A confusion about creating lambda function with filter function in python

Question

Here's the prompt for the problem:

Write code using zip and filter so that all provided lists are combined into one big list and assigned to the variable 'larger_than_3' if they are both longer than 3 characters each.

l1 = ['left', 'up', 'front']

l2 = ['right', 'down', 'back']

I am able to solve this by the following line of the code:

larger_than_3 = list(filter(lambda x: len(x[0]) > 3 and len(x[1]) > 3, list(zip(l1, l2))))

I understand that here python interpreter treats x as a tuple and uses[] to access each of the elements in original lists, respectively. Because lambda takes in only one parameter as input, I also created the following code:

l_lst = list(zip(l1, l2))       
larger_than_3 = list(filter(lambda (a,b): len(a)>3 and len(b)>3, l_lst))

But python says this line of code is invalid syntax. I can't quite figure out why it is wrong as the lambda function could take in a tuple as its parameter.

In the last bit of code, you added parentheses around the lambda parameters. The first lambda correctly has no parentheses around its parameter list. — luther
– luther, Commented Jun 27, 2022 at 20:38
Python used to allow that (putting a parenthesized sublist of parameters in parenthesis, to unpack a single provided parameter), but that was causing some sort of problem (debugger support, I think), so was removed. — jasonharper
– jasonharper, Commented Jun 27, 2022 at 20:39

Trevor Hurst · Accepted Answer · 2022-06-27 20:39:50Z

1

It's a syntax error, the lambda function can take multiple variables

lambda a,b: print(a,b)

but in this case it's not actually necessary because of the way the data is being passed to the lambda by filter, re-writing it to:

larger_than_3 = list(filter(lambda a: len(a[0])>3 and len(a[1])>3, l_lst))

makes it function the same way as the first part! because a is set to each item in the list, thing for a in l_list so a = ('left','right') so we can index it and get the expected output!

answered Jun 27, 2022 at 20:39

Trevor Hurst

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