README.md: sumab() --> sum_ab() for consistancy

CONTRIBUTING.md

@@ -82,7 +82,7 @@ We want your work to be readable by others; therefore, we encourage you to note
   ```python
   def sum_ab(a, b):
       """
-      Returns the sum of two integers a and b
+      Return the sum of two integers a and b
       >>> sum_ab(2, 2)
       4
       >>> sum_ab(-2, 3)
@@ -116,8 +116,8 @@ We want your work to be readable by others; therefore, we encourage you to note
   The use of [Python type hints](https://docs.python.org/3/library/typing.html) is encouraged for function parameters and return values.  Our automated testing will run [mypy](http://mypy-lang.org) so run that locally before making your submission.
 
   ```python
-  def sumab(a: int, b: int) --> int:
-      pass
+  def sum_ab(a: int, b: int) --> int:
+      return a + b
   ```
 
 - [__List comprehensions and generators__](https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions) are preferred over the use of `lambda`, `map`, `filter`, `reduce` but the important thing is to demonstrate the power of Python in code that is easy to read and maintain.

data_structures/linked_list/middle_element_of_linked_list.py

@@ -8,14 +8,14 @@ class LinkedList:
     def __init__(self):
         self.head = None
 
-    def push(self, new_data:int) -> int:
-        new_node = Node(new_data) 
-        new_node.next = self.head 
+    def push(self, new_data: int) -> int:
+        new_node = Node(new_data)
+        new_node.next = self.head
         self.head = new_node
         return self.head.data
 
     def middle_element(self) -> int:
-        '''
+        """
         >>> link = LinkedList()
         >>> link.middle_element()
         No element found.
@@ -44,11 +44,11 @@ def middle_element(self) -> int:
         >>> link.middle_element()
         12
         >>> 
-        '''
+        """
         slow_pointer = self.head
         fast_pointer = self.head
-        if self.head: 
-            while fast_pointer and fast_pointer.next: 
+        if self.head:
+            while fast_pointer and fast_pointer.next:
                 fast_pointer = fast_pointer.next.next
                 slow_pointer = slow_pointer.next
             return slow_pointer.data