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

@SandersLin Your review please?

dynamic_programming/climbing_stairs.py

@@ -1,27 +1,42 @@
+#!/usr/bin/env python3
+
+
 def climb_stairs(n: int) -> int:
-  """
-  LeetCdoe No.70: Climbing Stairs
-  Distinct ways to climb a n step staircase where
-  each time you can either climb 1 or 2 steps. 
+    """
+    LeetCdoe No.70: Climbing Stairs
+    Distinct ways to climb a n step staircase where
+    each time you can either climb 1 or 2 steps.
 
-  Args:
-      n: number of steps of staircase
+    Args:
+        n: number of steps of staircase
 
-  Returns:
-      Distinct ways to climb a n step staircase 
+    Returns:
+        Distinct ways to climb a n step staircase
 
-  Raises:
-      AssertionError: n not positive integer
+    Raises:
+        AssertionError: n not positive integer
 
-  >>> climb_stairs(3) 
-  3
-  >>> climb_stairs(1)
-  1
-  """
-  assert isinstance(n,int) and n > 0, "n needs to be positive integer, your input {0}".format(0)
-  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]
+    >>> climb_stairs(3)
+    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
+    """
+    fmt = "n needs to be positive integer, your input {}"
+    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()