Duncan Booth wrote:
Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote:
I was thrilled to learn a new trick, popping keyword arguments before
calling super, and wondered why I hadn't thought of that myself. How on
earth did I fail to realise that a kwarg dict was mutable and therefore
you can remove keyword args, or inject new ones in?
Probably because most of the time it is better to avoid mutating kwargs.
Instead of popping an argument you simply declare it as a named argument in
the method signature. Instead of injecting new ones you can pass them as
named arguments.
def foo(x=None, **kwargs):
bar(y=2, **kwargs)
def bar(**kwargs):
print(kwargs)
foo(x=1, z=3)
{'y': 2, 'z': 3}
foo(x=1, y=2, z=3)
Traceback (most recent call last):
File "<pyshell#8>", line 1, in <module>
foo(x=1, y=2, z=3)
File "<pyshell#4>", line 2, in foo
bar(y=2, **kwargs)
TypeError: bar() got multiple values for keyword argument 'y'
And the above error is exactly why you don't want to use named arguments
in MI -- because you don't know in what order the methods will be
called, you cannot know which named arguments to supply to the method
that super() will call next.
~Ethan~
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