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]
119 questions
2675
votes
23 answers
What does ** (double star/asterisk) and * (star/asterisk) do for parameters?
In the following method definitions, what does the * and ** do for param2?
def foo(param1, *param2):
def bar(param1, **param2):
Todd
- 28,050
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660
votes
5 answers
What does the star and doublestar operator mean in a function call?
What does the * operator mean in Python, such as in code like zip(*x) or f(**k)?
How is it handled internally in the interpreter?
Does it affect performance at all? Is it fast or slow?
When is it useful and when is it not?
Should it be used in a…
psihodelia
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122
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
- 10,877
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113
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
- 36,160
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67
votes
2 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
- 9,259
- 1
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- 41
56
votes
5 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
- 3,603
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52
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
- 7,006
- 8
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- 32
41
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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27
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
- 3,005
- 8
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26
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
- 289
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22
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
- 2,636
- 17
- 36
20
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
- 1,224
- 1
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- 36
17
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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14
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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14
votes
3 answers
Difference call function with asterisk parameter and without
I know what the meaning of an asterisk is in a function definition in Python.
I often, though, see asterisks for calls to functions with parameters like:
def foo(*args, **kwargs):
first_func(args, kwargs)
second_func(*args, **kwargs)
What…
Robert Moon
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