signature-dispatch 0.2.0

Installation

Install from PyPI:

$ pip install signature_dispatch

Version numbers follow semantic versioning.

Usage

Create a dispatcher and use it to decorate multiple functions. Note that the module itself is directly invoked to create a dispatcher:

>>> import signature_dispatch
>>> dispatch = signature_dispatch()
>>> @dispatch
... def f(x):
...    return x
...
>>> @dispatch
... def f(x, y):
...    return x, y
...

When called, all of the decorated functions will be tested in order to see if they match the given arguments. The first one that does will be invoked. A TypeError will be raised if no matches are found:

>>> f(1)
1
>>> f(1, 2)
(1, 2)
>>> f(1, 2, 3)
Traceback (most recent call last):
    ...
TypeError: can't dispatch the given arguments to any of the candidate functions:
arguments: 1, 2, 3
candidates:
(x): too many positional arguments
(x, y): too many positional arguments

Type annotations are taken into account when choosing which function to invoke:

>>> from typing import List
>>> dispatch = signature_dispatch()
>>> @dispatch
... def g(x: int):
...    return 'int', x
...
>>> @dispatch
... def f(x: List[int]):
...    return 'list', x
...

>>> g(1)
('int', 1)
>>> g([1, 2])
('list', [1, 2])
>>> g('a')
Traceback (most recent call last):
    ...
TypeError: can't dispatch the given arguments to any of the candidate functions:
arguments: 'a'
candidates:
(x: int): type of x must be int; got str instead
(x: List[int]): type of x must be a list; got str instead
>>> g(['a'])
Traceback (most recent call last):
    ...
TypeError: can't dispatch the given arguments to any of the candidate functions:
arguments: ['a']
candidates:
(x: int): type of x must be int; got list instead
(x: List[int]): type of x[0] must be int; got str instead

Each decorated function will be replaced by the same callable. To avoid confusion, then, it’s best to use the same name for each function. The docstring of the ultimate callable will be taken from the final decorated function.

Applications

Writing decorators that can optionally be given arguments is tricky to get right, but signature_dispatch makes it easy. For example, here is a decorator that prints a message to the terminal every time a function is called and optionally accepts an extra message to print:

>>> def log(*args, **kwargs):
...     import signature_dispatch
...     from functools import wraps, partial
...
...     dispatch = signature_dispatch()
...
...     @dispatch
...     def decorator(*, msg):
...         return partial(wrap, msg=msg)
...
...     @dispatch
...     def decorator(f):
...         return wrap(f)
...
...     def wrap(f, msg=None):
...
...         @wraps(f)
...         def wrapper(*args, **kwargs):
...             print(f.__name__)
...             if msg: print(msg)
...             return f()
...
...         return wrapper
...
...     return decorator(*args, **kwargs)

Using @log without an argument:

>>> @log
... def foo():
...     pass
>>> foo()
foo

Using @log with an argument:

>>> @log(msg="Hello world!")
... def bar():
...     pass
>>> bar()
bar
Hello world!

Alternatives

The dispatching library does almost the same thing as this one, with a few small differences:

• The API is slightly more verbose.

• Subscripted generic types (e.g. List[int]) are not supported.

• Annotations can be arbitrary functions.