[Python-ideas] PEP: Dict addition and subtraction

Jonathan Fine jfine2358 at gmail.com
Fri Mar 22 04:36:12 EDT 2019

Python fits well in the mind, because (1) by design it reduces
cognitive load, and (2) it encourages its users to reduce cognitive
load, and (3) we have a culture of reading code, taking pride in our
code. Readability counts.

Steven D'Aprano says that examples such as below don't help us discuss
the cognitive load associated with dict + dict.

> 1. Toy examples using generic names don't count.
>         eggs += spam

I assume he's referring to my example
    >>> items.update(points)
    >>> items += points
In this example, items.update gives useful additional information.

We expect, from duck typing and sensible naming, that points can be
iterated to give key value pairs. In Python, when
    >>> a + b
gives one of
    TypeError: unsupported operand type(s) for +: 'int' and 'str'
    TypeError: Can't convert 'int' object to str implicitly
we get a very strong hint to write instead something like
    a + int(b)
    str(a) + b
so that the nature of the addition is made clear to the next person
who reads the code (who might be ourselve, in a crisis, in ten years

(JavaScript does implicit conversion. This makes the code easier to
write, harder to read, and harder to maintain.)

For certain values of dct and lst we get
    >>> lst += dct
    >>> lst
    [('a', 1), ('b', 2), 'c', 'd']

For the same values of dct and lst (if proposal allowed)
    >>> dct += lst
    >>> dct
    {'a': 1, 'b': 2, 'c': 3, 'd': 4}

In these examples, dct is a dict, and lst is a list. This behaviour is
something Python users will have to learn, and have in their mind,
whenever they see '+=' in unfamiliar code.

I find this as much an unwelcome cognitive load as that produced by Javascript's
    > 2 * "8"
    > 2 + "8"

To be fair, this may in part be a problem with our expectations about +=.


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