Given a string that's a weekday name, how to figure out weekday as decimal (and the next date it occurs)?

Question

Given a string, reqDayOf, that's a weekday name, how do you figure out weekday as decimal (and then return the next instance of it based on a datetime object)?

Getting the datetime object's year and iso Week of Year and then using strptime with the year + week of year + the weekday name works, but it feels like a hack.

import datetime

def getDateFromDayOf(dateTimeObj,reqDayOf):
  #reqDayOf is one of ['monday','tuesday','wednesday','thursday','friday','saturday','sunday']
  #return the next instance of reqDayOf
  #after dateTimeObj
  #as a datetime object 
  #Get the WeekOfYear from dateTimeObj and then
  #get the date based on Year + WeekOfYear + reqDayOf 
  (dtoYear,dtoIsoWeek,x)=dateTimeObj.isocalendar()
  checkDate= '{}-{}-{}'.format( dtoYear,dtoIsoWeek,reqDayOf)
  dateOfDay=datetime.datetime.strptime(checkDate,"%Y-%U-%A")
  #return dateOfDay if it's greater than the original date
  if dateOfDay > dateTimeObj:
    return dateOfDay
  else:
    #this is needed on Sundays
    #add a week
    return dateOfDay + datetime.timedelta(days=7)  


>>> datetime.datetime.now().strftime('%a %Y %b %d')
'Fri 2013 Apr 05'
>>> getDateFromDayOf(datetime.datetime.now(),'Monday').strftime('%a %Y %b %d')
'Mon 2013 Apr 08'

Answer 1

The day of the week (as an integer) is returned by the weekday method:

import datetime as DT
dow = dict(zip('monday tuesday wednesday thursday friday saturday sunday'.split(),
           range(7)))

def getDateFromDayOf(dateTimeObj, reqDayOf):
    weekday = dateTimeObj.weekday()        
    return dateTimeObj + DT.timedelta(days=(dow[reqDayOf.lower()]-weekday-1)%7+1)

---

In [90]: getDateFromDayOf(DT.datetime.now(), 'Monday').date()
Out[90]: datetime.date(2013, 4, 8)

In [91]: getDateFromDayOf(DT.datetime.now(), 'Tuesday').date()
Out[91]: datetime.date(2013, 4, 9)

In [92]: getDateFromDayOf(DT.datetime.now(), 'Friday').date()
Out[92]: datetime.date(2013, 4, 12)

---

Or, using dateutil,

import datetime as DT
import dateutil
import dateutil.relativedelta as rdelta
import dateutil.rrule as rrule

dow = dict(zip('monday tuesday wednesday thursday friday saturday sunday'.split(),
               (getattr(rdelta, d) for d in 'MO TU WE TH FR SA SU'.split())))
def getDateFromDayOf(dateTimeObj, reqDayOf):
    rr = rrule.rrule(
        rrule.DAILY,                       # step by days
        byweekday = dow[reqDayOf.lower()], # return only this day of the week
        dtstart = dateTimeObj)             # start on this day 
    res = rr.after(dateTimeObj, inc=False) # inc=False means don't include the dtstart day
    return res

Using dateutil you can express the idea in a high-level manner, without having to worry about off-by-one errors in grubby details like

DT.timedelta(days=(dow[reqDayOf.lower()]-weekday-1)%7+1)