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Python Dates and Times in Python (2014) Let's Build a Timed Quiz App Harder Time Machine

Posted by Adnan Fateh
Adnan Fateh
4,396 Points

Harder Time Machine - Bummer: local variable 'td' referenced before assignment

My code yields the following error: Bummer: local variable 'td' referenced before assignment

When I flip the arguments in the "time_machine" function to "(num, time_string)", the code passes the challenge. I don't understand why. I think it might work the first way if I assigned the td variable as some initial value- but I get all kinds of errors for trying that. Can someone help me understand why it works one way and not the other.

time_machine.py 
import datetime

starter = datetime.datetime(2015, 10, 21, 16, 29)

# Remember, you can't set "years" on a timedelta!
# Consider a year to be 365 days.

## Example
# time_machine(5, "minutes") => datetime(2015, 10, 21, 16, 34)
def time_machine(time_string, num):
    if time_string == 'minutes':
        td = datetime.timedelta(minutes=num)
    elif time_string == 'hours':
        td = datetime.timedelta(hours=num)
    elif time_string == 'days':
        td = datetime.timedelta(days=num)
    elif time_string == 'years':
        td = datetime.timedelta(days=num*365)
    return starter + td

2 Answers

Rocket Dollar Invest
Rocket Dollar Invest
7,129 Points
Rocket Dollar Invest
Rocket Dollar Invest
7,129 Points

A simple solution for this problem is to first check if the string is 'years' and if so change the string to 'days' since timedelta will not except years as an argument. Then multiply the integer by 365 to get the equivalent number of days for the given number of years. Finally, return the duration as the difference of the starter datetime and the timedelta of the string and integer passed to the method as a literal dictionary with the ** prefix operator to unpack the dictionary. 

import datetime

starter = datetime.datetime(2015, 10, 21, 16, 29)

def time_machine(integer, string):
    if string == 'years':
        string = 'days'
        integer *= 365
    return starter + datetime.timedelta(**{string: integer})
Jeff Muday
Jeff Muday
Treehouse Moderator 28,716 Points
Jeff Muday
Jeff Muday
Treehouse Moderator 28,716 Points

I like how you think about this. You already have the instincts that will serve you well as a developer!

I definitely agree that capturing days/years together would work as a good default conversion unit. Though the Challenge unit test may include "hours" and "minutes" specifications, so it is good practice to include those cases.

Jeff Muday
MOD
Jeff Muday
Treehouse Moderator 28,716 Points
Jeff Muday
Jeff Muday
Treehouse Moderator 28,716 Points
  1. Change your last elif to a simple else clause, then td will be instantiated.
  2. Reverse the order of time_string and num
def time_machine(num, time_string):
    if time_string == 'minutes':
        td = datetime.timedelta(minutes=num)
    elif time_string == 'hours':
        td = datetime.timedelta(hours=num)
    elif time_string == 'days':
        td = datetime.timedelta(days=num)
    else:
        td = datetime.timedelta(days=num*365)
    return starter + td