Added doctest and more explanation about Dijkstra execution. (#1014)

* Added doctest and more explanation about Dijkstra execution.

* tests were not passing with python2 due to missing __init__.py file at number_theory folder

* Removed the dot at the beginning of the imported modules names because 'python3 -m doctest -v data_structures/hashing/*.py' and 'python3 -m doctest -v data_structures/stacks/*.py' were failing not finding hash_table.py and stack.py modules.

* Moved global code to main scope and added doctest for project euler problems 1 to 14.

* Added test case for negative input.

* Changed N variable to do not use end of line scape because in case there is a space after it the script will break making it much more error prone.

* Added problems description and doctests to the ones that were missing. Limited line length to 79 and executed python black over all scripts.

* Changed the way files are loaded to support pytest call.

* Added __init__.py to problems to make them modules and allow pytest execution.

* Added project_euler folder to test units execution

* Changed 'os.path.split(os.path.realpath(__file__))' to 'os.path.dirname()'

project_euler/problem_07/sol1.py

@@ -1,30 +1,61 @@
-'''
+# -*- coding: utf-8 -*-
+"""
 By listing the first six prime numbers:
-2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
-What is the Nth prime number?
-'''
+
+    2, 3, 5, 7, 11, and 13
+
+We can see that the 6th prime is 13. What is the Nth prime number?
+"""
 from __future__ import print_function
 from math import sqrt
+
+try:
+    raw_input  # Python 2
+except NameError:
+    raw_input = input  # Python 3
+
+
 def isprime(n):
-    if (n==2):
+    if n == 2:
         return True
-    elif (n%2==0):
+    elif n % 2 == 0:
         return False
     else:
-        sq = int(sqrt(n))+1
-        for i in range(3,sq,2):
-            if(n%i==0):
+        sq = int(sqrt(n)) + 1
+        for i in range(3, sq, 2):
+            if n % i == 0:
                 return False
     return True
-n = int(input())
-i=0
-j=1
-while(i!=n and j<3):
-    j+=1
-    if (isprime(j)):
-        i+=1
-while(i!=n):
-    j+=2
-    if(isprime(j)):
-        i+=1
-print(j)
+
+
+def solution(n):
+    """Returns the n-th prime number.
+ 
+    >>> solution(6)
+    13
+    >>> solution(1)
+    2
+    >>> solution(3)
+    5
+    >>> solution(20)
+    71
+    >>> solution(50)
+    229
+    >>> solution(100)
+    541
+    """
+    i = 0
+    j = 1
+    while i != n and j < 3:
+        j += 1
+        if isprime(j):
+            i += 1
+    while i != n:
+        j += 2
+        if isprime(j):
+            i += 1
+    return j
+
+
+if __name__ == "__main__":
+    print(solution(int(raw_input().strip())))