A default value lets a function work when a caller leaves out an optional argument. This keeps the common call short without removing the caller’s ability to choose a different value. Python’s print(), for example, separates items with spaces unless you supply another sep value. section 3.1.2
print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)
The print() signature shows its defaults together: a space separator, a newline ending, standard output as the destination, and flushing turned off. section 3.1.2
Make part of a name optional
Start with a full_name function that requires a first, middle, and last name. Give mname and lname defaults of None, and fname becomes the only required argument. A caller can then provide a last name without providing a middle name, as in full_name("John", lname="Smith"). The function’s body must still decide how to format a missing name. section 3.2.2
Put required parameters before parameters with defaults: once a parameter has a default, those following it must have defaults too. A related use appears in SQLAlchemy model columns, where a default can supply a value for a new row. A Post column uses get_next_sort_key to generate its sort order; that is a column default, not a function-parameter default. section 3.2.2 section 11.7.1
