Живой Журнал Максима Дементьева

$ python -m timeit -s 'a,b,c = "a", "b", "d"' '"%s%s%s" % (a, b, c)'
10000000 loops, best of 3: 0.153 usec per loop
$ python -m timeit -s 'a,b,c = "a", "b", "d"' 'a + b + c'
10000000 loops, best of 3: 0.0818 usec per loop
$ python -m timeit -s 'a,b,c = "a", "b", "d"' 'f"{a}{b}{c}"'
10000000 loops, best of 3: 0.0668 usec per loop
$ python -m timeit -s 'a,b,c = "a", "b", "d"' '"{}{}{}".format(a, b, c)'
1000000 loops, best of 3: 0.286 usec per loop

Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use None as the default, and explicitly test for it in the body of the function, e.g.:

def whats_on_the_telly(penguin=None):
    if penguin is None:
        penguin = []
    penguin.append("property of the zoo")
    return penguin