• pandas
  • numpy
>>> x = np.arange(10)
>>> if x<5: print('Small!')
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

That x<5 is not actually a boolean value, but an array of 10 bools, indicating which values are under 5.

If what the user wants is (x<5).any() or (x<5).all() and they just failed to read the error, close as a dup (separate questions for numpy and pandas).

If what the user wanted was to use x<5 as a mask array to do further array processing instead of whatever loop with an if or while they were attempting, that could be a good question, or a “I don’t want to read the numpy tutorial, I want you to write me a new numpy tutorial instead” question, but probably not a dup.