Python program outputting different results, even though no random is used

Question

I am trying to solve Project Euler Problem 19.

I am not here to ask for the answer to the problem, but I noticed that every time my program runs, its output is different.

Please can someone explain why for me

"""
    Author: Luke Spademan
    Calculates answer to project euler problem id19
    https://projecteuler.net/problem=19
"""


def calculate():
    ans = 0
    months = {
        "jan": 31,
        "feb": 28,
        "mar": 31,
        "apr": 30,
        "may": 31,
        "jun": 30,
        "jul": 31,
        "aug": 31,
        "sep": 30,
        "oct": 31,
        "nov": 30,
        "dec": 31,
    }
    i = 1
    for year in range(1901, 2001):
        for month in months:
            months["feb"] = 28
            if year % 4 == 0 and not (year % 100 == 0 and year % 400 != 0):
                months["feb"] = 29
            if i % 7 == 0:
                ans += 1
            i += months[month]
    return ans

if __name__ == "__main__":
    answer = calculate()
    print(answer)

Answer 1

The problem is that the result of the computation depends on the order in which months is iterated. As of Python 3.3, string hashing is randomized by default, meaning that this order will not be deterministic (until Python 3.6). You can read about how to make this program run deterministically here, although I think that your intention is to iterate months in a predefined order always, in which case it should probably be a list or an OrderedDict.