Example of set subtraction in python

Question

I'm taking a data structures course in Python, and a suggestion for a solution includes this code which I don't understand.

This is a sample of a dictionary:

vc_metro = {
    'Richmond-Brighouse': set(['Lansdowne']),
    'Lansdowne': set(['Richmond-Brighouse', 'Aberdeen'])
}

It is suggested that to remove some of the elements in the value, we use this code:

vc_metro['Lansdowne'] -= set(['Richmond-Brighouse'])

I have never seen such a structure, and using it in a basic situation such as:

my_list = [1, 2, 3, 4, 5, 6]
other_list = [1, 2]
my_list -= other_list

doesn't work. Where can I learn more about this recommended strategy?

Answer 1

You can't subtract lists, but you can subtract set objects meaningfully. Sets are hashtables, somewhat similar todict.keys() , which allow only one instance of an object.

The -= operator is equivalent to the difference method, except that it is in-place. It removes all the elements that are present in both operands from the left one.

Your simple example with sets would look like this:

>>> my_set = {1, 2, 3, 4, 5, 6}
>>> other_set = {1, 2}
>>> my_set -= other_set
>>> my_set
{3, 4, 5, 6}

Curly braces with commas but no colons are interpreted as a set object. So the direct constructor call

set(['Richmond-Brighouse'])

is equivalent to

{'Richmond-Brighouse'}

Notice that you can't do set('Richmond-Brighouse') : that would add all the individual characters of the string to the set, since strings are iterable.

The reason to use -= / difference instead of remove is that differencing only removes existing elements, and silently ignores others. The discard method does this for a single element. Differencing allows removing multiple elements at once.

The original line vc_metro['Lansdowne'] -= set(['Richmond-Brighouse']) could be rewritten as

vc_metro['Lansdowne'].discard('Richmond-Brighouse')