How to specify argument type in lambda? [duplicate]

Question

For usual functions, you can annotate the type as follows

def my_function(arg1: int)

How do I do this in lambda?

lambda a: int, b: int : a+b

gives a syntax error.

Answer 1

The question was: "How to specify argument type in python lambda?". Although the other answers are good, they do not provide answer to this main question.

The answer is: you cannot.

From the PEP specification of function annotations:

lambda 's syntax does not support annotations. The syntax of lambda could be changed to support annotations, by requiring parentheses around the parameter list. However it was decided [12] not to make this change because:

It would be an incompatible change. Lambda's are neutered anyway. The lambda can always be changed to a function.

For reference: https://stackoverflow.com/a/33833896/7051394

Answer 2

The problem is that reduce will compute

result1 = func(x1, x2)
result2 = func(result1, x3)
...

your function accepts two lists and returns a number, so on the second call the function fails (unless you only provide two elements).

To be able to use reduce the function should accept two arguments and return a value of the same type of the arguments because the output of the function will be re-used as input at next iteration.