I propose that the + sign merge two python dictionaries such that if there are
conflicting keys, a KeyError is thrown.
This way, d1 + d2 isn’t just another obvious way to do {**d1, **d2}. The second
syntax makes it clear that a new dictionary is being constructed and that d2
overrides keys from d1.
One can reasonably expect or imagine a situation where a section of code that
expects to merge two dictionaries with non-conflicting keys commits a semantic
error if it merges two dictionaries with conflicting keys.
To better explain, imagine a program where options is a global variable storing
parsed values from the command line.
def verbose_options():
if options.quiet
return {'verbose': True}
def quiet_options():
if options.quiet:
return {'verbose': False}
If we were to define an options() function, return {**quiet_options(),
**verbose_options()} implies that verbose overrules quiet; whereas return
quiet_options() + verbose_options() implies that verbose and quiet cannot be
used simultaneously. I am not aware of another easy way in Python to merge
dictionaries while checking for non-conflicting keys.
Compare:
def settings():
return {**quiet_options(), **verbose_options()}
def settings():
try:
return quiet_options() + verbose_options()
except KeyError:
print('conflicting options used', sys.stderr')
sys.exit(1)
***
This is a simple scenario, but you can imagine more complex ones as well. Does
—quiet-stage-1 loosen —verbose? Does —quiet-stage-1 conflict with
—verbose-stage-1?Does —verbosity=5 override —verbosity=4 or cause an error?
Having {**, **} and + do different things provides a convenient and Pythonic
way to model such relationships in code. Indeed, you can even combine the two
syntaxes in the same expression to show a mix of overriding and exclusionary
behavior.
Anyways, I think it’s a good idea to have this semantic difference in behavior
so Python developers have a good way to communicate what is expected of the two
dictionaries being merged inside the language. This is like an assertion
without
Again, I propose that the + sign merge two python dictionaries such that if
there are conflicting keys, a KeyError is thrown, because such “non-conflicting
merge” behavior would be useful in Python. It gives clarifying power to the +
sign. The + and the {**, **} should serve different roles.
In other words, explicit + is better than implicit {**, **#, unless explicitly
suppressed. Here + is explicit whereas {**, **} is implicitly allowing
inclusive keys, and the KeyError is expressed suppressed by virtue of not using
the {**, **} syntax. People expect the + operator to be commutative, while the
{**, **} syntax prompts further examination by virtue of its “weird” syntax.
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