100 Days of DevOps — Day 88-Lists in Python

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Welcome to Day 88 of 100 Days of DevOps, Focus for today is Lists in Python

Lists in Python

• The list is a value that contains multiple values(mutable sequence)

>>> x = [1,2,’abc’,2.5]
>>> x
[1, 2, 'abc', 2.5]

• We can access the item in the list with an integer index that start with 0(not 1)

>>> x[0]
1

• We can use the negative index to refer to the last item in the list

>>> x[-1]
2.5

• To get multiple items from the list use slice

>>> x[0:2]
[1, 2]
#Basically grab every value of the list
>>> x[0:]
[1, 2, 'abc', 2.5]

• All the function that works with strings works in the same way with list eg:len()

>>> len(x)
4

• To delete the value from the list use del

>>> del x[0]
>>> x
[2, ‘abc’, 2.5]

• We can convert a value into a list by passing it to the list() function

>>> list(‘hello’)
[‘h’, ‘e’, ‘l’, ‘l’, ‘o’]

• To find out the value in list use in operator

>>> ‘h’ in ‘hello’
True
#Opposite of that is not in operator
>>> 'h' not in 'hello'
False

• Use of for loop with a list, so as you can see for loop iterate over the values in a list

>>> for i in x:
… print(i)
…
2
abc
2.5

• range() function returns a list-like value which we can pass to the list() function if we need an actual value

>>> list(range(0,4))
[0, 1, 2, 3]

Let’s take one more example

>>> items = [“a”,”b”,”c”,”d”]
>>> for i in range(len(items)):
...     print("Value at "+ str(i) + " is equal to: " + items[i])
...
Value at 0 is equal to: a
Value at 1 is equal to: b
Value at 2 is equal to: c
Value at 3 is equal to: d

Swapping Variables

>>> a,b,c = 1,2,3
>>> a
1
>>> b
2
>>> c
3

Lists Methods

Methods are functions that are called on values. List has several methods

• index() list method returns the index of an item in the list

>>> test = [“a”,”b”,”c”,”d”]
>>> test.index(“a”)
0
# Index method will raise the exception if it doesn't find the value
>>> test.index("e")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: 'e' is not in list
# Now in case of duplicate list it only return the index of first value
>>> test = ["a","b","c","d","a","b"]
>>> test.index("b")
1

• append() list method adds value to the end of a list

>>> test.append(“z”)
>>> test
[‘a’, ‘b’, ‘c’, ‘d’, ‘a’, ‘b’, ‘z’]

• insert() list method adds value anywhere inside a list

>>> test.insert(0,”hola”)
>>> test
[‘hola’, ‘a’, ‘b’, ‘c’, ‘d’, ‘a’, ‘b’, ‘z’]

• remove() list method removes an item, specified by the value from a list

>>> test.remove(“a”)
>>> test  #Only first incident of that value is removed
[‘hola’, ‘b’, ‘c’, ‘d’, ‘a’, ‘b’, ‘z’]

• reverse(): A list can be reversed by calling its reverse() method

>>> a = [2,5,1,3,6,4]
>>> a.reverse()
>>> a
[4, 6, 3, 1, 5, 2]

• sort() list method sorts the items in a list

>>> test1 = [8,3,9,4,6,3]
>>> test1.sort()
>>> test1
[3, 3, 4, 6, 8, 9]

But if we want to sort a combination of numbers and strings, Python throws an error as it doesn’t know how to sort it

>>> test2 = [1,2,3,”a”,”b”]
>>> test2 = [1,3,2,6,5,”a”,”b”]
>>> test2.sort()
Traceback (most recent call last):
File “<stdin>”, line 1, in <module>
TypeError: ‘<’ not supported between instances of ‘str’ and ‘int’

The way sorting works is using ASCII-betical order that is the upper case first

>>> test3 = [‘a’,’b’,’A’,’B’]
>>> test3.sort()
>>> test3
[‘A’, ‘B’, ‘a’, ‘b’]

This doesn’t look good as A should be followed by a, so to do that pass str.lower which is technically converting everything to lowercase

>>> test3.sort(key=str.lower)
>>> test3
[‘A’, ‘a’, ‘B’, ‘b’]
# The important use case of key, where we use len as a key(here sorting happens based on the length)
>>> b
['hello', 'how', 'are', 'u', 'Mr', 'Prashant']
>>> b.sort(key=len)
>>> b
['u', 'Mr', 'how', 'are', 'hello', 'Prashant']

NOTE: These list methods operate on the list “in place”, rather than returning a new list value

• Sorted(): built-in function sorts any iterable series and returns a list

>>> x = [5,2,3,1]
>>> sorted(x)
[1, 2, 3, 5]
>>> x
[5, 2, 3, 1]

• count(): Count returns the number of matching elements

>>> a = [1,2,3,4,1,23,5,6]
>>> a.count(1)
2

New few things we need to be really careful about lists

>>> test = [1,2,3,4]
>>> test1 = test
>>> test1
[1, 2, 3, 4]
>>> test1[0] = 9
>>> test1
[9, 2, 3, 4]
>>> test
[9, 2, 3, 4]

We only change the value of test1 but the value of the test got changed?

The Reason for that when we created this list(test), Python created this list in computer memory but it’s assigned a reference to test. Now when we run this

test1 = test

A reference get copied to test1 and they referencing the same list and that will cause all sort of weirds error,so please be really careful when dealing with lists

We don’t have this kind of issues with immutable values like strings/tuples as we can’t replace it by new values

>>> a = “abc”
>>> b = a
>>> b = “efg”
# Change in b will not impact a
>>> b
‘efg’
>>> a
‘abc

Let’s go much deeper into the same concept, as you see it’s just the reference which is changing here. Now we have no way to reach x = 100 so at some point of time Python garbage collector will take care of it.

• id(): returns a unique identifier for an object

>>> x = 100
>>> id(x)
4340331440
>>> x = 50
>>> id(x)
4340329840

So how to take care of this kind of issues in which we want a completely separate list, for that we have a module called copy(a copy has a module called deep copy which creates a brand new list and returns a reference to new list)

>>> x = [1,2,3]
>>> import copy
>>> y = copy.deepcopy(x)
>>> y
[1, 2, 3]
>>> y[0] = 4
>>> y
[4, 2, 3]
>>> x
[1, 2, 3]

OR(full slice technique)

>>> a = [4,5,6]
>>> b = a[:]
>>> b
[4, 5, 6]
>>> id(a)
4325208200
>>> id(b)
4325208328
>>> b[0] = 10
>>> b
[10, 5, 6]
>>> a
[4, 5, 6]

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Reference

100 Days of DevOps — Day 0

100 Days of DevOps — Day 87-While/For Loop Python