Usage

This package aims to offer two additional options to fix this issue:

• With a handy function decorator @fix_mutable_kwargs to fix a certain function.

• With a metaclass ImmutableDefaultArguments to fix all methods, classmethods and staticmethods at once.

Using the decorator:

from immutable_default_args import fix_mutable_kwargs

@fix_mutable_kwargs
def foo(a=[]):
    a.append(5)
    return a

>>> foo()
[5]
>>> foo()
[5]
...

It doesn’t matter if the iterable is empty or not:

@fix_mutable_kwargs
def foo(a=[1, 2, {'key': 'value'}, 3, 4]):
    a.append(5)
    return a

>>> foo()
[1, 2, {'key': 'value'}, 3, 4, 5]
>>> foo()
[1, 2, {'key': 'value'}, 3, 4, 5]
...

Fixing all mutable default values for all methods of an object via the ImmutableDefaultArguments metaclass:

class Foo(object):

    __metaclass__ = ImmutableDefaultArguments  # Py2 syntax

    def foo(self, a=[]):
        a.append(5)
        return a

    @classmethod  # staticmethods work as well
    def foo_classmethod(cls, a=[]):
        a.append(5)
        return a

instance_of_foo = Foo()
>>> instance_of_foo.foo()
[5]
>>> instance_of_foo.foo()
[5]
...
>>> Foo.foo_classmethod()
[5]
>>> Foo.foo_classmethod()
[5]

Compatibility

The immutable_default_args package is tested against Py2/3 and is supported from Py2.7 upstream.

Changelog

License

You are free to do whatever you like with the code. Please note that I am not accountable for anything that might have happened as a result of executing the code from the immutable_default_args package….ever.