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sachin soman
sachin soman

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Python Tuples and List

Python tuple and list are really interesting, if we don't keep a keen eye
on the documentation, we may miss several important fine prints.
I will in this page not include the common stuff only those things I see as stuff that I may forget will be presented.

First lets initialize a list

x = [1,2,3,4]
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  • list is mutable
  • it is dynamic
  • it is ordered
  • have any type of objects as elements eg an int element and some user defined object in same list

we can retrieve the last element in a list without knowing its length by negative index slicing

last_element_x = x[-1]
Output:4
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Lets have an another negative index slicing

x = [1,2,3,4,5]
last_element_x = x[-5:-1]
Output:[1, 2, 3, 4]
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We can slice in steps

x = [1,2,3,4,5]
slice_in_steps_x = x[len(x):0:-2]
Output:[6, 4, 2]
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So we can do same for reversing the string

x = [1,2,3,4,5]
reversed_x = x[::-1]
Output:[5, 4, 3, 2, 1]
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Other things in list include mutability , nested lists and all the functions associated with list object which I can use docs to access and
nothing new there




Tuples

Tuples are really interesting and I have some important observations which I would like to share including how kwargs in functions work.

Tuple has a lot of similarity with a list as in data can be accessed, sliced and of course, like data in list it is ordered; The major difference between list and tuple
is that tuple is immutable and is much faster than a list. The access times will only matter in large data and in most trivial situations the access time shouldn't matter much.

Tuple defined by use of parenthesis

x = (1,2,3,)
type(x)
Output:tuple
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However look at this example

x = (1)
type(x)
Output:int
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Here we expected to get type tuple but python interprets it as a simple int. So what happens here is that remember we use () in other stuff like
in if condition etc. So python thinks that the () used here is for something like if(1) and thinks that x = (1) is just an int.Instead, if
we define x as shown below.

x = (1,)
type(x)
Output:tuple
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Here we added a comma after which the interpreter has identified the variable to be of type tuple. One thing to note is that this quirk occurs only when we
define a single items

Tuple Unpacking

When we define a tuple with multiple items its as if the object is packed and we can subsequently assign different items in the tuple to different
variables this is an important concept to remember as we dive into some interesting stuff now.
imageIgnore my crude drawings

Lets try to to unpack the tuple x = [1,2,3] to variables a, b, c:

x = (1,2,3)
type(x)
Output:tuple
a,b,c = x
print(a)
Output:1
print(b)
Output:2
print(c)
Output:3
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imageUnpacking in action

So here we can see that each item in tuple gets assigned to each variable.
An interesting thing to note is that

x = (1,2,3)
a,b,c = x
is same as 
(a,b,c) = x
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In such situations python doesn't care for the () parentheses. This is an important observation when we discuss
stuff later in the section.
With this new found observation lets look at one more example

x = 1,2,3,4
type(x)
Output:tuple
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So here we assigned a tuple without the need for a parantheses. With that we might be able recognize the the famous example
everyone gives to show how easy python is. swapping elements without a temporary variable

x1 , x2 = 1 , 2
x1 , x2 = x2 , x1
print(x1)
Output: 2
print(x2)
Output: 1
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This is because we used tuples without parentheses and tuple unpacking. I know MIND == BLOWN!!


*args

Now I need to show one more thing before I conclude this blog. Infact it all come down to this

insert drumrolls...

Remember that in python there are functions where we can enter as many paramenters as we want like max(1,2,3,4,5,6,7) and get a result.
How does the function know how many arguments were passed?
Before we go into details here some examples we need to see

x = 1,2,3,4,5
type(x)
Output:tuple
a,b,c = x
Output: ValueError: too many values to unpack
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Here we see that when I try to unpack tuple x to variables a,b,c since I only have 3 variables and for unpacking I have 5 values we get a ValueError
in such cases we can do the following

x = 1,2,3,4,5
type(x)
Output:tuple
a,b,*c = x
print(a)
Output: 1
print(b)
Output: 2
print(c)
Output: [3,4,5]
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Here we see that when I used * on variable c the tuple elements gets added to c as a list and I can now iterate through it. Pretty neat!.
This is known as unpacking of tuple.
lets look at some more examples

x = 1,2,3,4,5
type(x)
Output:tuple
a,*b,c = x
print(a)
Output: 1
print(b)
Output: [2,3,4]
print(c)
Output: 5
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This is self explanatory the interpreter does its best to assign values by dividing them.

x = 1,2,3,4,5
type(x)
Output:tuple
*a,*b,c = x
Output: SyntaxError: two starred expressions in assignment
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Here the interpretor doesn't know how to split data so it throws a SyntaxError

Now lets look at an example code

def test_function(*args):
    for x in args:
        print(x," ")

test_function(1,2,3,4,5)
Output: 1 2 3 4 5
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So what happened here. Like our previous examples, the arguments 1,2,3,4,5 get passed to *args which will be an unpacked tuple
and therefore we can iterate through the args like we would on a normal tuple or a list.
With this information, we can now make methods which can have a varying number of parameters.

These are some of the interesting observations I had while studying about tuples and list. If I made some egregious mistakes do send me a mail
or put a pull request on my repo. Also do check out my other blogs as well links in About section :)

Reference

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