Python - How to check list monotonicity

Question

What would be an efficient and pythonic way to check list monotonicity?
i.e. that it has monotonically increasing or decreasing values?

Examples:

[0, 1, 2, 3, 3, 4]   # This is a monotonically increasing list [4.3, 4.2, 4.2, -2]  # This is a monotonically decreasing list [2, 3, 1]            # This is neither 

Answer 1

It's better to avoid ambiguous terms like "increasing" or "decreasing" as it's not clear if equality is acceptable or not. You should always use either for example "non-increasing" (clearly equality is accepted) or "strictly decreasing" (clearly equality is NOT accepted).

def strictly_increasing(L):     return all(x<y for x, y in zip(L, L[1:]))  def strictly_decreasing(L):     return all(x>y for x, y in zip(L, L[1:]))  def non_increasing(L):     return all(x>=y for x, y in zip(L, L[1:]))  def non_decreasing(L):     return all(x<=y for x, y in zip(L, L[1:]))  def monotonic(L):     return non_increasing(L) or non_decreasing(L) 

Answer 2

If you have large lists of numbers it might be best to use numpy, and if you are:

import numpy as np  def monotonic(x):     dx = np.diff(x)     return np.all(dx <= 0) or np.all(dx >= 0) 

should do the trick.