When is not a good time to use python generators?

This is rather the inverse of What can you use Python generator functions for?: python generators, generator expressions, and the itertools module are some of my favorite features of python these days. They're especially useful when setting up chains of operations to perform on a big pile of data--I often use them when processing DSV files.

So when is it not a good time to use a generator, or a generator expression, or an itertools function?

• When should I prefer zip() over itertools.izip(), or
• range() over xrange(), or
• [x for x in foo] over (x for x in foo)?

Obviously, we eventually need to "resolve" a generator into actual data, usually by creating a list or iterating over it with a non-generator loop. Sometimes we just need to know the length. This isn't what I'm asking.

We use generators so that we're not assigning new lists into memory for interim data. This especially makes sense for large datasets. Does it make sense for small datasets too? Is there a noticeable memory/cpu trade-off?

I'm especially interested if anyone has done some profiling on this, in light of the eye-opening discussion of list comprehension performance vs. map() and filter(). (alt link)

Use a list instead of a generator when:

1. You need to access the data multiple times (i.e. cache the results instead of recomputing them):

   for i in outer: # used once, okay to be a generator or return a list for j in inner: # used multiple times, reusing a list is better ...

2. You need random access (or any access other than forward sequential order):

   for i in reversed(data): ... # generators aren't reversible

   s[i], s[j] = s[j], s[i] # generators aren't indexable

3. You need to join strings (which requires two passes over the data):

   s = ''.join(data) # lists are faster than generators in this use case

4. You are using PyPy which sometimes can't optimize generator code as much as it can with normal function calls and list manipulations.

In general, don't use a generator when you need list operations, like len(), reversed(), and so on.

There may also be times when you don't want lazy evaluation (e.g. to do all the calculation up front so you can release a resource). In that case, a list expression might be better.

Profile, Profile, Profile.

Profiling your code is the only way to know if what you're doing has any effect at all.

Most usages of xrange, generators, etc are over static size, small datasets. It's only when you get to large datasets that it really makes a difference. range() vs. xrange() is mostly just a matter of making the code look a tiny little bit more ugly, and not losing anything, and maybe gaining something.

Profile, Profile, Profile.

You should never favor zip over izip, range over xrange, or list comprehensions over generator comprehensions. In Python 3.0 range has xrange-like semantics and zip has izip-like semantics.

List comprehensions are actually clearer like list(frob(x) for x in foo) for those times you need an actual list.

As you mention, "This especially makes sense for large datasets", I think this answers your question.

If your not hitting any walls, performance-wise, you can still stick to lists and standard functions. Then when you run into problems with performance make the switch.

As mentioned by @u0b34a0f6ae in the comments, however, using generators at the start can make it easier for you to scale to larger datasets.

Regarding performance: if using psyco, lists can be quite a bit faster than generators. In the example below, lists are almost 50% faster when using psyco.full()

import psyco
import time
import cStringIO

def time_func(func):
    """The amount of time it requires func to run"""
    start = time.clock()
    func()
    return time.clock() - start

def fizzbuzz(num):
    """That algorithm we all know and love"""
    if not num % 3 and not num % 5:
        return "%d fizz buzz" % num
    elif not num % 3:
        return "%d fizz" % num
    elif not num % 5:
        return "%d buzz" % num
    return None

def with_list(num):
    """Try getting fizzbuzz with a list comprehension and range"""
    out = cStringIO.StringIO()
    for fibby in [fizzbuzz(x) for x in range(1, num) if fizzbuzz(x)]:
        print >> out, fibby
    return out.getvalue()

def with_genx(num):
    """Try getting fizzbuzz with generator expression and xrange"""
    out = cStringIO.StringIO()
    for fibby in (fizzbuzz(x) for x in xrange(1, num) if fizzbuzz(x)):
        print >> out, fibby
    return out.getvalue()

def main():
    """
    Test speed of generator expressions versus list comprehensions,
    with and without psyco.
    """

    #our variables
    nums = [10000, 100000]
    funcs = [with_list, with_genx]
    
    #  try without psyco 1st
    print "without psyco"
    for num in nums:
        print "  number:", num
        for func in funcs:
            print func.__name__, time_func(lambda : func(num)), "seconds"
        print
        
    #  now with psyco
    print "with psyco"
    psyco.full()
    for num in nums:
        print "  number:", num
        for func in funcs:
            print func.__name__, time_func(lambda : func(num)), "seconds"
        print

if __name__ == "__main__":
    main()

Results:

without psyco
  number: 10000
with_list 0.0519102208309 seconds
with_genx 0.0535933367509 seconds

  number: 100000
with_list 0.542204280744 seconds
with_genx 0.557837353115 seconds

with psyco
  number: 10000
with_list 0.0286369007033 seconds
with_genx 0.0513424889137 seconds

  number: 100000
with_list 0.335414877839 seconds
with_genx 0.580363490491 seconds

You should prefer list comprehensions if you need to keep the values around for something else later and the size of your set is not too large.

For example: you are creating a list that you will loop over several times later in your program.

To some extent you can think of generators as a replacement for iteration (loops) vs. list comprehensions as a type of data structure initialization. If you want to keep the data structure then use list comprehensions.

As far as performance is concerned, I can't think of any times that you would want to use a list over a generator.

I've never found a situation where generators would hinder what you're trying to do. There are, however, plenty of instances where using generators would not help you any more than not using them.

For example:

sorted(xrange(5))

Does not offer any improvement over:

sorted(range(5))

A generator builds and enumerable list of values. enumerables are useful when iterative process can use the values on demand. It takes time to build your generator, so if the list is millions of records in size, it may be more useful to use sql server to process the data in sql.