2, 2, 1, 1]
## [3, 1, 1, 1]

[John Machin]

Re: The Yield statement

On Jul 1, 8:31 am, Alex Bryan <alexnbr...@gma il.comwrote:

Okay, so i don't really understand the Yield thing and i know it is
useful. I've read a few things about it but it is all programming
jargon and so basically it is hard for me to understand. So can anyone
give me a description or link me to a site that has a good definition
and/or examples of it? If you could I would really appreciate it.

Try this:

http://www.dabeaz.com/generators/

(ignore the "tricks" word in the title)

[inhahe]

Re: The Yield statement


"Alex Bryan" <alexnbryan@gma il.comwrote in message
news:mailman.10 09.1214865088.1 044.python-list@python.org ...

Okay, so i don't really understand the Yield thing and i know it is
useful. I've read a few things about it but it is all programming jargon
and so basically it is hard for me to understand. So can anyone give me a
description or link me to a site that has a good definition and/or
examples of it? If you could I would really appreciate it.

Really short answer:

def f():
yield 1
yield 2
yield 3

for x in f(): print x,
# should print 1 2 3

def f():
for x in xrange(10):
yield x

for x in f(): print x,
# should print 0 1 2 3 4 5 6 7 8 9

note that this won't work;
def f():
for x in xrange(10):
for y in xrange(10):
yield (x,y)

yield just doesn't work right with multiple levels of loops. i had to
discover that the hard way.

[Mark Tolonen]

Re: The Yield statement


"inhahe" <inhahe@gmail.c omwrote in message
news:z6_bk.2496 0$AJ6.1423@bign ews8.bellsouth. net...

>
"Alex Bryan" <alexnbryan@gma il.comwrote in message
news:mailman.10 09.1214865088.1 044.python-list@python.org ...

>Okay, so i don't really understand the Yield thing and i know it is
>useful. I've read a few things about it but it is all programming jargon
>and so basically it is hard for me to understand. So can anyone give me
>a description or link me to a site that has a good definition and/or
>examples of it? If you could I would really appreciate it.

>
Really short answer:
>
def f():
yield 1
yield 2
yield 3
>
for x in f(): print x,
# should print 1 2 3
>
def f():
for x in xrange(10):
yield x
>
for x in f(): print x,
# should print 0 1 2 3 4 5 6 7 8 9
>
note that this won't work;
def f():
for x in xrange(10):
for y in xrange(10):
yield (x,y)
>
yield just doesn't work right with multiple levels of loops. i had to
discover that the hard way.

Huh? Works fine:

>>def f():

.... for x in xrange(10):
.... for y in xrange(10):
.... yield (x,y)
....

>>for x in f(): print x,

....
(0, 0) (0, 1) (0, 2) (0, 3) (0, 4) (0, 5) (0, 6) (0, 7) (0, 8) (0, 9) (1, 0)
(1,
1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6) (1, 7) (1, 8) (1, 9) (2, 0) (2, 1)
(2, 2)
(2, 3) (2, 4) (2, 5) (2, 6) (2, 7) (2, 8) (2, 9) (3, 0) (3, 1) (3, 2) (3,
3) (3
, 4) (3, 5) (3, 6) (3, 7) (3, 8) (3, 9) (4, 0) (4, 1) (4, 2) (4, 3) (4, 4)
(4, 5
) (4, 6) (4, 7) (4, 8) (4, 9) (5, 0) (5, 1) (5, 2) (5, 3) (5, 4) (5, 5) (5,
6) (
5, 7) (5, 8) (5, 9) (6, 0) (6, 1) (6, 2) (6, 3) (6, 4) (6, 5) (6, 6) (6, 7)
(6,
8) (6, 9) (7, 0) (7, 1) (7, 2) (7, 3) (7, 4) (7, 5) (7, 6) (7, 7) (7, 8) (7,
9)
(8, 0) (8, 1) (8, 2) (8, 3) (8, 4) (8, 5) (8, 6) (8, 7) (8, 8) (8, 9) (9, 0)
(9,
1) (9, 2) (9, 3) (9, 4) (9, 5) (9, 6) (9, 7) (9, 8) (9, 9)

-Mark

[Jorgen Grahn]

Re: The Yield statement

On Sun, 6 Jul 2008 00:29:26 -0700, Mark Tolonen <M8R-yfto6h@mailinat or.comwrote:

>
"inhahe" <inhahe@gmail.c omwrote in message
news:z6_bk.2496 0$AJ6.1423@bign ews8.bellsouth. net...

....

>note that this won't work;
>def f():
> for x in xrange(10):
> for y in xrange(10):
> yield (x,y)
>>
>yield just doesn't work right with multiple levels of loops. i had to
>discover that the hard way.

>
Huh? Works fine: [...]

Yes -- as is expected. Yield would be much less useful if it only
work with strange restrictions on the structure of the function
containing it.

As far as I can tell, you can pick any function, stick zero or more
yield statements into it, and it will work as expected: hit a yield,
and you get another value for your sequence.

/Jorgen

--
// Jorgen Grahn <grahn@ Ph'nglui mglw'nafh Cthulhu
\X/ snipabacken.se R'lyeh wgah'nagl fhtagn!

[Paul Hankin]

Re: The Yield statement

On Jul 1, 12:45 am, Mensanator <mensana...@aol .comwrote:

Here's an example.
...
def partition_gener ator(depth,widt h):
  """creates all partions of a given depth,widtth (depth>=width)
  depth objects in width bins such that each bin has at least 1 object
  this function is a generator (does comb(depth-1,width-1) partitions)
>
  partition_gener ator(depth,widt h)
  depth:  total inverse rule 1 count (p of 2**p)
  width:  total inverse rule 2 count (q of 3**q)
  sv:     sequence vector (the partition)
  returns sequence vector [sv]
  """
  def move_col(c):
    sv[c-1] += 1
    sv[c] -= 1
  def find_c():
    i = -1
    while i<0:
      if sv[i]>1:
        return i
      i -= 1
  def rollover(c):
    move_col(c)
    sv[-1] = sv[c]
    sv[c] = 1
  if depth<width:                # error
    print 'depth',depth,' must be >= to width',width
    return                       # kills generator
  max_element = depth - width + 1
  sv = [1 for i in range(width)]
  sv[-1] = max_element
  yield sv[:]                    # first partition
  while sv[0]<max_element:       # rest are generated by this loop
    c = find_c()
    if c < -1:
      rollover(c)
      yield sv[:]   # must yield copy of sv, not sv
    else:
      move_col(c)
      yield sv[:]

It's just an example I know, but you can write this much more
cleanly...

def partitions(n, length):
"""Generate all sequences of positive numbers of the given
length that sum to n."""
assert 0 < length <= n
if length == 1:
yield [n]
return
for i in xrange(1, n + 2 - length):
for p in partitions(n - i, length - 1):
yield [i] + p

--
Paul Hankin