genericfuncs 0.1.1

genericfuncs allows you to cleanly implement functions which execute different implementations depending on the arguments.

This module can be seen as a powerful improvement over Python 3’s singledispatch:

• Allows dispatch over any boolean callable, not just type checks.

• Allows dispatch over any number of arguments, not just the first argument.

Example usage:

# define a generic function
@genericfuncs.generic
def func(a):
    # default implementation
    raise TypeError()

# dispatch on type
@func.when(int)
def _when_int(a):
    return a * a

# any boolean callable can be a predicate
@func.when(lambda a: a == 'magic')
def _when_magic_word(a):
    return a.upper()

# multiple predicates
@func.when([float, lambda a: a < 0])
def _when_float_and_negative(a):
    return a * -1

func(10) --> 100  # _when_int() invoked
func('magic') --> 'MAGIC'  # _when_magic_word() invoked
func(-5.5) --> 5.5  # _when_float_and_negative() invoked
func(Something()) --> TypeError raised  # default implementation invoked

The first predicate that returns True has its mapped implementation invoked. Predicates are checked in order of definition.

Arguments are injected into predicates and implementations by their name. This means a predicate or implementation is able to specify only the arguments it needs. For example:

@generic
def multiple_params_func(a, b, c):
    return a + b + c  # default implementation

@multiple_params_func.when(lambda b: b > 10)  # only inject argument `b` to the predicate
def _when_b_greater_than_10(a):  # only inject `a` to the implementation
    return a * 10

@multiple_params_func.when(lambda a, b: a % b == 0)  # only inject `a` and `b`
def _when_a_divisible_by_c(a, b, c):  # use all arguments
    return a / b * c

However the call site must list all mandatory arguments, as usual in Python:

multiple_params_func(10, 20, 30) --> 100  # _when_b_great_than_10() invoked
multiple_params_func(4, 2, 'bla') --> 'blabla'  # _when_a_divisible_by_c() invoked
multiple_params_func(0, 0, 0) --> 0  # default implementation invoked

When defining a predicate, one can list exception types that should not propagate if raised inside the predicate. For example:

@my_generic_func.when(lambda a: a > 10, ignored_errors=[TypeError])
def _implementation(a):
    ...

When invoking my_generic_func(MyThing()), a TypeError will be raised inside the predicate because MyThing doesn’t support > operator. Normally, the error would propagate and crash the program. Specifying ignored_errors=[TypeError] makes the error be silently ignored, moving on to the next predicate.