Python 减少 Boolean 行为不符合预期

Question

Can someone explain why this won't determine whether an array is monotonic? Not sure I understand reduce() completely. I have used it for aggregating sums, but figured it should be able to aggregate a boolean here, right?

from functools import reduce
def isMonotonic(array):
    # Write your code here.
    if not array or len(array) == 1:
        return True
    return reduce(lambda x, y: bool(x <= y), array, True) or reduce(lambda x, y: bool(x >= y), array, True)

Answer 1

From functools doc: functools.reduce(function, iterable[, initializer]) Apply function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). The left argument, x, is the accumulated value and the right argument, y, is the update value from the iterable. If the optional initializer is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. If initializer is not given and iterable contains only one item, the first item is returned.

Thus you get result of the comparison of all of the values before and the last one. Because of duck typing the comparison between an int and a bool is possible. False is 0, True is 1.Just try

print(True >= 2)

In strictly typed languages, this would cause an error.

What I would do:

def isMonotonic(a):
    # Write your code here.
    if not a or len(a) == 1:
        return True
    # create an list of tuples of the pairs, the last value in the second list is ignored because of the laziness of zip. Use a list so it can be used multiple times.
    a = list(zip(a[1:],a))
# create a list of comparison results. Then check if all comparisons are true.
return all(x <= y for x,y in a) or all(x >= y for x,y in a)

print(isMonotonic([1,2,3,4,5]))
print(isMonotonic([1,2,3,7,5]))
print(isMonotonic([5,3,2,1]))
print(isMonotonic([5,3,3,4,1]))