Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

What does single(not double) asterisk * means when unpacking dictionary in Python?

Can anyone explain the difference when unpacking the dictionary using single or double asterisk? You can mention their difference when used in function parameters, only if it is relevant here, which I don't think so.

However, there may be some relevance, because they share the same asterisk syntax.

def foo(a,b)
    return a+b

tmp = {1:2,3:4}
foo(*tmp)        #you get 4
foo(**tmp)       #typeError: keyword should be string. Why it bothers to check the type of keyword? 

Besides, why the key of dictionary is not allowed to be non-string when passed as function arguments in THIS situation? Are there any exceptions? Why they design Python in this way, is it because the compiler can't deduce the types in here or something?

Thanks!

like image 672
Han XIAO Avatar asked Nov 05 '18 03:11

Han XIAO


People also ask

What is double * in Python?

For numeric data types double asterisk (**) is defined as exponentiation operator >>> a=10; b=2 >>> a**b 100 >>> a=1.5; b=2.5 >>> a**b 2.7556759606310752 >>> a=3+2j >>> b=3+5j >>> a**b (-0.7851059645317211+2.350232331971346j)

What is the significance of unpacking the element in tuple through asterisk (*)?

The * operator is an unpacking operator that will unpack the values from any iterable object, such as lists, tuples, strings, etc… And that's it! The asterisk, *, or unpacking operator, unpacks num_list, and passes the values, or elements, of num_list as separate arguments to the num_sum function.

What happens when we prefix a parameter with an asterisk * What about two asterisks * * As parameter prefix?

In a function call the '**' unpacks data structure of dictionary into positional or keyword arguments to be received by function definition. In a function definition the '*' packs positional arguments into a tuple. In a function definition the '**' packs keyword arguments into a dictionary.

What happens when we prefix a parameter with an asterisk (*)?

It is used to pass a variable number of arguments to a function, it is mostly used to pass a non-key argument and variable-length argument list.


3 Answers

When dictionaries are iterated as lists the iteration takes the keys of it, for example

for key in tmp:
    print(key)

is the same as

for key in tmp.keys():
    print(key)

in this case, unpacking as *tmp is equivalent to *tmp.keys(), ignoring the values. If you want to use the values you can use *tmp.values().

Double asterisk is used for when you define a function with keyword parameters such as

def foo(a, b):

or

def foo(**kwargs):

here you can store the parameters in a dictionary and pass it as **tmp. In the first case keys must be strings with the names of the parameter defined in the function firm. And in the second case you can work with kwargs as a dictionary inside the function.

like image 52
vlizana Avatar answered Nov 03 '22 01:11

vlizana


def foo(a,b)
   return a+b

tmp = {1:2,3:4}
foo(*tmp)        #you get 4
foo(**tmp) 

In this case:
foo(*tmp) mean foo(1, 3)
foo(**tmp) mean foo(1=2, 3=4), which will raise an error since 1 can't be an argument. Arg must be strings and (thanks @ Alexander Reynolds for pointing this out) must start with underscore or alphabetical character. An argument must be a valid Python identifier. This mean you can't even do something like this:

def foo(1=2, 3=4):
   <your code>

or

def foo('1'=2, '3'=4):
   <your code>

See python_basic_syntax for more details.

like image 44
enamoria Avatar answered Nov 03 '22 01:11

enamoria


It is a Extended Iterable Unpacking.

>>> def add(a=0, b=0):
...     return a + b
...
>>> d = {'a': 2, 'b': 3}
>>> add(**d)#corresponding to add(a=2,b=3)
5

For single *,

def add(a=0, b=0):
    ...     return a + b
    ...
    >>> d = {'a': 2, 'b': 3}
    >>> add(*d)#corresponding to add(a='a',b='b')
    ab

Learn more here.

like image 27
Ashutosh Chapagain Avatar answered Nov 03 '22 00:11

Ashutosh Chapagain