It's twice as slow, doesn't work with more than one dictionary, you can't easily control the merge prescience and you can't (easily) use expressions/variables in the keys:
{**x, 'fo'+'o': 'bar', **y} {**x, 'fo'+'o': 'bar', **y}Personally I hate the new {} for set syntax.
Is {} an empty set, or an empty dict?
return dict(x, **{'fo'+'o': 'bar'}, **y)
The expanded unpacking works in all cases.
Just skip all this and use Perl instead. You can write far more idiomatic, succinct readable code in Perl than you can in Python.
The whole point of Python is to not write code this way.
Python 2.7.12 (default, Dec 4 2017, 14:50:18)
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> x = {'foo': 5, 'bar': 6}
>>> y = {'foo': 7, 'baz': 9}
>>> dict(x, **{'fo'+'o': 'bar'}, **y)
File "<stdin>", line 1
dict(x, **{'fo'+'o': 'bar'}, **y)
^
SyntaxError: invalid syntax
Python 3.5.2 (default, Nov 23 2017, 16:37:01)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x = {'foo': 5, 'bar': 6}
>>> y = {'foo': 7, 'baz': 9}
>>> dict(x, **{'fo'+'o': 'bar'}, **y)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: type object got multiple values for keyword argument 'foo'
>>> {**x, 'fo'+'o': 'bar', **y}
{'foo': 7, 'baz': 9, 'bar': 6}X ++ {'fo'+'o': 'bar'} ++ y
Where '++' here means dictionary union, the choice of symbol is not relevant.