Function Parameters Default Values

We have covered default values for parameters during our discussion of function parameters.
We now want to explain some more details about that.

Python IS dynamic

• If you are coming from static programming languages (C, C++, Java, C#, Rust, Golang etc..) then you should remind yourself that in python EVERYTHING happens in runtime.
• For example:

1num=5
2if num > 10:
3    def test_func_A():
4        print('test func A')
5else:
6    def test_func_B():
7        print('test func B')

This code creates a function called test_func_B (and not test_func_A).

• This is not a recommended way to write code, but it is a perfect demonstration of Python syntax.
  There is a specific moment of a function creation during run-time.
  This is the time when the def command is evaluated.

Default values evaluation

• Python documentation says:
  Default parameter values are evaluated from left to right when the function definition is executed. (look for that here)
• Here's an example, to make it more concrete:

1n = 7
2
3def test_func(num=5+n):
4    print(num)
5
6n = 9
7test_func()

This code fixes the default value of the num parameter to 7.
This is done just once, so calling the function prints 12 (5+7).

• Since the evaluation of the default value happens just once, setting a default value to a mutable value will also happen once:

1def add_to_list(something, lst=[]):
2    lst.append(something)
3    print(lst)
4
5add_to_list('a')
6add_to_list('b')
7add_to_list('c')

This will print:

1['a']
2['a', 'b']
3['a', 'b', 'c']