[Tutor] Anonymous function

[Alan Gauld]

On 17/03/2022 00:00, Devanshu Nalavade wrote:

> a user-defined function. This function has an anonymous function that takes
> the same arguments and does the summation. However for some reason the code
> is not giving me the desired output, there are no syntax errors so I'm not
> understanding what I'm doing wrong.

There are so many issues here it is hard to know where to start,
so my apologies if I over simplify things.

Caveat: My code below is untested so may not be 100% correct...

>>>> def sum(arg1,arg2,arg3=10):
>   ...  m = lambda arg1,arg2,arg3:arg1+arg2+arg3

That creates a function object(aka a lambda) and
assigns it the name m

It is identical to:

def sum(arg1,arg2,arg3=10):
   def m(arg1,arg2,arg3):
       return arg1+arg2+arg3


But sum() does nothing except define the function - which
is then just thrown away when sum() finishes.

And because sum has no return value Python returns None - the
default for functions.

But of course calling the function sum means you have hidden
the built-in function sum(), which does a similar thing but
for an unlimited number of args and without the default
value of 10 for arg3.

Hiding the built in functions is not usually a good idea.
Perhaps calling it sum3() or some-such would be better.

Also you use the names arg1,arg2,arg3 as the definition of the
parameters to lambda. But that creates 3 new names inside the
function, it does not pass the parameters of sum() into the
lambda as I suspect you think it does! It potentially then
becomes very confusing so I suggest using different names
in the lambda, say a1,a2,a3?

>>>> print(sum(5,6))
> 
> The output comes out as 'None'

Yes because your function does not return anything, not
even the lambda.

To get this to work you'd need to modify it like so:

def sum3(arg1,arg2,arg3=10):
    m = lambda a1,a2,a3:a1+a2+a3
    return m(arg1,arg2,arg3)

print( sum3(5,6) )  # prints 21

That now defines the lambda and then calls it.
but since the lambda is not stored anywhere you might
as well just execute the code directly:

def sum3(arg1,arg2,arg3=10):
    return arg1+arg2+arg3

Or even

def sum3(arg1,arg32,arg3=10):
    return sum(arg1,arg2,arg3)

Normally we use lambdas when we want the return value
to be a function that the caller will use later. Or where
we want a function passed in and the caller provides a
lambda. We don't usually use them to define a function
that we immediately call then throw away.

For example here is a function that returns a function
that sums but with a user defined default value:

def sum3N(N):
   return lambda a1,a2,a3=N: sum(a1,a2,a3)


sum10 = sum3N(10)
sum10(4,5)  # returns 4+5+10 = 19

sum666 = sum3N(666)
sum666(10,20)   # returns 696
sum666(1,2,3)   # returns 6


HTH
-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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