10 examples of 'python program to find prime number' in Python

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def prime(num):
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    # num is actually a string because input() returns strings. We'll convert it to int
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    num = int(num)
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    if num < 0:
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        print("Negative integers can not be prime")
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        quit()
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    if num is 1:
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        print("1 is neither prime nor composite")
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        # See how I lazily terminated program otherwise it'd forward "None"(default behaviour of python when function
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        # returns nothing) rather than True or False. Which could mess up the program.
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        # If we hit this if statement above statement is printed then program exits.
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        quit()  # Now you don't need to get sys.exit() to exit python has quit to handle the same thing
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    if num in [2, 3]:
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        # if given argument is 2 or 3, it is prime. We used list without defining a variable which is perfectly valid
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        return True
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    if num % 2 == 0:  # excluding all even numbers except two.
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        return False
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    else:
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        # Here we are starting counter variable from 3 in range. Second argument excludes numbers above one third
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        # of the given argument. Third argument in range sets steps to take to 2. This makes loop to iterate odds
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        for x in range(3, int(num/3), 2):
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            # Checking if argument is divisible by counter. % is modulus operator which returns remainder of division
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            if num % x == 0:
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                return False
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    # It's okay to have more than one return statement when program hits return statement it exits the function.
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    return True

12
def primes():
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    yield 2
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    yield 3
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    for i in itertools.count(start=5, step=2):
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        if is_prime(i):
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            yield i

7
def isPrime(num):
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    # Returns True if num is a prime number, otherwise False.
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    # Note: Generally, isPrime() is slower than primeSieve().
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    # all numbers less than 2 are not prime
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    if num < 2:
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        return False
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    # see if num is divisible by any number up to the square root of num
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    for i in range(2, int(math.sqrt(num)) + 1):
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        if num % i == 0:
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            return False
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    return True

23
def findprimes(start, end):
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	for i in range(start, end):
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		if i not in Checked:
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			Checked.append(i)
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			if is_prime(i):
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				Primes.append(i)

6
def is_prime(n):
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   '''
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   checks if a number is prime
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   '''
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   if n < 2:
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      return False
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   if n == 2:
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      return True
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   for x in range(2, int(n**0.5)+1, 2):
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      if n % x == 0:
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         return False
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   return True

3
def is_prime(n):
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    i = 3
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    while i * i <= n:
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        if n % i == 0: return False
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        i += 2
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    return True

1
def is_prime(n):
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	if n < 2:
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		return False
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	i = 2
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	while i * i <= n:
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		if n % i == 0:
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			return False
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		i += 1
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	return True

6
def countPrimes(self, n):
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    notPrimes = []
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    for k in range(n + 1):
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        notPrimes.append(False)
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    count = 0
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    for i in range(2, n):
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        if notPrimes[i]:
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            continue
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        count += 1
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        for j in range(i * i, n, i):
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            notPrimes[j] = True
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    return count

21
def is_prime(n):
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    for i in xrange(2, int(sqrt(n)) + 1):
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        if n % i == 0:
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            return False
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    return True

3
def is_prime(n):
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    if n <= 1:
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        return False
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    elif n == 2:
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        return True
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    elif n % 2 == 0:
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        return False
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    for i in xrange(3, int(sqrt(n))+1, 2):
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        if n % i == 0:
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            return False
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    return True