Use with blocks in python to assign a shorter name to a variable

with_rename.py

class mutate(Exception):
    pass

class alias:
    def __init__(self, obj):
        self.obj = obj

    def __enter__(self):
        return self.obj

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type is mutate:
            alias.mutate(self.obj, exc_value.args[0])
            return True

    @staticmethod
    def mutate(obj, newobj):
        if hasattr(obj, '__dict__'):
            obj.__dict__ = newobj.__dict__
        else:
            obj.__init__(newobj)

# Testing

this_is_quite_a_long_name = [1, 2, 3]
with alias(this_is_quite_a_long_name) as foo:
    assert(foo == [1, 2, 3])
    assert(foo.pop() == 3)
    assert(this_is_quite_a_long_name == [1, 2])
    assert(foo == [1, 2])
assert(this_is_quite_a_long_name == [1, 2])


immutable_value = 'Hello, world!'
with alias(immutable_value) as bar:
    assert(bar == 'Hello, world!')
    bar = bar.replace('world', 'Steve')
    assert(bar == 'Hello, Steve!')
    assert(immutable_value == 'Hello, world!')

this_set_gets_changed_by_assignment = {1, 2, 4, 8, 16}
with alias(this_set_gets_changed_by_assignment) as baz:
    baz = {3, 6, 9}
    raise mutate(baz)
assert(this_set_gets_changed_by_assignment == {3, 6, 9})

# Immutable values will NOT be modified, however
hello = 'hello'
with alias(hello) as x:
    x = 'goodbye'
    raise mutate(x)
assert(hello == 'hello')

# Weird's constructor doesn't support a single Weird object as an argument.
# However, it is not a builtin, so it supports __dict__.
class Weird:
    def __init__(self, height, color):
        self.height = height
        self.color = color
    def __str__(self):
        return 'My height is {}, and I am {}'.format(self.height, self.color)

weird_instance = Weird(42, 'blue')
with alias(weird_instance) as w:
    w = Weird(10, 'red')
    raise mutate(w)
assert(str(weird_instance) == 'My height is 10, and I am red')


# Examples

list_with_long_name = ['apples', 'pears']

with alias(list_with_long_name) as x:
    print(x.count('apples'))
    x = ['chickens', 'cows']
    # Mutate is necessary because x was modified by assignment
    raise mutate(x)

set_with_long_name = {1, 4, 9, 16, 25}
with alias(set_with_long_name) as foo:
    print(foo & {1, 2, 3, 4, 5})
    foo.intersection_update(range(7,22))

Of course, this is terrible.