*args implementation and clarification about tuple status

[amjass12]

Hi!

I don't think this should go in the code checking section as I am also asking for clarification on implementation so hope this thread is ok here.

I have a function that starts as follows:

def fun(*args):
     list_args = []
     for i in args:
          list_args.append(i)
      #continue with function

then run:
fun(arg1, arg2, arg3, arg4)#say they are simply integers, 1,2,3,4

This works and does allow me to add a variable number of arguments and then continue downstream with the rest of the function definition.

My two question are:

1. is this valid and correct?

2. Reason for question, *args is specified as tuple - I know what tuples are, but not sure about what it means in this context.. does it prohibit the use of the *args in specific elements of code (say appending to a list as i have done)?

Would appreciate any clarification on what this actually means.

thank you!

[Gribouillis]

You can use args directly. It is indeed a tuple, which is created when the function is called

>>> def func(*args):
...     print(args)
... 
>>> func(1, 2, 3, 4)
(1, 2, 3, 4)
>>> 

Sometimes one needs to convert it to a list, it can be done faster this way

def func(*args):
    args_list = list(args)
    ....

[amjass12]

You can use args directly. It is indeed a tuple, which is created when the function is called

>>> def func(*args):
...     print(args)
... 
>>> func(1, 2, 3, 4)
(1, 2, 3, 4)
>>> 

Sometimes one needs to convert it to a list, it can be done faster this way

def func(*args):
    args_list = list(args)
    ....

thank you! this makes sense, I didn't know *args was converted directly to a tuple object inside the function. Which then leads to another question which is, is it safer to iterate of a tuple or a list? As tuples are immutable its likely safer, but i dont know if its safer.

I generate a for loop to compare one value to each element of the newly created list from *args - i suppose it doesn't make a difference since nothing is being done to the list itself apart from saying compare value x to element 1, do something.. then compare to element 2, do something etc.

[Gribouillis]

Iterating a tuple or a list are equaly safe unless you try to update the structure during the iterations. Doing so with a tuple raises an exception because the tuple is immutable while doing so with a list yields unpredictable results. Most of the time you won't need to convert args to a list because you can read the arguments directly with expressions such as args[1] or perhaps x, y = args[:2], etc.

[amjass12]

Iterating a tuple or a list are equaly safe unless you try to update the structure during the iterations. Doing so with a tuple raises an exception because the tuple is immutable while doing so with a list yields unpredictable results. Most of the time you won't need to convert args to a list because you can read the arguments directly with expressions such as args[1] or perhaps x, y = args[:2], etc.

got it! thank you so much this very clear!

And yes, that is how i iterate over the lists when carrying out the comparisons either for

i in list:

or when doing individually,

 list[1], list[2]

etc..

[buran]

cross posted at https://stackoverflow.com/q/68269203/4046632
Please, don't cross-post or if you do so - provide a link.

[amjass12]

cross posted at https://stackoverflow.com/q/68269203/4046632
Please, don't cross-post or if you do so - provide a link.

this is weird - this has been copied and pasted..

the user that posted this is not me... i posted this here this morning and nowhere else. also- this has been asked 44mins ago - WELL after I asked and also got an answer 2 hours ago!

[deanhystad]

This means the same in all contexts:

2. Reason for question, *args is specified as tuple - I know what tuples are, but not sure about what it means in this context.. does it prohibit the use of the *args in specific elements of code (say appending to a list as i have done)?

When used as below, args is a tuple and it has all the features and limitations of a tuple. likewise kwargs is a dictionary and it works exactly like a dictionary.

def func(*args, **kwargs):
    kwargs['this is'] = 'a bad idea'
    print(type(args), args, type(kwargs), kwargs)

func()

Output:
<class 'tuple'> () <class 'dict'> {'this is': 'a bad idea'}

So you can do anything with args that you can do with a tuple, and since args is a tuple you cannot append things to args.

def func(*args):
    augmented_args = [1, 2, 3]
    augmented_args.extend(args)
    print(augmented_args)
    args.append('Not allowed')

func(4, 5, 6)

Output:
[1, 2, 3, 4, 5, 6]
Traceback (most recent call last):
  File "c:\Users\hystadd\Documents\python\sandbox\junk.py", line 7, in <module>
    func(4, 5, 6)
  File "c:\Users\hystadd\Documents\python\sandbox\junk.py", line 5, in func
    args.append('Not allowed')
AttributeError: 'tuple' object has no attribute 'append'

[buran]

this is weird - this has been copied and pasted..

OK, I agree. It's really weird.
I checked all three questions asked by this SO user and all of them were copy/paste verbatim from threads here (by different users).
I will revoke the warning for cross-posting

[amjass12]

This means the same in all contexts:

2. Reason for question, *args is specified as tuple - I know what tuples are, but not sure about what it means in this context.. does it prohibit the use of the *args in specific elements of code (say appending to a list as i have done)?

When used as below, args is a tuple and it has all the features and limitations of a tuple. likewise kwargs is a dictionary and it works exactly like a dictionary.

def func(*args, **kwargs):
    kwargs['this is'] = 'a bad idea'
    print(type(args), args, type(kwargs), kwargs)

func()

Output:
<class 'tuple'> () <class 'dict'> {'this is': 'a bad idea'}

So you can do anything with args that you can do with a tuple, and since args is a tuple you cannot append things to args.

def func(*args):
    augmented_args = [1, 2, 3]
    augmented_args.extend(args)
    print(augmented_args)
    args.append('Not allowed')

func(4, 5, 6)

Output:
[1, 2, 3, 4, 5, 6]
Traceback (most recent call last):
  File "c:\Users\hystadd\Documents\python\sandbox\junk.py", line 7, in <module>
    func(4, 5, 6)
  File "c:\Users\hystadd\Documents\python\sandbox\junk.py", line 5, in func
    args.append('Not allowed')
AttributeError: 'tuple' object has no attribute 'append'

thank you - so the solution to manipulating the tuple of variable arguments would be to say make a list etc as others have suggested - i know it seems silly.. but one thing i wasn't sure of was if this 'pythonic' and 'allowed'