====== Python yield ======

=== Iterables ===
<code>
>>> list = [1,2,3,4,5,6]
list is iterable
>>> list = [x for x in range(7)]
Iterables are handy but stores all values in memory
</code>
=== Generators ===
<code>
>>> listgenerator = (x for x in range(7))
>>> for i in listgenerator: print(i)
</code>
Generators are iterators, but you can only iterate over them once. It's because they do not store all the values in memory, they generate the values on the fly.
=== yield ===
Yield is a keyword that is used like return, except the function will return a generator.

<code python>
def createGenerator():
    print("not running until next() called")
    for i in range(7):
       yield i*i

x = createGenerator()
for t in x:
    print(t)
</code>

=== Practical example ===

tail -f access.log
<code python>
import time
def follow(thefile):
    thefile.seek(0,2) # Go to the end of the file
    while True:
        line = thefile.readline()
        if not line:
            time.sleep(0.1) # Sleep briefly
            continue
        yield line

logfile = open("access-log")
for line in follow(logfile):
    print(line)
</code>
=== Pipeline ===

<code python>
def grep(pattern,lines):
    for line in lines:
        if pattern in line:
            yield line
# Set up a processing pipe : tail -f | grep python
logfile = open("access-log")
loglines = follow(logfile)
pylines = grep("python",loglines)
# Pull results out of the processing pipeline
for line in pylines:
    print line,

</code>