Use this tag for questions related with argument unpacking, a technique that allows Arrays and Traversable objects to be extracted/unpacked into argument lists/sequences.
argument-unpacking is usually found in PHP, Python and Go.
Questions tagged [argument-unpacking]
144 questions
3315
votes
27 answers
What does ** (double star/asterisk) and * (star/asterisk) do for parameters?
What do *args and **kwargs mean in these function definitions?
def foo(x, y, *args):
pass
def bar(x, y, **kwargs):
pass
See What do ** (double star/asterisk) and * (star/asterisk) mean in a function call? for the complementary question…
Todd
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786
votes
4 answers
What do ** (double star/asterisk) and * (star/asterisk) mean in a function call?
In code like zip(*x) or f(**k), what do the * and ** respectively mean? How does Python implement that behaviour, and what are the performance implications?
See also: Expanding tuples into arguments. Please use that one to close questions where OP…
psihodelia
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176
votes
4 answers
Go Unpacking Array As Arguments
So in Python and Ruby there is the splat operator (*) for unpacking an array as arguments. In Javascript there is the .apply() function. Is there a way of unpacking an array/slice as function arguments in Go? Any resources for this would be great as…
eatonphil
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134
votes
4 answers
Unpacking, extended unpacking and nested extended unpacking
Consider the following expressions. Note that some expressions are repeated to present the "context".
(this is a long list)
a, b = 1, 2 # simple sequence assignment
a, b = ['green', 'blue'] # list asqignment
a, b…
treecoder
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76
votes
4 answers
Class that acts as mapping for **unpacking
Without subclassing dict, what would a class need to be considered a mapping so that it can be passed to a method with **.
from abc import ABCMeta
class uobj:
__metaclass__ = ABCMeta
uobj.register(dict)
def f(**k): return k
o =…
dskinner
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- 43
67
votes
3 answers
How to extract parameters from a list and pass them to a function call
What is a good, brief way to extract items from a list and pass them as parameters to a function call, such as in the example below?
Example:
def add(a,b,c,d,e):
print(a,b,c,d,e)
x=(1,2,3,4,5)
add(magic_function(x))
falek.marcin
- 9,679
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66
votes
6 answers
Passing an array/list into a Python function
I've been looking at passing arrays, or lists, as Python tends to call them, into a function.
I read something about using *args, such as:
def someFunc(*args)
for x in args
print x
But not sure if this is right/wrong. Nothing seems to…
Jack Franklin
- 3,765
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60
votes
3 answers
Cannot unpack array with string keys
The error message is:
FATAL ERROR Uncaught Error: Cannot unpack array with string keys
I know I can simply run the method fetch() twice and pass the ['q'] and ['bind'], but I am trying to get to grips with using the new ... to unpack values. I…
Jaquarh
- 6,493
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- 86
55
votes
2 answers
Python 3: starred expression to unpack a list
Example use:
def f(a, b, c, d):
print(a, b, c, d, sep = '&')
f(1,2,3,4)
>>> 1&2&3&4
f(*[1, 2, 3, 4])
>>> 1&2&3&4
Where in the python documentation is * explained?
Kifsif
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32
votes
3 answers
Python: Splat/unpack operator * in python cannot be used in an expression?
Does anybody know the reasoning as to why the unary (*) operator cannot be used in an expression involving iterators/lists/tuples?
Why is it only limited to function unpacking? or am I wrong in thinking that?
For example:
>>> [1,2,3, *[4,5,6]]
File…
Har
- 3,727
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31
votes
3 answers
Unpack NumPy array by column
If I have a NumPy array, for example 5x3, is there a way to unpack it column by column all at once to pass to a function rather than like this: my_func(arr[:, 0], arr[:, 1], arr[:, 2])?
Kind of like *args for list unpacking but by column.
jeff_new
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31
votes
3 answers
Why this unpacking of arguments does not work?
I get an error type object argument after ** must be a mapping, not tuple.
I have this code:
create_character = player.Create(**generate_player.generate())
this is player.py module:
class Create(object):
def __init__(self,name,age,gender):
…
user3056783
- 2,265
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23
votes
5 answers
Python keyword arguments unpack and return dictionary
I have a function definition as below and I am passing keyword arguments. How do I get to return a dictionary with the same name as the keyword arguments?
Manually I can do:
def generate_student_dict(first_name=None, last_name=None , birthday=None,…
Karan Kumar
- 2,655
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18
votes
4 answers
python - iterating list of dictionaries and unpacking
Given a flat list of simple dictionaries
lst = [{'key1': 1}, {'key2': 2}, {'key3': 3}]
I'd like to find the dict that yields the minimum value evaluated using a method not detailed here.
My first idea was to iterate the list to check dict by dict,…
jake77
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18
votes
1 answer
How are python's unpacking operators * and ** used?
The unpacking/splat operators * and ** differ widely in their applicability across python versions (2.7, 3.x < 3.5 and 3.x >= 3.5).
For example:
| 2.7 | 3.1-3.4 | 3.5 …
cs95
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