python - Filtering another filter object -

i trying generate prime endlessly,by filtering out composite numbers. using list store , test primes makes whole thing slow, tried use generators.

from itertools import count def chk(it,num):     in it:         if i%num:             yield(i) genstore = [count(2)] primestore = [] while 1:     prime = next(genstore[-1])     primestore.append(prime)     genstore.append(chk(genstore[-1],num)) 

it works quite well, generating primes, until hit maximum recursion depth. found ifilter (or filter in python 3). documentation of python standard library:

make iterator filters elements iterable returning predicate true. if predicate none, return items true. equivalent to:

def ifilter(predicate, iterable):     # ifilter(lambda x: x%2, range(10)) --> 1 3 5 7 9     if predicate none:         predicate = bool     x in iterable:         if predicate(x):             yield x 

so following:

from itertools import count genstore = [count(2)] primestore = [] while 1:     prime = next(genstore[-1])     primestore.append(prime)     genstore.append(filter(lambda x:x%num,genstore[-1])) 

i expected get:

2 3     5 7 11 13 17 ... 

what is:

2 3 4 5 6 7 ... 

it seems next() iterate through count(), not filter. object in list should point object, expected works filter(lambda x: x%n,(.... (filter(lambda x:x%3,filter(lambda x:x%2,count(2)))). experiment , noticed following characteristic:

1. filter(lambda x:x%2,filter(lambda x:x%3,count(0))) #works, filter 2*n , 3*n
2. genstore = [count(2)]; genstore.append(filter(lambda x:x%2,genstore[-1])); genstore.append (filter(lambda x:x%2,genstore[-1])) - works, filter 2*n , 3*n
3. next(filter(lambda x:x%2,filter(lambda x:x%3,count(2)))) - works, printing out 5

on contrast:

from itertools import count genstore = [count(2)] primestore = [] while 1:     prime = next(genstore[-1])     print(prime)     primestore.append(prime)     genstore.append(filter(lambda x:x%prime,genstore[-1]))     if len(genstore) == 3:         in genstore[-1]:             print(i) #it doesn't work, filtering out 4*n. 

questions:

• why doesn't work?
• is feature of python, or made mistakes somewhere?
• is there way fix it?

i think problem stems fact lambdas not evaluated it's 'too late' , prime same of them of them point @ same variable.

you can try add custom filter , use normal function instead of lambda:

def myfilt(f, i, p):     n in i:         print("gen:", n, i)         if f(n, p):             yield n  def byprime(x, p):     if x % p:         print("pri:", x, p)         return true  f = myfilt(byprime, genstore[-1], prime) 

this way avoid problems of lambdas being same