Default Argument Values

For some functions, you may want to make some of its parameters as optional and use default values if
the user does not want to provide values for such parameters. This is done with the help of default argument
values. You can specify default argument values for parameters by following the parameter name
in the function definition with the assignment operator (=) followed by the default value.
Note that the default argument value should be a constant. More precisely, the default argument value
should be immutable - this is explained in detail in later chapters. For now, just remember this.
Using Default Argument Values

Example 7.5. Using Default Argument Values
#!/usr/bin/python
# Filename: func_default.py
def say(message, times = 1):
print message * times
say('Hello')
say('World', 5)
Output
$ python func_default.py
Hello
WorldWorldWorldWorldWorld
How It Works
The function named say is used to print a string as many times as want. If we don't supply a value, then
by default, the string is printed just once. We achieve this by specifying a default argument value of 1 to
the parameter times.
In the first usage of say, we supply only the string and it prints the string once. In the second usage of
say, we supply both the string and an argument 5 stating that we want to say the string message 5
times.
Important
Only those parameters which are at the end of the parameter list can be given default argument
values i.e. you cannot have a parameter with a default argument value before a parameter
without a default argument value in the order of parameters declared in the function parameter
list.
This is because the values are assigned to the parameters by position. For example, def
func(a, b=5) is valid, but def func(a=5, b) is not valid.