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Using Python standard libraries, i.e. without dateutil or others, and solving the 'February 31st' problem:

import datetime
import calendar

def add_months(date, months):
    months_count = months + date.month

    # Calculate the year
    year = date.year + int(months_count / 12)

    # Calculate the month
    month = (months_count % 12)
    if month == 0:
        month = 12

    # Calculate the day
    day = date.day
    last_day = calendar.monthrange(year, month)[1]
    if day > last_day:
        day = last_day

    new_date = datetime.date(year, month, day)
    return new_date

Testing:

>>>date = datetime.date(2018, 11, 30)

>>>print(date, add_months(date, 3))
(datetime.date(2018, 11, 30), datetime.date(2019, 2, 28))

>>>print(date, add_months(date, 14))
(datetime.date(2018, 12, 31), datetime.date(2020, 2, 29))

Using Python standard libraries, i.e. without dateutil or others, and solving the 'February 31st' problem:

import datetime
import calendar

def add_months(date, months):
    months_count = date.month + months

    # Calculate the year
    year = date.year + int(months_count / 12)

    # Calculate the month
    month = (months_count % 12)
    if month == 0:
        month = 12

    # Calculate the day
    day = date.day
    last_day_of_month = calendar.monthrange(year, month)[1]
    if day > last_day_of_month:
        day = last_day_of_month

    new_date = datetime.date(year, month, day)
    return new_date

Testing:

>>>date = datetime.date(2018, 11, 30)

>>>print(date, add_months(date, 3))
(datetime.date(2018, 11, 30), datetime.date(2019, 2, 28))

>>>print(date, add_months(date, 14))
(datetime.date(2018, 12, 31), datetime.date(2020, 2, 29))

Using standard way, i.e. without dateutil :

import datetime
import calendar

def add_months(date, months):
    target_month = months + date.month

    # Calculate the year
    year = date.year + int(target_month / 12)

    # Calculate the month
    month = (target_month % 12)
    if month == 0:
        month = 12

    # Calculate the day
    day = date.day
    last_day = calendar.monthrange(year, month)[1]
    if day > last_day:
        day = last_day

    new_date = datetime.date(year, month, day)
    return new_date

Testing:

>>>date = datetime.date(2018, 11, 30)

>>>print(date, add_months(date, 3))
(datetime.date(2018, 11, 30), datetime.date(2019, 2, 28))

>>>print(date, add_months(date, 14))
(datetime.date(2018, 12, 31), datetime.date(2020, 2, 29))

Using Python standard libraries, i.e. without dateutil or others, and solving the 'February 31st' problem:

import datetime
import calendar

def add_months(date, months):
    months_count = months + date.month

    # Calculate the year
    year = date.year + int(months_count / 12)

    # Calculate the month
    month = (months_count % 12)
    if month == 0:
        month = 12

    # Calculate the day
    day = date.day
    last_day = calendar.monthrange(year, month)[1]
    if day > last_day:
        day = last_day

    new_date = datetime.date(year, month, day)
    return new_date

Testing:

>>>date = datetime.date(2018, 11, 30)

>>>print(date, add_months(date, 3))
(datetime.date(2018, 11, 30), datetime.date(2019, 2, 28))

>>>print(date, add_months(date, 14))
(datetime.date(2018, 12, 31), datetime.date(2020, 2, 29))

Using standard way, i.e. without dateutil :

import datetime
import calendar

def add_months(date, months):
    target_month = months + date.month
    year = date.year + int(target_month / 12)
    month = (target_month % 12)
    if month == 0:
        month = 12
    day = date.day
    last_day = calendar.monthrange(year, month)[1]
    if day > last_day:
        day = last_day
    new_date = datetime.date(year, month, day)
    return new_date

Testing:

>>>date = datetime.date(2018, 11, 30)

>>>print(date, add_months(date, 3))
(datetime.date(2018, 11, 30), datetime.date(2019, 2, 28))

>>>print(date, add_months(date, 14))
(datetime.date(2018, 12, 31), datetime.date(2020, 2, 29))

Using standard way, i.e. without dateutil :

import datetime
import calendar

def add_months(date, months):
    target_month = months + date.month 

    # Calculate the year
    year = date.year + int(target_month / 12)

    # Calculate the month
    month = (target_month % 12)
    if month == 0:
        month = 12

    # Calculate the day
    day = date.day
    last_day = calendar.monthrange(year, month)[1]
    if day > last_day:
        day = last_day 

    new_date = datetime.date(year, month, day)
    return new_date

Testing:

>>>date = datetime.date(2018, 11, 30)

>>>print(date, add_months(date, 3))
(datetime.date(2018, 11, 30), datetime.date(2019, 2, 28))

>>>print(date, add_months(date, 14))
(datetime.date(2018, 12, 31), datetime.date(2020, 2, 29))