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_02/sol3.py

@@ -1,18 +1,47 @@
-'''
+"""
 Problem:
-Each new term in the Fibonacci sequence is generated by adding the previous two terms. 
-                0,1,1,2,3,5,8,13,21,34,55,89,..
-Every third term from 0 is even So using this I have written a simple code
-By considering the terms in the Fibonacci sequence whose values do not exceed n, find the sum of the even-valued terms.
-e.g. for n=10, we have {2,8}, sum is 10.
-'''
-"""Python 3"""
-n = int(input())
-a=0
-b=2
-count=0
-while 4*b+a<n:
-    a, b = b, 4*b+a
-    count+= a
-print(count+b)
-   
+Each new term in the Fibonacci sequence is generated by adding the previous
+two terms. By starting with 1 and 2, the first 10 terms will be:
+
+    1,2,3,5,8,13,21,34,55,89,..
+
+By considering the terms in the Fibonacci sequence whose values do not exceed
+n, find the sum of the even-valued terms. e.g. for n=10, we have {2,8}, sum is
+10.
+"""
+from __future__ import print_function
+
+try:
+    raw_input  # Python 2
+except NameError:
+    raw_input = input  # Python 3
+
+
+def solution(n):
+    """Returns the sum of all fibonacci sequence even elements that are lower
+    or equals to n.
+    
+    >>> solution(10)
+    10
+    >>> solution(15)
+    10
+    >>> solution(2)
+    2
+    >>> solution(1)
+    0
+    >>> solution(34)
+    44
+    """
+    if n <= 1:
+        return 0
+    a = 0
+    b = 2
+    count = 0
+    while 4 * b + a <= n:
+        a, b = b, 4 * b + a
+        count += a
+    return count + b
+
+
+if __name__ == "__main__":
+    print(solution(int(raw_input().strip())))