```
# prime numbers are only divisible by unity and themselves
# (1 is not considered a prime number by convention)
def isprime(n):
'''check if integer n is a prime'''
# make sure n is a positive integer
n = abs(int(n))
# 0 and 1 are not primes
if n < 2:
return False
# 2 is the only even prime number
if n == 2:
return True
# all other even numbers are not primes
if not n & 1:
return False
# range starts with 3 and only needs to go up the squareroot of n
# for all odd numbers
for x in range(3, int(n**0.5)+1, 2):
if n % x == 0:
return False
return True
# test ...
print isprime(1) # False
print isprime(2) # True
print isprime(3) # True
print isprime(29) # True
print isprime(345) # False
print isprime(999979) # True
print isprime(999981) # False
# extra test ...
print isprime(49) # False
print isprime(95) # False
```

*Edited 5 Years Ago by vegaseat*: 49 55

*Edited 5 Years Ago by kungfujam*: Previous incorrect

*Edited 4 Years Ago by pyTony*: Code block by TAB

*Edited 3 Years Ago by vegaseat*: slow algorithm

*Edited 3 Years Ago by vegaseat*: spell

*Edited 4 Years Ago by pyTony*

*Edited 3 Years Ago by vegaseat*: timing

*Edited 4 Years Ago by woooee*

*Edited 4 Years Ago by Gribouillis*

*Edited 3 Years Ago by Gribouillis*