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


y = (2*x for x in range(10000))
print len(x)
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-5-e45ca0296d00>", line 1, in <module>
    print len(x)
TypeError: object of type 'int' has no len()
print len(w)
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(w)
99990000


print sum(y)
99990000

print sum(y)
0