Question

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What does *args and **kwargs mean?

I’ just reading Mining the social web and encountered a python syntax that I can’t figure out:

transforms = [(', Inc.', ''), (', Inc', ''), (', LLC', ''), (', LLP', '')]

"google, Inc.".replace(*transforms[0])

But if I type

*transforms[0]

in the interpreter, it says it is invalid syntax. I googled it, but the python docu really is not up for the job.

So what does the asterisk means here please? Thank you all.

Answer 1

The *argument format in python means: use all elements in the sequence argument and pass them as arguments to the function.

In this specific case, that translates to:

"google, Inc.".replace(', Inc.', '')

This is easiest demonstrated:

>>> def foo(arg1, arg2):
...     print arg1, arg2
...
>>> arguments = ('spam', 'eggs')
>>> foo(*arguments)
spam, eggs

You can also use the **kw double star format to pass in keyword arguments:

>>> def foo(arg1='ham', arg2='spam'):
...     print arg1, arg2
...
>>> arguments = dict(arg2='foo', arg1='bar')
>>> foo(**arguments)
bar, foo

and you can use the same spelling in a function definition to capture arbitrary positional and keyword arguments:

>>> def foo(*args, **kw):
...     print args, kw
...
>>> foo('arg1', 'arg2', foo='bar', spam='eggs')
('arg1', 'arg2'), {'foo': 'bar', 'spam': 'eggs'}