A python module is a container for some definitions and statements. You generally call it like this:
import math
or like that
from math import ceil
or
import math as mymath
Python modules can also be written in C or C++, but I'll only explain how to write the module in Python. Modules can be written in C++ for performance reasons. Just take a look at /usr/lib/python3.1/lib-dynload with all the *.so files (shared libraries).
Python Paths
When you try to import a module, Python looks at these directories in the given order:
- the PYTHONPATH
- the current working directory
- the default search path
You get your PYTHONPATH and your default search path like this:
import os
os.environ["PATH"].split(os.pathsep)
os.environ["PYTHONPATH"].split(os.pathsep)
Example
I've just searched for a Python module for primes. It seems as if no such module existed. So I wrote the module primes.py.
"""
This module offers some functions related to primes.
"""
def miller_rabin(n):
import random
""" Source: http://en.literateprograms.org/
Miller-Rabin_primality_test_(Python)
"""
d = n - 1
s = 0
while d % 2 == 0:
d >>= 1
s += 1
for repeat in range(20):
a = 0
while a == 0:
a = random.randrange(n)
if not miller_rabin_pass(a, s, d, n):
return False
return True
def miller_rabin_pass(a, s, d, n):
a_to_power = pow(a, d, n)
if a_to_power == 1:
return True
for i in range(s - 1):
if a_to_power == n - 1:
return True
a_to_power = (a_to_power * a_to_power) % n
return a_to_power == n - 1
def getPrimeFactors(n):
"""Return the prime factors of n.
If the result is small enough to fit in an int, return an int.
Else return a long.
>>> [getPrimeFactors(n) for n in range(11)]
[[], [], [2], [3], [2, 2], [5], [2, 3], [7], [2, 2, 2], [3, 3], [2, 5]]
>>> getPrimeFactors(36)
[2, 2, 3, 3]
"""
import math
if not n >= 0:
raise ValueError("n must be >= 0")
if math.floor(n) != n:
raise ValueError("n must be exact integer")
elif n <= 2147483647:
n = int(n)
else:
n = long(n)
fact = []
if n == 0:
return fact
while n % 2 == 0:
fact.append(2)
n /= 2
if n == 1:
return fact
if miller_rabin(n):
fact.append(n)
return fact
check = 3
rootn = n ** 0.5
while n != 1:
while n % check == 0:
fact.append(check)
n /= check
check += 2
return fact
if __name__ == "__main__":
import doctest
doctest.testmod()