PrintLet us continue with the “greet” example, but let’s modify it to be a bit simpler again with a single parameter for picking the language, and instead of using last name and form of address we just go with first names. However, we now want to be able to not only greet a single person but arbitrarily many persons, like this:
greet('English', 'Jim', 'Michelle')
Output: Hello Jim! Hello Michelle!
greet('Spanish', 'Jim', 'Michelle', 'Sam')
Output: Hola Jim! Hola Michelle! Hola Sam!
To achieve this, the parameter list of the function needs to end with a special parameter that has a * symbol in front of its name. If you look at the code below, you will see that this parameter is treated like a list in the body of the function:
def greet(language, *names):
greetings = { 'English': 'Hello', 'Spanish': 'Hola' }
for n in names:
print('{0} {1}!'.format(greetings[language], n))
What happens is that all values given to that function from the one corresponding to the parameter with the * on will be placed in a list and assigned to that parameter. This way you can provide as many parameters as you want with the call and the function code can iterate through them in a loop. Please note that for this example we changed things so that the function directly prints out the greetings rather than returning a string.
We also changed language to a positional parameter because if you want to use keyword arguments in combination with an arbitrary number of parameters, you need to write the function in a different way. You then need to provide another special parameter starting with two stars ** and that parameter will be assigned a dictionary with all the keyword arguments provided when the function is called. Here is how this would look if we make language a keyword parameter again:
def greet(*names, **kwargs):
greetings = { 'English': 'Hello', 'Spanish': 'Hola' }
language = kwargs['language'] if 'language' in kwargs else 'English'
for n in names:
print('{0} {1}!'.format(greetings[language], n))
If we call this function as
greet('Jim', 'Michelle')
the output will be:
Hello Jim! Hello Michelle!
And if we use
greet('Jim', 'Michelle', 'Sam', language = 'Spanish')
we get:
Hola Jim! Hola Michelle! Hola Sam!
Yes, this is getting quite complicated, and it’s possible that you will never have to write functions with both * and ** parameters, still here is a little explanation: All non-keyword parameters are again collected in a list and assigned to variable names. All keyword parameters are placed in a dictionary using the name appearing before the equal sign as the key, and the dictionary is assigned to variable kwargs. To really make the ‘language’ keyword argument optional, we have added line 5 in which we check if something is stored under the key ‘language’ in the dictionary (this is an example of using the ternary "... if ... else ..." operator). If yes, we use the stored value and assign it to variable language, else we instead use ‘English’ as the default value. In line 9, language is then used to get the correct greeting from the dictionary in variable greetings while looping through the name list in variable names.