Re: [Libguestfs] Extending the nbdkit python plugin
This may be confusing at first but is not that difficult once you wrap
your head around it.
There are two types of arguments in Python: positional(*) and
keyword(**). There is a rule that all positional arguments are defined
before the keyword arguments. Also once an argument has a default value
it is considered keyword (not positional). There are some nuances for
which is best to reach to some documentation.
Another possibility is we could encourage existing Python plugins to
add **kwargs to all functions where we might plausibly add extra
parameters in future, ie. the above would become:
def pwrite (h, buf, offset, **kwargs):
This still requires all Python plugins to change, but at least they
would remain backwards compatible with old and new nbdkit. However I
couldn't actually work out how to make this work because:
>>> def test(a, **kwargs):
Here you defined single positional argument 'a'.
... pass
...
>>> test(1)
>>> test(1,2)
You are passing two positional arguments. A keyword argument has to have
a name. E.g. this will work: test(1, foo=2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: test() takes 1 positional argument but 2 were given
Check the attached script for some examples. Hopefully that will help
you get a better grasp.
Tomas
--
Tomáš Golembiovský <tgolembi(a)redhat.com>
Attachments:
- args.py (text/plain — 1.3 KB)