Late

Late binding for Python default arguments

What is it?

Late provides decorators and functions to work around the issues that early binding of default argument values produces in Python.

What follows is not intuitive for newcomers to Python, but it's something that everyone learns quickly:

>>> def f(x=[]):
...     x.append(1)
...     return x
...
>>> f()
[1]
>>> f()
[1, 1]
>>> f()
[1, 1, 1]

The behavior in Python is that the same [] initializer value is passed on every function invocation.

The coding pattern to work around the above is to use None as the initializer, and check for the parameter value at the start of the function code:

>>> def f(x=None):
...     if x is None:
...         x = []
...     x.append(1)
...     return x
...
>>> f()
[1]
>>> f()
[1]
>>> f()
[1]

It's ugly, but it works.

Now comes the other ugly part. When using type annotations, the above function must be declared in away so that type checkers do not complain about using None as the default value:

>>> def f(x: list[Any] | None = None) -> list[Any]:

Late provides a way to solve the above ugliness with some decorator magic. This is how the code looks with some of that magic:

from late import latebinding, __


@latebinding
def f(x: list[Any] = __([])) -> list[Any]:
    x.append(1)
    return x

assert f() == [1]
assert f() == [1]
assert f() == [1]

Working with classes

Late also works with classes and dataclass. The @latebinding decorator must be the outer one:

@latebinding
@dataclass
class C:
    x: list[Any] = __([])  # noqa

c = C()
assert c.x == []

d = C()
assert d.x == []
c.x = [1]
assert c.x == [1]
assert d.x == []

assert d.x is not c.x

Working with generators

Late allows passing a generator as a default argument value, and it will provide the next value on each function call. The usefulness of this feature is unknown, but it's something that came up during the discussions about default arguments, so Late implements it.

    def fib() -> Iterator[int]:
        x, y = 0, 1
        while True:
            yield x
            x, y = y, x + y


    @latebinding
    def f(x: int = __(fib())) -> int:
        return x

    assert f() == 0
    assert f() == 1
    assert f() == 1
    assert f() == 2
    assert f() == 3
    assert f() == 5

About name choice

The names of what Late exports are chosen to be explicit where it matters, and to not get in the way of the visuals of a declaration. In particular, __() was chosen to interfere the least possible with reading a function declaration (late() is another name for it, and __ is seldom used in Python code).

At any rate, Late is so simple and so small that you can apply any changes you like and use it as another part of your code instead of installing it as a library.

License

Late is under the GNU GENERAL PUBLIC LICENSE Version 3, as reads in the LICENSE file.