3.4.6. Positional and keyword arguments

Up to now, we called functions using an ordered list of arguments: each argument was assigned using its position. For instance, in this function:

def print_full_name(first_name, last_name):
    """Print the full name of a person."""
    print(f"{first_name} {last_name}")

We know that the first argument should be the first name, followed by the last name. This is known as positional arguments. We would call this function using:

print_full_name("Catherina", "Smith")

Catherina Smith

In Python, it is also possible to call functions using keyword arguments. To do so, we directly assign values to the argument names using = signs. This can make the code still clearer:

print_full_name(first_name="Catherina", last_name="Smith")

Catherina Smith

Note that keyword arguments can be assigned in any order:

print_full_name(last_name="Smith", first_name="Catherina")

Catherina Smith

For very simple functions, using keyword arguments is not that useful. However, some functions may have lots of arguments, with many of them being optional. For example, let’s look at the signature of Pandas’ pd.read_csv function (we will use Pandas later), in Fig. 3.5.

Fig. 3.5 Signature of pandas.read_csv.

It has an awful lot of arguments, most of them being optional. In this case, it makes no sense to remember their order: using keyword arguments is much preferred.

Tip

We can use both positional and keyword arguments in a same function call. We begin with the positional arguments, which are being assigned based on their order, and end with the keyword arguments:

pandas.read_csv(filename, delimiter=',')

Note

In some function signatures, you may sometimes see these symbols: / and *.

• Any argument that comes before a / symbol must be a positional argument.

• Any argument that comes after a * symbol must be a keyword argument.