What does it mean to pass a function to another function?

Question

I have the following code:

def print_function(values, test):
    for x in values:
        print('f(', x,')=', test(x), sep='')


def poly(x):
    return 2 * x**2 - 4 * x + 2


print_function([x for x in range(-2, 3)], poly)

I understand what the poly function does. What I don't understand is the test(x)? What does this do and where is the connection to the poly function?

Functions are first class member, poly is passed as the argument to test, so when the function calls test it is really calling poly — Robin Gertenbach, Oct 03 '21 at 11:38
Does this answer your question? [What are "first class" objects?](https://stackoverflow.com/questions/245192/what-are-first-class-objects) — Robin Gertenbach, Oct 03 '21 at 11:40
Besides the above answers, you could try to run here - https://pythontutor.com/ to see the visual running steps. — Daniel Hao, Oct 03 '21 at 11:43
You could add this line after `for loop` - `print(f' test: {test} & {type(test}} ')` to confirm your understanding — Daniel Hao, Oct 03 '21 at 11:48

Balaji · Accepted Answer · 2021-10-03T11:55:57.920

In python, functions can be stored in other variables and then called as functions themselves.

Example

def myfunction():
        print('hello')
    
myfunction() #prints hello
    
alias_myfunction = myfunction # store function object in other variable
    
alias_myfunction() #prints hello as well

# use id to test if myfunction and alias_myfunction point to same object.
print(id(myfunction) == id(alias_myfunction))

Output

hello
hello
True

Both myfunction and alias_myfunction store the reference to the function object

This can be confirmed using id

In this particular case

# values -> [x for x in range(-2, 3)]
# test -> poly
def print_function(values, test):
    for x in values:
        print('f(', x,')=', test(x), sep='') 
        # When test(x) is called, it is essentially calling poly(x)    

def poly(x):
    return 2 * x**2 - 4 * x + 2
    
print_function([x for x in range(-2, 3)], poly)

What does it mean to pass a function to another function?

1 Answers1