lambda arguments unpack error

Question

In Python 2 this code is OK:

f = lambda (m, k): m + k

m = [1,2,3,4]
k = [5,6,7,8]

print(map(f, zip(m, k)))

but in Python 3 the following

Answer 1

You can use the same syntax in both Python 2 and Python 3 if you use itertools.starmap instead of map which unpacks the tuple items for us:

>>> from itertools import starmap
>>> f = lambda m, k: m + k
>>> list(starmap(f, zip(m, k)))
[6, 8, 10, 12]

Answer 2

You may find this solution easier to read:

lambda mk: (lambda m,k: m + k)(*mk)

Additionally, I'd argue that the unpacking makes this more (1) Pythonic and (2) consistent with the manual unpacking of tuple arguments for named functions, required in Python 3 by PEP 3113.

Answer 3

Or you can just sum() to add numbers without unpack:

f = lambda args: sum(args)

Answer 4

The removal of tuple unpacking is discussed in PEP 3113. Basically, you can't do this in Python 3. Under the headline Transition plan, you see that the "suggested" way of doing this is as your final code block:

lambda x_y: x_y[0] + x_y[1]

Answer 5

You cannot use parentheses in Python3 to unpack arguments in lambda functions (PEP 3113), Try:

f = lambda m, k: m + k

---

To make it work with your code, you should use:

lambda mk: mk[0] + mk[1]

Answer 6

Just use

map(f, m, k)

Note that f can be

from operator import add
map(add, m, k)