Question

>>> def itself_and_plusone(x):
...     return x, x+1
... 
>>> itself_and_plusone(1)
(1, 2)

>>> (lambda x: x,x+1)(10)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

why? and workaround with lambda? not by

>>> (lambda x: (x,x+1))(10)
(10, 11)

because it returns a tuple(or list..) and unpacking the tuple would be required

Answer 1

Without the parentheses it is interpreted as follows:

((lambda x: x),x+1)(10)

This fails because the second x is outside the lambda expression. And even if x were defined, it would still fail because you can’t use a tuple as if it were a function.

This simple variation shows what is going on:

>>> x=42
>>> (lambda x: x,x+1)
(<function <lambda> at 0x00000000022B2648>, 43)

Notice that the 43 is because the x in the outer scope is used, not the x in the lambda function.

The correct way to write it is lambda x: (x,x+1). This does indeed, as you point out, return a tuple, but so does your original function:

>>> type(itself_and_plusone(10))
<class 'tuple'>