27.02.19 20:48, Guido van Rossum пише:
On Wed, Feb 27, 2019 at 10:42 AM Michael Selik
<m...@selik.org
<mailto:m...@selik.org>> wrote > The dict subclass collections.Counter overrides the update method
for adding values instead of overwriting values.
https://docs.python.org/3/library/collections.html#collections.Counter.update
Counter also uses +/__add__ for a similar behavior.
>>> c = Counter(a=3, b=1)
>>> d = Counter(a=1, b=2)
>>> c + d # add two counters together: c[x] + d[x]
Counter({'a': 4, 'b': 3})
At first I worried that changing base dict would cause confusion for
the subclass, but Counter seems to share the idea that update and +
are synonyms.
Great, this sounds like a good argument for + over |. The other argument
is that | for sets *is* symmetrical, while + is used for other
collections where it's not symmetrical. So it sounds like + is a winner
here.
Counter uses + for a *different* behavior!
>>> Counter(a=2) + Counter(a=3)
Counter({'a': 5})
I do not understand why we discuss a new syntax for dict merging if we
already have a syntax for dict merging: {**d1, **d2} (which works with
*all* mappings). Is not this contradicts the Zen?
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