list() strange behaviour

[danilob]

Hi,
I'm an absolute beginner in Python (and in English too ;-)
Running this code:


--------------
# Python 3.9.0

a = [[1, 2, 0, 3, 0],
      [0, 4, 5, 0, 6],
      [7, 0, 8, 0, 9],
      [2, 3, 0, 0, 1],
      [0, 0, 1, 8, 0]]


b = ((x[0] for x in a))

print(list(b))
print(list(b))
---------------


I get this output:

[1, 0, 7, 2, 0]
[]


I don't know why the second print() output shows an empty list.
Is it possible that the first print() call might have changed the value 
of "b"?

Thank you in advance.