Python function (advanced): What is a default argument?

The following is typical in Python.

def f(x):
    print(x)


f('book')
# book

In Python, you can set the default argument in function.

def f(x='pen'):
    print(x)


f('book')
# book

f()
# pen

pen is the defualt argument of function f. If you set x book, book is printed. If you set x nothing, pen is printed because the default value of x is pen.

Most Python codes, built-in functions, modules, libs use the defualt argument in defining functions.