combining 2 dictionaries

[Skaperen]

i have 2 dictionary expressions. i want to combine them into one so that the one has each item from both. if one is already in a variable i can use .update() on that variable. but both dictionaries are expressions that are comprehensions. if the | operator could be used for dictionaries like it can for sets then i could do something like:
(warning: invalid code)

fun( {foo(x):x for x in range(256)} | {bar(x):x for x in range(256)} )

but i am unable to come up with a way to do this without forming the dictionary in another variable using more than one line of code. i want to have it all in one comprehension.

IMHO: every operation for sets should be allowed on dictionary keys and work the same way.

[bowlofred]

What version are you on? This works in 3.9. Prior to that you can use a different syntax.

def double(x):
    return 2 * x

def triple(x):
    return 3 * x

print({**{double(x):x for x in range(10)}, **{triple(x):x for x in range(10)}})
# works on 3.9+
print({double(x):x for x in range(10)} | {triple(x):x for x in range(10)})

The PEP that added this to dicts in 3.9 only added union. There is a section in the link that talks about the other set operators, but they were not added at the same time.

[Skaperen]

does not work in 3.6.

the engine logic for this should be the same for sets and dictionaries. that's what i did in my C libraries (not the language i was building long long ago). the C stuff i did was a library for C that implemented a namespace that only used byte string for keys. but it had a no-value assigned capability, somewhat like how None gets used in Python. the C stuff just didn't implement a bunch of types. these were mappings. to create a node you call an add function with just a key. then you call an assign function with the value. it assigned to the last referenced node. you could do a lookup with a key and that was the last reference. if the lookup failed, the last reference was empty and assign would fail. it had other features like forward/reverse stepping (a way to iterate over the mapping).

one of these days, i will look and see how CPython does dictionaries and sets. if Python didn't have sets, i'd just use dictionaries for that.

[bowlofred]

does not work in 3.6.

If you're referring to the syntax that I pointed out only works in 3.9 and later, I'd expect that. If you're referring to the other syntax, it works fine for me on 3.6.13.

[Skaperen]

other? which is which?

[bowlofred]

print({**{double(x):x for x in range(10)}, **{triple(x):x for x in range(10)}})
# works on 3.9+
print({double(x):x for x in range(10)} | {triple(x):x for x in range(10)})

The line on the bottom (under the "works on 3.9+") only works on 3.9 and later.

The line on top works on (I think) 3.5 and later. The oldest I have right now is 3.6.13 and it works on that one.

[Skaperen]

OK, yes, that works on 3.6.9.