Using lambda inside a function

Question

I'm learning about lambda functions in Python through tutorials online. I understand how it works but I came across an example that puzzles me (on this page):

def myfunc(n):
    return lambda a : a * n

mydoubler = myfunc(2)
print(mydoubler(11))

I don't understand how mydoubler function works here. How does it take 11 as an argument when we didn't define it before?

Answer 1

mydoubler is what myfunc(2) returns. It returns a lambda that accepts a single argument, a.

When you call on a function like this: myfunction(this_argument), it is going to resolve to what is returned in that spot. So this is effectively the same as writing mydoubler = lambda a : a * 2

Answer 2

A lambda function is a small anonymous function. In your example

myfunc(2) # lambda a: a * 2

You translate it as apply a function on each input element. It is quite obvious when an input is just a scalar, for example

mydoubler(11) #outputs 11 * 2 = 22

But what do you expect when the input is an array or a string?

mydoubler([1,1,1]) #outputs [1,1,1] * 2 = [1,1,1,1,1,1]
mydoubler("str") #outputs "str" * 2 = "strstr"

Answer 3

Your example has two functions: the outer function myfunc and the inner function lambda. Normally you can call a lambda function directly:

n = 2
print((lambda a: a * n)(11))
# 22

Or you can assign some variable to this function and call it through this variable:

inner = lambda a: a * n
print(inner(11))
# 22

You can also define some outer function, which will return the inner lambda function:

def myfunc():
    n = 2
    return lambda a: a * n

mydoubler = myfunc()
print(mydoubler(11))
# 22

What is equivalent to:

mydoubler = lambda a: a * 2
print(mydoubler(11))
# 22

In the example above the variable n was declared inside myfunc and in your case n is the parameter of myfunc, which is passed to the lambda function. The function myfunc returns thelambda function with n equal to the argument, which you pass to myfunc by the function call. So the function call myfunc(2) returns the fuction lambda a: a * 2.