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Start your free trialJohn Forbes
10,879 PointsWhy won't the timedelta function take a variable as a keyword?
When I run the code below it gives me this response, Bummer! 'the_string' is an invalid keyword argument for this function. I think I know how to fix it but just more of a general question as to why this is happening.
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.
def time_machine(length_time, the_string):
if(the_string == 'years'):
return starter + datetime.timedelta(days=(length_time * 365))
else:
return starter + datetime.timedelta(the_string=length_time)
## Example
# time_machine(5, "minutes") => datetime(2015, 10, 21, 16, 34)
2 Answers
Chris Freeman
Treehouse Moderator 68,441 PointsIt's a syntax issue. A keyword in an argument list to a function cannot be a variable or an expression.
You can construct a dict to use as a kwargs argument:
return starter + datetime.timedelta(**{the_string: length_time})
Denis Watida Machaka
7,311 Pointsdef time_machine(integer, the_string): if(the_string == 'years'): return starter + datetime.timedelta(days=(length_time * 365)) else: return starter + datetime.timedelta(**{the_string: integer})
John Forbes
10,879 PointsJohn Forbes
10,879 PointsAhh very cool thanks.
daniel steinberg
14,651 Pointsdaniel steinberg
14,651 PointsChris two quick questions based on:
1. why does this work:
if(the_string == 'years'): return starter + datetime.timedelta(days=(length_time * 365))
and this not
else: return starter + datetime.timedelta(the_string = length_time)
is it because
(the_string = length_time)
is evaluated as an expression?2. When you unpack the dictionary, how does python know to evaluate the datetime function with an "=" sign?
return starter + datetime.timedelta(**{the_string: length_time})
so for ex. with "weeks" and 5, how does it know to evaluate this as (where does the "=" sign come from)?
return starter + datetime.timedelta(weeks = 5)
Thanks much Dan
[MOD: added ```python formatting -cf]
Chris Freeman
Treehouse Moderator 68,441 PointsChris Freeman
Treehouse Moderator 68,441 Pointsdaniel steinberg, For 1, keyword names are not interpreted. They are parsed as literal strings. In the first case, βdaysβ is the keyword. In the second βthe_stringβ is the keyword which will be rejected by
timedelta
as an invalid keyword.For 2, the magic is in the packing/unpacking operator **. When ** is used on a dictionary it converts the dict key/value pairs into keyword/argument pairs. This is where the equals sign is created.
For example:
[MOD: added ```python formatting -cf]
Post back if you need more help. Good luck!!!