Pythonic Way to Solve Prefix Sum

Alex — Thu, 12 Mar 2026 10:05:10 +0000

Problem

Given an array nums. We define a running sum of an array as runningSum[i] = sum(nums[0]…nums[i]).

Return the running sum of nums.

Example

Input: nums = [1,2,3,4]
Output: [1,3,6,10]
Explanation: Running sum is obtained as follows: [1, 1+2, 1+2+3, 1+2+3+4].

Pythonic Solution

Use itertools.accumulate

from itertools import accumulate

class Solution:
    def runningSum(self, nums: List[int]) -> List[int]:
        return list(accumulate(nums))

Time Complexity: O(N)
Space Complexity: O(1)

Result:

Memory: Beats 94.67%
Runtime: Beats 100%

Converting an aware datetime object from one timezone to another in python

Alex — Tue, 15 Oct 2019 12:13:25 +0000

Problem

I recently ran into a beginner problem at work where I had to write a utility method that would receive a timestamp and the timezone of the timestamp and convert it into a human readable form for UTC timezone. Now, this looked straightforward except that I hadn't worked with the datetime library before. So after much searching, I came across the very useful datetime.astimezone() method which helped solve the problem.

About the `datetime.astimezone(tz=None)` method

The astimezone(tz=None) takes in a datetime object and returns a new datetime object with the time set to the equivalent time in the datetime.tzinfo attribute tz. If no tz object is not provided, it defaults to UTC timezone.

>>> from datetime import datetime
>>> from pytz import timezone

# Convert the local time in London to the local time in Sydney
>>> datetime.now(timezone('Europe/London')).astimezone(timezone('Australia/Sydney'))
datetime.datetime(2019, 10, 15, 22, 48, 4, 449019, tzinfo=<DstTzInfo 'Australia/Sydney' AEDT+11:00:00 DST>)

# Convert the time in London to it's equivalent in UTC
>>> datetime.now(timezone('Europe/London')).astimezone()
datetime.datetime(2019, 10, 15, 11, 54, 58, 952388, tzinfo=datetime.timezone(datetime.timedelta(0), 'UTC'))

Final Solution to my problem

Below is the solution I came up with which made use of the datetime.astimezone() method in the datetime library. Note that the code has been simplified to allow the reader to grasp it more easily.

from datetime import datetime
from pytz import timezone

def change_timezone_of_datetime_object(date_time_object, new_timezone_name):
    """Return the *date_time_object* with it's timezone changed to *new_timezone_name*

    :param date_time_object: The datetime object whose timezone is to be changed
    :type date_time_object: datetime
    :param new_timezone_name: The name of the timezone to which the *date_time_object* is to be changed to
    :type new_timezone_name: str
    :rtype: datetime
    """
    # Create a pytz.timezone object for the new_timezone
    new_timezone_object = timezone(new_timezone_name)
    # Update the timezone of the datetime object
    date_time_object = date_time_object.astimezone(new_timezone_object)
    # Return the converted datetime object
    return date_time_object

# Testing
>>> dtobj = datetime.now(timezone('UTC'))
>>> change_timezone_of_datetime_object(dtobj, 'Europe/London')
datetime.datetime(2019, 10, 15, 13, 56, 30, 220369, tzinfo=<DstTzInfo 'Europe/London' BST+1:00:00 DST>)

References

Python datetime library documentation
10 things you need to know about Date and Time in Python with datetime, pytz, dateutil & timedelta - Example 4 turned out to be very useful. Thanks @eleroy.

DEV Community: Alex

Pythonic Way to Solve Prefix Sum

Problem

Example

Pythonic Solution

Converting an aware datetime object from one timezone to another in python

Problem

About the datetime.astimezone(tz=None) method

Final Solution to my problem

References

About the `datetime.astimezone(tz=None)` method