Working of *(arg1, arg2) in python in function call.

Question

>>def f(a, b):
print a, b

>>f(a=1, *(2, ))
error: f() has got multiple values for a

>>f(1, *(2, ))
1 2

>>f( *(1,2))
1 2
>>f(a=1, *( ,2))
error: invalid syntax

------>>>>>
can someone please explain all these 4 cases and its output.
also, please explain the working behind the *( arg1, arg2). and *(arg1, ).??????
thanx in advance. :-)

Answer 1

While testing I found this text related to "variable unpacking", which is faster:

http://caisbalderas.com/blog/JUMP_LINK__&&__python__&&__JUMP_LINK-function-unpacking-args-and-kwargs/

Specifically: "arbitrary arguments will try to >>fill function arguments in order<< therefore may conflict with named arguments". That's what's wrong in your first test.

Test 2 is an unnamed argument + a single-value tuple. I believe this allows Python to apply all of them as a single expansion, leading to no conflict. Notice the linked example:

assert(z=2, *t)

succeeds. z is third in line; the 2-value tuple tries to expand into the first arguments (x, y) and never clobbers z.

Your third example is just star expansion, working as designed; starting at parameter 1 and filling to the right.

Example 4 is an invalid tuple; (,2) is a syntax error--unrelated to star expansion.

Answer 2

I'll be able to answer more; mobile's just slow. Take a look at this related example:

def expander(*vars):
... for var in vars:
... print(var)
...
>>> expander(1,2,3,4)
1
2
3
4

Edit: This was intended to show "expansion" of a variable that collects multiple values.

Answer 3

I'm working on it--I have to edit in another app; I don't think you're following 'star syntax' rules and I'm trying to prove it without a lot of text.

Answer 4

but...I am not getting its connection with my question.

Answer 5

oh thanks..@ kirk. now this is something understandable to me.