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

Ryan Dyke
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.a{fill-rule:evenodd;}techdegree
Ryan Dyke
Full Stack JavaScript Techdegree Student 6,630 Points

Harder Time Machine - can you pass strings to timedelta?

Instead of making 4 if statements, I'd like to pass time_str ("days", "hours", or "minutes") directly into the timedelta method. Is this possible, or are the quotation marks holding me back form doing so?

time_machine.py
import datetime

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

def time_machine(time_int, time_str):
    if time_str == "years":
        time_int = time_int * 365
        time_str = "days"
        finisher = starter + datetime.timedelta(time_str=time_int)
    else:
        finisher = starter + datetime.timedelta(time_str=time_int)

2 Answers

Steven Parker
Steven Parker
228,026 Points

It looks like you forgot to return the result value!
And the keys in the "key=value" syntax can't be variables, but you can do a little trick with a dictionary literal and the unpacking operator:

def time_machine(time_int, time_str):
    if time_str == "years":
        time_int *= 365
        time_str = "days"
    return starter + datetime.timedelta(**{time_str : time_int})
boi
boi
14,241 Points

Steven, you genius son of a programmer. Hold my upvote. What took me 13 lines of code, you got it in 5.

boi
boi
14,241 Points

Steven, I can't figure out the core building block of your (**{time_str : time_int}), I can't seem to wrap my head around this concept.

def time_machine(time_int, time_str): 👈#Here we're passing a "string" and an "int"
    if time_str == "years":
        time_int *= 365
        time_str = "days"
    return starter + datetime.timedelta(**{time_str : time_int})👈#How'd you change "hours" into hours.

Aren't we passing in "string"?. how did you manage to convert "string" into variable? I still don't quite get that last line of code, could you please break that up for me, so I could see how it works.

return starter + datetime.timedelta(**{time_str : time_int}) #Please explain the working

Would appreciate your help, GREATLY.

Jonathan Gonzalez
Jonathan Gonzalez
8,544 Points

Hey boi,

Chris Freeman explained it really good here: https://teamtreehouse.com/community/why-wont-the-timedelta-function-take-a-variable-as-a-keyword

I think what happens is python takes the variables passed into the function and places them in the dictionary

datetime.timedelta(**{5 : "days"})

then when it runs, it unpacks the dictionary into the key/value pair which timedelta likes

time_machine(5, "days") ------> (datetime.timedelta(day=5)

not 100% so if that's off let me know

boi
boi
14,241 Points

Hey Jonathan,

YES, you're correct, it's the ** that makes it into keyword arguments. I cleared my doubt already by Kenneth's explanation here. Chris Freeman's explanation was more detailed. Thank you for the help, Jonathan 👍🙂