Can you assign variables in a lambda?

Question

I was using a lambda statement to perform math, and happened to repeatedly use one certain value. Therefore I was wondering if it was possible to assign and use

Answer 1

Im no expert at this, but the way i did it was by modifying globals() or locals() like this:

lambda: globals().__setitem__('some_variable', 'some value')

or if it's inside a function:

lambda: locals().__setitem__('some_variable', 'some value')

you could also use update() instead of __setitem__() if you wanted to, but that's a bit redundant.

Answer 2

You can create 2 different lambda functions and pass one to the other. For example,

b = lambda x: 3+2*x
a = lambda y: [my math here using variable b(y)]

Answer 3

I've cooked up this recipe for python 3.8+ using PEP572 Assignment Expressions to assign arbitrary variables and execute arbitrary expressions.

# python 3.8b1
lambda a: (
    (bool(b:=a**2) or 1)
    and (bool(c:=a-b) or 1)
    and not print(f'compute: {a} + {b} + {c}')
    and (
        (ret:=a + b + c) or ret)
    )
)
tst(0) 
# prints: "compute: 0 + 0 + 0"; returns: 0
tst(1)
# prints: "compute: 1 + 1 + 0"; returns: 2
tst(8)
# prints: "compute: 8 + 64 + -56"; returns: 16

So the pattern is:

lambda: [*vars]: (
    (bool(a:=[expr(*vars)]) or 1)
    and (bool([expr]) or 1)
    and bool([always true expr])
    and not bool([always false expr])
    and (
        # parentheses required so `result:=` doesn't capture the `or result` part
        (result:=[result expr]) or result
    )
)

This may be simplified if you know the truthiness of any particular expression.

That being said, if you want to assign a variable to reuse inside a lambda, you probably should consider writing a normal function.

Answer 4

You can instead use a bit of creativity, for example if you want to do some evaluation to an equation and assign the result to a variable it can be done like this:

class Results:
    res = 0

clas= Results()
setattr(clas, 'res', 3+2*4)
print(clas.res)

Answer 5

You can assign variables in lambda functions is you use exec:

>>> a = lambda: exec('global x; x = 1')
>>>a()
>>>x
1

Answer 6

You can just pass your lambda an argument which passes it along to another argument if you wish:

>>> b = lambda x: 3 + 2*x
>>> a = lambda y: y * b(y)
>>> a(1)
5
>>> a(2)
14