Defining a function with input

[perfringo]

First, radius is going to be a string and you want an integer.

It's correct that input returns str but radius can be integer as well as float.

For OP I suggest to follow conventions set in PEP8 for function naming (circle_area).

Regarding original question:

>>> def my_func():
...     print('This is my_func')
...
>>> my_func
<function my_func at 0x7fce15136c10>
>>> my_func()
This is my_func