A consequence of defining general functions is the introduction of additional arguments. Functions with many arguments can be awkward to call and difficult to read.

In Python, we can provide default values for the arguments of a function. When calling that function, arguments with default values are optional. If they are not provided, then the default value is bound to the formal parameter name instead. For instance, if an application commonly computes pressure for one mole of particles, this value can be provided as a default:

>>> def pressure(v, t, n=6.022e23):
        """Compute the pressure in pascals of an ideal gas.

        v -- volume of gas, in cubic meters
        t -- absolute temperature in degrees kelvin
        n -- particles of gas (default: one mole)
        """
        k = 1.38e-23  # Boltzmann's constant
        return n * k * t / v

The = symbol means two different things in this example, depending on the context in which it is used. In the def statement header, = does not perform assignment, but instead indicates a default value to use when the pressure function is called. By contrast, the assignment statement to k in the body of the function binds the name k to an approximation of Boltzmann's constant.

>>> pressure(1, 273.15)
2269.974834
>>> pressure(1, 273.15, 3 * 6.022e23)
6809.924502

The pressure function is defined to take three arguments, but only two are provided in the first call expression above. In this case, the value for n is taken from the def statement default. If a third argument is provided, the default is ignored.

As a guideline, most data values used in a function's body should be expressed as default values to named arguments, so that they are easy to inspect and can be changed by the function caller. Some values that never change, such as the fundamental constant k, can be bound in the function body or in the global frame.