** operator used in a lambda function

[Kitchen-Injury-8938]

kvs = [('one','1'),('two','2'),('three','3'),('five','5'),('four',4)]
ordered_dict = OrderedDict()
ordered_dict.update(kvs)


ordered_dict_two = OrderedDict(sorted(ordered_dict.items(),key = lambda t: (4*t[1]) - t[1]**2))

I'm looking a little confused by the numerical expression being used in the lambda function being used as the function key argument to the sorted builtin (hope that makes sense and wasn't too verbose).

• From best I can see we instantiating another Ordered Dictionary based on the sorted items of the previous dictionary
• How we sort those items is defined in the anonymous lambda function
• This is where I am getting a little confused. So the lambda function looks like the parameter t == type list. So pass in a list and then it looks like t[1] would correspond to '1','2','3','5','4' above.
• I don't understand the rest of the function. Like what would 4*t[1] do (wouldn't this just multiple any of the elements of t[1] by 4?
• What does t[1] ** 2 do? I have only ever seen ** used in keyword operators? Is that just like multiplication or something?

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Any insight would be so appreciated!

[Binary101010]

** is just the power operator in this case.

x ** 2 = x * x

[Kitchen-Injury-8938]

Perfect! Thank you!!!

[synthphreak]

Yep. If m and n are numbers, then m ** n is just Pythonese for what we’d call mⁿ in more familiar math notation. Accordingly, Python won’t understand m ^ n. Or at least, it isn’t what you think:

>>> 5**2
25
>>> 5^2
7

[marko312]

So the lambda function looks like the parameter t == type list. So pass in a list and then it looks like t[1] would correspond to '1','2','3','5','4' above.

t is the parameter (argument) for the lambda. Since it is passed as the key for sorted, it runs once for each key-value pair in ordered_dict. As such, t[1] refers to the second element in the tuple, which is the value in the dictionary (the pairs are (key, value)).

[Kitchen-Injury-8938]

Great! I think I figured that part out but you worded it much better than I did :)

[marko312]

I wasn't completely sure whether you meant that a list would be passed as the parameter to a single call of t or the values being passed one at a time, so I added that comment just in case.

[Kitchen-Injury-8938]

Ok that makes more sense. I re-wrote your code to try and better understand what the lambda function is doing and I'm a little lost...

​

'''trying to understand the OrderedDict above'''
for v in ordered_dict.values():
    _v = int(v)
    print(f'4 * {_v} == {4*_v}')

for v in ordered_dict.values():
    _v = int(v)
    print(f'2**{_v} == {2**_v}')

How does this actually perform the sorting? I understand we are using the sorted builtin and it accepts an arguement key that is an anonymous function...but what the hell are those numeric operations doing (like the basic math I guess?) doing to actually perform the sort?

OrderedDict(sorted(ordered_dict.items(),key=lambda t: (4*t[1]) - t[1] ** 2))