This article was first published here

What are first class functions ?

Functions in programming language are considered first class citizens if they exhibit the following characteristics.

• They can be stored in a variables.
• They can be stored in data structures.
• They can be passed as an argument to a function.
• They can be returned as values from another function.

Python supports all these features so functions are treated as first class citizens.

This is not a python only feature by any means, many other languages treat functions as their first class citizens, namely JavaScript, PHP, Perl. They are pretty much a necessity for the functional programming style.

Defining functions

As you must already know we define functions in Python with the def keyword.

def foo():
    pass

The function gets stored at a memory location as shown below at the address 0x0376CB28.

>>> foo
<function foo at 0x0376CB28>

Storing functions in variables

As function in python are first class citizens you can store the function in a variable.

>>> foo
<function foo at 0x0376CB28>
>>> bar = foo
>>> bar
<function foo at 0x0376CB28>

Even though the function is assigned to a separate variable bar notice that the location of the function still stays the same. Its merely 2 variables storing the location of the same function in the memory.

Not only you can store them in simple variables but you can store functions in a list, dict and other data structures as shown below.

# storing in list
>>> mylist = []
>>> mylist.append(foo)

# storing in dict
>>> mydict = {}
>>> mydict['function_foo']= foo

Passing functions as arguments

Functions can be passed as arguments to another function. Check out the sample below.

def main(func):
    print(func)

def foo():
    pass

print(foo)

main(foo)

Output shown below...

<function foo at 0x0163CD68>
<function foo at 0x0163CD68>

The function foo is passed as an argument to the function main and we can confirm that it is the same function foo which main has received as the argument by printing the memory address (feature of CPython) of the function globally as well as within the main function.

Returning functions

Similarly a function can return another function.

def foo():
    pass

def main():
    return foo  # returns a function

bar = main()

As you see the memory address is still the same for foo and bar.

>>> foo
<function foo at 0x009CF6F0>
>>> bar
<function foo at 0x009CF6F0>

Advantages of first class functions

There can be numerous benefits of this approach, some of them are listed below. You can check out the below topics to further dig deeper.

• Decorators
• Dynamic Code Inspection
• Callbacks