python - lambda arguments unpack error

10
votes

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 error occurred:

f = lambda (m, k): m + k
^
SyntaxError: invalid syntax

If I remove parentheses in lambda expression then another error occurred:

TypeError: <lambda>() missing 1 required positional argument: 'k'

Also approach with tuple as single lambda argument works in Python 3, but it's not clear (hard for reading):

f = lambda args: args[0] + args[1]

How can I unpack values in the right way in Python 3?

6
pythonpython-3.xlambdatuplesunpack
map() applies f to each element of the provided iterable; in your given example, map() tries to pass (1, 5) into your function, which takes two arguments, but only one is provided, so it fails and gives you the error. - dazedconfused
They removed it from functions, to play nicely with type hints (among others). See also my answer here: stackoverflow.com/a/54991720/1338797 - Tomasz Gandor

6 Answers

9
votes

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]
8
votes

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]
4
votes

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]
4
votes

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.

1
votes

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

f = lambda args: sum(args)
1
votes

Just use

map(f, m, k)

Note that f can be

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