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Comment by rat9988 | Hacker News Reader

d = d1 + d2 is way easier to parse.

As mentioned elsewhere in this thread, the mathematical + operator is commutative: d1 + d2 == d2 + d1. How does the + operator maintain that commutativity in the face of duplicate keys?

The + symbol doesn't denote any particural operation mathematics. it widely used for vector space where commutativity is a condition though.

A symbol's property depend on the operation, the multiplication symbol for example is commutative in some cases and not in others.

It is a convention, not a rule, in mathematics to use (+) to represent operations that are commutative (i.e. abelian) groups.

But this convention is not at all respected in programming (e.g. + for string concatenation in many languages) so I have to admit that it's pretty irrelevant.

The war to keep programming operators at all consistent with mathematics was lost long ago, with `=` and `==` being the worst offenders.

To be fair, mathematics uses = to indicate assignment as well, e.g. for summation limits.

Operators are polymorphic. While the + operator is commutative when using on integers/float, it is not currently when used on lists:

>>> [1,2,3] + [4,5,6]

[1, 2, 3, 4, 5, 6]

>>> [4,5,6] + [1,2,3]

[4, 5, 6, 1, 2, 3]

In dictionaries it would behave differently as well (duplicated keys, etc.)

It's also not commutative on strings and that is the case in many languages, not just Python.

That already falls apart when the + operator is used for string concatenation so I think the answer is people are using it in a less rigorous sense already.

The + here is not a mathematical + per se.

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