Python 2.7
If we look at the CPython source related to creating the AST(ast_for_call) for a function call the order of argument evaluation turns out to be:
return Call(func, args, keywords, vararg, kwarg, func->lineno,
func->col_offset, c->c_arena);
ie. args --> keywords --> vararg --> kwarg
So, in your case the keyword argument is evaluated first and then the star based expression(vararg) is evaluated.
Byte code:
>>> dis.dis(lambda: func(1, 2, *('k', 'j', 'l'), z=1, y =2, three=x.pop(), **{kwarg:1}))
1 0 LOAD_GLOBAL 0 (func)
3 LOAD_CONST 1 (1) # arg
6 LOAD_CONST 2 (2) # arg
9 LOAD_CONST 3 ('z') # keyword
12 LOAD_CONST 1 (1)
15 LOAD_CONST 4 ('y') # keyword
18 LOAD_CONST 2 (2)
21 LOAD_CONST 5 ('three') # keyword
24 LOAD_GLOBAL 1 (x)
27 LOAD_ATTR 2 (pop)
30 CALL_FUNCTION 0
33 LOAD_CONST 9 (('k', 'j', 'l')) #vararg
36 BUILD_MAP 1
39 LOAD_CONST 1 (1)
42 LOAD_GLOBAL 3 (kwarg) #kwarg
45 STORE_MAP
46 CALL_FUNCTION_V
Hence in your case the pop() call will happen first followed by the varargs evaluation.
So, if three is a part of kwargs then we will get an error with map:
>>> x = [1, 2, 3]
>>> count(*map(float, x), **{'three': x.pop()})
Traceback (most recent call last):
File "<ipython-input-133-e8831565af13>", line 1, in <module>
count(*map(float, x), **{'three': x.pop()})
TypeError: count() got multiple values for keyword argument 'three'
It will work if we do it *lazily:
>>> x = [1, 2, 3]
>>> count(*(float(y) for y in x), **{'three': x.pop()})
1.0, 2.0, 3
*The reason why generator works and map or list comprehension fails is explained at the end.
Python 3.5
The ast_for_call function here only maintains two lists: args and keywords.
Here the varargs are inserted into the args list and kwargs go to the keywords list. So, in the end the call looks like:
return Call(func, args, keywords, func->lineno, func->col_offset, c->c_arena);
Byte code:
>>> dis.dis(lambda: func(1, 2, *('k', 'j', 'l'), z=1, y =2, three=x.pop(), **{kwarg:1}))
1 0 LOAD_GLOBAL 0 (func)
3 LOAD_CONST 1 (1)
6 LOAD_CONST 2 (2)
9 LOAD_CONST 9 (('k', 'j', 'l'))
12 LOAD_CONST 6 ('z')
15 LOAD_CONST 1 (1)
18 LOAD_CONST 7 ('y')
21 LOAD_CONST 2 (2)
24 LOAD_CONST 8 ('three')
27 LOAD_GLOBAL 1 (x)
30 LOAD_ATTR 2 (pop)
33 CALL_FUNCTION 0 (0 positional, 0 keyword pair)
36 LOAD_GLOBAL 3 (kwarg)
39 LOAD_CONST 1 (1)
42 BUILD_MAP 1
45 CALL_FUNCTION_VAR_KW 770 (2 positional, 3 keyword pair)
48 RETURN_VALUE
Now things can get a little exciting if the expression yielding the varargs is lazy:
>> def count(one, two, three):
print (one, two, three)
...
>>> x = [1, 2, 3]
>>> count(*map(float, x), three=x.pop()) # map is lazy in Python 3
1.0 2.0 3
>>> x = [1, 2, 3]
>>> count(*[float(y) for y in x], three=x.pop())
Traceback (most recent call last):
File "<ipython-input-25-b7ef8034ef4e>", line 1, in <module>
count(*[float(y) for y in x], three=x.pop())
TypeError: count() got multiple values for argument 'three'
Byte code:
>>> dis.dis(lambda: count(*map(float, x), three=x.pop()))
1 0 LOAD_GLOBAL 0 (count)
3 LOAD_GLOBAL 1 (map)
6 LOAD_GLOBAL 2 (float)
9 LOAD_GLOBAL 3 (x)
12 CALL_FUNCTION 2 (2 positional, 0 keyword pair)
15 LOAD_CONST 1 ('three')
18 LOAD_GLOBAL 3 (x)
21 LOAD_ATTR 4 (pop)
24 CALL_FUNCTION 0 (0 positional, 0 keyword pair)
27 CALL_FUNCTION_VAR 256 (0 positional, 1 keyword pair)
30 RETURN_VALUE
>>> dis.dis(lambda: count(*[float(y) for y in x], three=x.pop()))
1 0 LOAD_GLOBAL 0 (count)
3 LOAD_CONST 1 (<code object <listcomp> at 0x103b63930, file "<ipython-input-28-1cc782164f20>", line 1>)
6 LOAD_CONST 2 ('<lambda>.<locals>.<listcomp>')
9 MAKE_FUNCTION 0
12 LOAD_GLOBAL 1 (x)
15 GET_ITER
16 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
19 LOAD_CONST 3 ('three')
22 LOAD_GLOBAL 1 (x)
25 LOAD_ATTR 2 (pop)
28 CALL_FUNCTION 0 (0 positional, 0 keyword pair)
31 CALL_FUNCTION_VAR 256 (0 positional, 1 keyword pair)
34 RETURN_VALUE
The lazy call works because unpacking(aka actual evaluation of the generator) doesn't happen until the function is actually called, hence in this case pop() call will remove the 3 first and then later on map will only pass 1, 2.
But, in the case of list comprehension the list object already contains 3 items and then even though pop() removed 3 later on we are still passing two values for the third argument.
