Update binary_exponentiation.py
diff --git a/maths/binary_exponentiation.py b/maths/binary_exponentiation.py
index 05de939d1bde..7eeca89262a9 100644
--- a/maths/binary_exponentiation.py
+++ b/maths/binary_exponentiation.py
@@ -5,6 +5,12 @@
 
 
 def binary_exponentiation(a: int, n: int) -> int:
+    """
+    >>> binary_exponentiation(3, 5)
+    243
+    >>> binary_exponentiation(10, 3)
+    1000
+    """
     if n == 0:
         return 1
 
@@ -17,6 +23,10 @@ def binary_exponentiation(a: int, n: int) -> int:
 
 
 if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
     try:
         BASE = int(input("Enter Base : ").strip())
         POWER = int(input("Enter Power : ").strip())

### Describe your change:

Added doctests.

* [ ] Add an algorithm?
* [ ] Fix a bug or typo in an existing algorithm?
* [X] Documentation change?

### Checklist:
* [X] I have read [CONTRIBUTING.md](https://github.com/TheAlgorithms/Python/blob/master/CONTRIBUTING.md).
* [X] This pull request is all my own work -- I have not plagiarized.
* [X] I know that pull requests will not be merged if they fail the automated tests.
* [X] This PR only changes one algorithm file.  To ease review, please open separate PRs for separate algorithms.
* [X] All new Python files are placed inside an existing directory.
* [X] All filenames are in all lowercase characters with no spaces or dashes.
* [X] All functions and variable names follow Python naming conventions.
* [X] All function parameters and return values are annotated with Python [type hints](https://docs.python.org/3/library/typing.html).
* [X] All functions have [doctests](https://docs.python.org/3/library/doctest.html) that pass the automated testing.
* [ ] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
* [X] If this pull request resolves one or more open issues then the description above includes the issue number(s) with a [closing keyword](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue): "Fixes #ISSUE-NUMBER".
