Explaining this primes generator function, i cannot understand [python]
def primes(n):
    if n==2: return [2]
    elif n<2: return []
    s=range(3,n+1,2)
    mroot = n ** 0.5
    half=(n+1)/2-1
    i=0
    m=3
    while m <= mroot:
        if s[i]:
            j=(m*m-3)/2
            s[j]=0
            while j<half:
                s[j]=0
                j+=m
        i=i+1
        m=2*i+3
    return [2]+[x for x in s if x]

def primes(n):
    if n==2: return [2]

elif n<2: return []

s=range(3,n+1,2)

mroot = n ** 0.5

half=(n+1)/2-1

i=0
    m=3

while m <= mroot:
        if s[i]:

j=(m*m-3)/2
            s[j]=0

while j<half:
                s[j]=0
                j+=m

i=i+1
        m=2*i+3

return [2]+[x for x in s if x]

>>> numbers = range(40)
>>> numbers[1] = 0    # 1 isn't prime
>>> for i in numbers:
...     if i:
...         for j in range(i + i, len(numbers), i):
...             numbers[j] = 0
...
>>> [n for n in numbers if n]
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37]

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10...]

[0, 0, 2, 3, 4, 5, 6, 7, 8, 9, 10...]
[0, 0, 2, 3, 0, 5, 0, 7, 0, 9, 0...]
[0, 0, 2, 3, 0, 5, 0, 7, 0, 0, 0...]