Fix error message and format with python/black (#1025)

@SandersLin Your review please?

dynamic_programming/climbing_stairs.py

@@ -1,3 +1,6 @@
+#!/usr/bin/env python3
+
+
 def climb_stairs(n: int) -> int:
     """
     LeetCdoe No.70: Climbing Stairs
@@ -17,11 +20,23 @@ def climb_stairs(n: int) -> int:
     3
     >>> climb_stairs(1)
     1
+    >>> climb_stairs(-7)  # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+        ...
+    AssertionError: n needs to be positive integer, your input -7
     """
-  assert isinstance(n,int) and n > 0, "n needs to be positive integer, your input {0}".format(0)
+    fmt = "n needs to be positive integer, your input {}"
-  if n == 1: return 1 
+    assert isinstance(n, int) and n > 0, fmt.format(n)
+    if n == 1:
+        return 1
     dp = [0] * (n + 1)
     dp[0], dp[1] = (1, 1)
     for i in range(2, n + 1):
         dp[i] = dp[i - 1] + dp[i - 2]
     return dp[n]
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()