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

David Sampimon
David Sampimon
12,025 Points

Why does this work? (Harder time machine code challenge)

I got stuck on this code challenge due to the keyword argument being used in timedelta and having to pass in a variable to make it work.

After searching the community forum I found the solution to pass in a dictionary and unpacking it with double asterix in the return statement. This passes the code challenge, but I do not understand what is happening. I read the python docs regarding keyword arguments, but am still puzzled: Why is for example: datetime.timedelta**{6 : 'hours'} understood as datetime.timedelta(hours=6)? in the last line?


import datetime

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

def time_machine(time_amount, time_metric):
    if time_metric == 'years':
        # Ensure years are converted to days
        return time_machine(time_amount * 365, 'days')

    return starter + datetime.timedelta(**{time_metric: time_amount})

2 Answers

Jeff Muday
Jeff Muday
Treehouse Moderator 28,372 Points

The ** in front of a dictionary means to use the dictionary as the keyword arguments. This is a "clever" way of coding the question. I probably wouldn't write it this way since it violates the spirit of PEP 20.


But!! writing it this way saves quite a bit of typing and additional conditional branching.

I feel it can be written more simply and clear using the keyword arguments like below... The choice is yours to make!

def time_machine(time_amount, time_metric):
    if time_metric == 'minutes':
        time_delta = datetime.timedelta(minutes=time_amount)
    elif time_metric == 'hours':
        time_delta = datetime.timedelta(hours=time_amount)
    elif time_metric == 'days':
        time_delta = datetime.timedelta(days=time_amount)
    elif time_metric == 'years':
        time_delta = datetime.timedelta(days=time_amount*365)
        raise ValueError("time_machine() requires time_metric be minutes, hours, days, or years")

    return starter + time_delta
David Sampimon
David Sampimon
12,025 Points

Thank you Jeff!

I wasn't trying to be clever, I thought the answer was 'looking' for this approach. But then again, I am Dutch so maybe that gives me a bit of an excuses regarding the spirit of PEP20.

Jeff Muday
Jeff Muday
Treehouse Moderator 28,372 Points

I am praising you for brevity! Good job!

Although the course is geared toward beginners who would understand the "explicit" parameters of the way that I wrote the code.

The Challenge was written in a way that was "reversed" from what you would expect.

The dictionary would be {'hours' : 6} which would then be unpacked to hours=6 with the ** operator.

Enjoy your Python journey! GuidoVan Rossum (inventor of Python) is your fellow countryman, so you are well on your way to mastery!