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/sol1.py

@@ -1,24 +1,47 @@
-'''
+"""
 Problem:
-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.
-'''
+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
+    raw_input  # Python 2
 except NameError:
     raw_input = input  # Python 3
 
-n = int(raw_input().strip())
-i=1
-j=2 
-sum=0
-while(j<=n):
-    if j%2 == 0:
-        sum+=j
-    i , j = j, i+j
-print(sum)
+
+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
+    """
+    i = 1
+    j = 2
+    sum = 0
+    while j <= n:
+        if j % 2 == 0:
+            sum += j
+        i, j = j, i + j
+
+    return sum
+
+
+if __name__ == "__main__":
+    print(solution(int(raw_input().strip())))