Expect The Unexpected

Like many other languages, Python has the notion of default values for function parameters, for example:

def compute_final_price(cost, sales_tax_rate=0.08):
    return cost * (1.0 + sales_tax_rate)

This code behaves as you would expect:

compute_final_price(10.0, 0.07)  # returns 10.7
compute_final_price(10.0)  # returns 10.8

However, interesting things happen in more complex cases, because the default value is evaluated once, at the time the function is defined, and not when it’s executed.

my_rate = 0.05

def compute_final_price(cost, sales_tax_rate=my_rate):
    return cost * (1.0 + sales_tax_rate)

compute_final_price(10.0)  # returns 10.5
my_rate = 0.06
compute_final_price(10.0)  # still returns 10.5

You can prove to yourself the value is evaluated at definition time by running help(compute_final_price), and you’ll notice the default value has already been computed:

Help on function compute_final_price in module __main__:

compute_final_price(cost, sales_tax_rate=0.05)

The above can be pernicious if the default value is an object that’s modified inside the function:

def add_to_list(item, my_list=[]):
    my_list.append(item)
    return my_list

add_to_list(4, [1, 2, 3])  # returns [1, 2, 3, 4]
add_to_list(1)  # returns [1]
add_to_list(1)  # returns [1, 1]

A common workaround for the above is to do the following:

def add_to_list(item, my_list=None):
    if my_list is None:
        my_list = []
    my_list.append(item)
    return my_list

add_to_list(1)  # returns [1]
add_to_list(1)  # returns [1]

I’ve encountered folks that insist one should never use default values except for None and the above pattern, but I’ve found most of the time I’m not mutating such defaults, and the behavior described here is fine. But still, the more you know.