immutable_default_args

=========================== Immutable default arguments

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This module provides facilities for turning mutable default function arguments into immutable ones. It is fairly lightweight and has no non-standard dependencies. You can install this package with the standard pip command::

$ pip install immutable_default_args

The issue with mutable argument default values <http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument>_ is pretty well known in Python. Basically mutable default values are assigned once at define time and can then be modified within the function body which might come as a surprise. Here is the example from the stackoverfow <http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument>_ thread::

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

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

The default way of preventing this behaviour is to use None as the default and check for it in the function body, like so::

def foo(a=None):
    a = a if (type(a) is list) else []
    a.append(5)
    return a

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

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

0.0.8 (08.05.2016)

• Add .landscape.yml and badges and more jazz.

0.0.7 (08.05.2016)

• Add CI and badges and all that jazz.

0.0.5 (08.05.2016)

• Fixed documentation

0.0.2 (08.05.2016)

• Added @fix_mutable_kwargs decorator
• Refactorings/Cleanup

0.0.1 (08.05.2016)

• First release. Included only ImmutableDefaultArguments metaclass

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.