x = [2*x for x in range(10000)]
print x[-10:]
[19980, 19982, 19984, 19986, 19988, 19990, 19992, 19994, 19996, 19998]


y = (2*x for x in range(10000))
print y
<generator object <genexpr> at 0x0000000003F1B1B0>

print len(x)
10000


print len(y)
Traceback (most recent call last):
  File "C:\Users\Susan\AppData\Local\Enthought\Canopy\User\lib\site-packages\IPython\core\interactiveshell.py", line 2885, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-7-6c8f20a5bcf6>", line 1, in <module>
    print len(y)
TypeError: object of type 'generator' has no len()

print sum(x)
99990000

print sum(y)
99990000

print sum(y)
0