How to convert Year and Day of Year to Date?

>>> import datetime
>>> datetime.datetime.strptime('2010 120', '%Y %j')
datetime.datetime(2010, 4, 30, 0, 0)
>>> _.strftime('%d/%m/%Y')
'30/04/2010'

The toordinal() and fromordinal() functions of the date class could be used:

from datetime import date
date.fromordinal(date(year, 1, 1).toordinal() + days - 1)

>>>import datetime
>>>year = int(input())
>>>month = int(input())
>>>day = int(input())
data = datetime.datetime(year,month,day)
daynew = data.toordinal()
yearstart = datetime.datetime(year,1,1)
day_yearstart = yearstart.toordinal()
print ((daynew-day_yearstart)+1)

Using the mx.DateTime module to get the date is similar to what has been proposed above using datetime and timedelta. Namely:

import mx.DateTime as dt 
date = dt.DateTime(yyyy,mm,dd) + dt.DateTimeDeltaFromDays(doy-1)

So, given that you know the year (say, 2020) and the doy (day of the year, say 234), then:

date = dt.DateTime(2020,1,1) + dt.DateTimeFromDays(233) 

which returns

2020-08-21 00:00:00.00

The advantage of the mx.DateTime library is that has many useful features. As per description in its homepage:

• Parses date/time string values in an almost seamless way.
• Provides conversion routines to and from many different alternative date/time
  storage formats.
• Includes an easy-to-use C API which makes
  integration a breeze.
• Fast, memory efficient, accurate.
• Georgian and Julian calendar support.
• Vast range of valid dates (including B.C. dates).
• Stable, robust and portable (mxDateTime has been around for almost 15 years
  now).
• Fully interoperates with Python’s time and datetime modules.

since it is pretty common these days, a pandas option, using pd.to_datetime with specified unit and origin:

import pandas as pd

day, year = 21, 2021

print(pd.to_datetime(day-1, unit='D', origin=str(year)))
# 2021-01-21 00:00:00