[Tutor] I am having difficulty grasping 'generators'

[Degreat Yartey]

I really love this explanation... that means functions just run till it
finishes its duty, then return...and generators just generate one at a time
until the 'for' statement asks for __next__().
On May 28, 2014 8:37 AM, "Cameron Simpson" <cs at zip.com.au> wrote:

> On 27May2014 15:27, Degreat Yartey <yarteydegreat2 at gmail.com> wrote:
>
>> I am studying python on my own (i.e. i am between the beginner and
>> intermediate level) and i haven't met any difficulty until i reached the
>> topic 'Generators and Iterators'.
>> I need an explanation so simple as using the expression 'print ()', in
>> this
>> case 'yield'.
>> Python 2.6 here!
>> Thank you.
>>
>
> Generators are functions that do a bit of work and then yield a value,
> then a bit more and so on. This means that you "call" them once. What you
> get back is an iterator, not the normal function return value.
>
> Whenever you use the iterator, the generator function runs until it hits a
> "yield" statement, and the value in theyield statement is what you get for
> that iteration. Next time you iterate, the function runs a bit more, until
> it yields again, or returns (end of function, and that causes end of
> iteration).
>
> So the function doesn't even run until you ask for a value, and then it
> only runs long enough to find the next value.
>
> Example (all code illstrative only, untested):
>
> Suppose you need to process every second line of a file.
>
> You might write it directly like this:
>
>   def munge_lines(fp):
>     ''' Do stuff with every second line of the already-open file `fp`.
>     '''
>     lineno = 0
>     for line in fp:
>       lineno += 1
>       if lineno % 2 == 0:
>         print lineno, line,
>
> That should read lines from the file and print every second one with the
> line number.
>
> Now suppose you want something more complex than "every second line",
> especially something that requires keeping track of some state. In the
> example above you only need the line number, and using it still consumes 2
> of the 3 lines in the loop body.
>
> A more common example might be "lines between two markers".
>
> The more of that you embed in the "munge_lines" function, the more it will
> get in the way of seeing what the function actually does.
>
> So a reasonable thing might be to write a function that gets the requested
> lines:
>
>   def wanted_lines(fp):
>     wanted = []
>     between = False
>     for line in fp:
>       if between:
>         if 'end_marker' in line:
>           between = False
>         else:
>           wanted.append(line)
>       elif 'start_maker' in line:
>         between = True
>     return wanted
>
> This reads the whole file and returns a line of the wanted lines, and
> "munge_lines: might then look like this:
>
>   for line in wanted_lines(fp):
>     print line
>
> However:
>
>   - that reads the whole file before returning anything
>
>   - has to keep all the lines in the list "wanted"
>
> Slow in response, heavy in memory cost, and unworkable if "fp" actually
> doesn't end (eg reading from a terminal, or a pipeline, or...)
>
> What you'd really like is to get each line as needed.
>
> We can rewrite "wanted_lines" as a generator:
>
>   def wanted_lines(fp):
>     between = False
>     for line in fp:
>       if between:
>         if 'end_marker' in line:
>           between = False
>         else:
>           yield line
>       elif 'start_maker' in line:
>         between = True
>
> All we've done is used "yield" instead of the "append" and removed the
> "wanted" list and the return statement. The calling code is the same.
>
> To see the difference, put a "print" in "wanted_lines" as the first line
> of the for loop. With the "list" version you will see all the prints run
> before you get the array back. With the generator you will see the print
> run just before each value you get back.
>
> Cheers,
> Cameron Simpson <cs at zip.com.au>
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