unpacking elements in python - any tips u want to share ?

[Stephan Houben]

Hi Ganesh,

Op 2017-07-27, Ganesh Pal schreef <ganesh1pal at gmail.com>:

> I have a list with say 7 elements  say if I need to unpack  first 3
> elements  in the list and pass it an argument to the new fuction, here is
> my elementary code

One way to do exactly what you request here is:

new_function(*my_list[:3])

Explanation:
1. my_list[:3] produces a list of the first 3 elements of my_list
2. new_function(*some_sequence) 
   (Note the * in the calling syntax!)
   is equivalent to calling new_function with all the elements of
   some_sequence individually:

   new_function(some_sequence[0], some_sequence[1], ...)

But I observe that this is slightly different from the code you
actually present:

> ...    var1,var2,var3,var4,var5,var6,var7 = my_list
> ...    var8 =  get_eighth_element(var1,int(var2),int(var3))

This code actually converts var2 and var3 into an integer first.
(By calling int).

Short intermezzo: I notice that you are apparently using Python 2,
because of this code:

> ….print my_list

This code would be print(my_list) in Python 3. 
If you are just learning Python and there is no particular reason you
need Python 2, I would recommend Python 3 at this point. Python 2 is
still supported but only until 2020 so you may want to invest in
something more future-proof.

Anyway, the reason I bring it up is because in Python 3 you can do:

  var1,var2,var3,*_ = my_list
  var8 =  get_eighth_element(var1,int(var2),int(var3))

This will work for any my_list which has at least 3 elements.
The syntax *_ binds the rest of the list to the variable _.
_ is a conventional name for a variable in which value you are not
interested.

If backward-compatibility with Python 2 is important, you can do instead:

  var1,var2,var3 = my_list[:3]
  var8 =  get_eighth_element(var1,int(var2),int(var3))


Finally, I would want to point out that if these fields of the list
have some specific meaning (e.g. my_list[0] is first name, my_list[1]
is last name, my_list[2] is address, etc..), then a more idiomatic
Python solution would be to define a class with named members for these
things:

  class Person:
     def __init__(self, first_name, last_name, address, city):
         self.first_name = first_name
         self.last_name = last_name
         self.address = address
         self.city = city


Greetings,

Stephan