Python中列表中生成器的多种用法

Question

Basically, I'm in following situation - I generate a list, eg

l = [2*x for x in range(10)]

which I iterate through later on multipletimes, eg

for i in l: print i    # 0,2,4,6,8,10,12,14,16,18
for i in l: print i    # 0,2,4,6,8,10,12,14,16,18
for i in l: print i    # 0,2,4,6,8,10,12,14,16,18

The problem is that the list is way too large to fit into memory, hence I use its generator form, ie:

l = (2*x for x in range(10))

However, after this construction only first iteration works:

for i in l: print i    # 0,2,4,6,8,10,12,14,16,18
for i in l: print i    #
for i in l: print i    #

Where is the problem? How may I iterate through it multipletimes?

Answer 1

Your generator is exhausted the first time. You should recreate your generator each time to renew it:

l = (2*x for x in range(10))
for i in l: print i
l = (2*x for x in range(10))
for i in l: print i

(Note: you should use xrange in python 2 because range creates a list in memory)

You can create also a shortcut function to help you or even a generator function :

def gen():
    for i in range(10):
        yield 2 * i

and then:

 for i in gen(): print i
 for i in gen(): print i

Answer 2

You can also iterate on the generator directly:

for i in (2*x for x in range(10)): print i
for i in (2*x for x in range(10)): print i
...