How to solve lambda() takes 1 positional argument but 2 were given

Question

I have the following code snippet but it is giving me the type error above. What I basically wanted to do is add two to every element and reduct it to a single sum, but it didnt work. How can we achieve this??

import functools
list1 = [1,2,3,4]
result = functools.reduce(lambda x:x+2, list1)
print(result)

Update:

It works when I do the following though:

list1 = [1,2,3,4]
result = functools.reduce(lambda x,y:x+y, list1)
print(result)

Asked By: Reactoo

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Source

Answer 1

reduce() requires a lambda function which takes 2 arguments.
The first argument is the collector.
But here, so that +2 is applied also on the first argument, you have to give the initial value 0.

import functools
list1 = [1,2,3,4]
result = functools.reduce(lambda acc, el: acc + el + 2, list1, 0)
print(result) ## sum([3, 4, 5, 6]) = 18

You basically want to do a normal summing reduce(lambda x, y: x + y, lst) => sum(lst)
and a map(lambda x: x + 2, l) which is expressed in a more pythonic way as a list comprehension:

l = [1, 2, 3, 4]
sum([x + 2 for x in l])            # most pythonic!
## 18
sum(map(lambda x: x + 2, l))
## 18

or:

import functools
l = [1, 2, 3 ,4]
result = functools.reduce(lambda x, y: x + y, [x + 2 for x in l])
result ## 18

or:

import functools
l = [1, 2, 3 ,4]
result = functools.reduce(lambda x, y: x + y, map(lambda x: x + 2, l))
result ## 18

Answered By: Gwang-Jin Kim

Answer 2

The callable passed to the reduce function needs to take two arguments to aggregate the current result with the next item from the given iterable:

import functools

list1 = [1, 2, 3, 4]
result = functools.reduce(lambda x, y: x + y + 2, list1, 0)
print(result)

This outputs:

Answered By: blhsing

How to solve lambda() takes 1 positional argument but 2 were given

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