Python Lists Creating Properties Items Length Data Types Constructor Collections syntax

1. Python Lists

   • Lists are used to store multiple items in a single variable.
   • One of the four built-in data types in Python used to store collections of data (the others are Tuple, Set, and Dictionary).

2. Creating Lists

   • Lists are created using square brackets ([]).
   • Example:

     thislist = ["apple", "banana", "cherry"]
     print(thislist)
     

3. List Properties

   • Ordered: Items have a defined order, which doesn’t change unless explicitly altered.
   • Changeable: You can add, remove, and modify items after creation.
   • Allow Duplicates: Lists can have items with duplicate values.

4. List Items

   • Items are indexed: The first item has index [0], the second [1], and so on.
   • Example of duplicate values:

     thislist = ["apple", "banana", "cherry", "apple", "cherry"]
     print(thislist)
     

5. List Length

   • Use len() to determine the number of items in a list.
   • Example:

     thislist = ["apple", "banana", "cherry"]
     print(len(thislist))
     

6. List Items - Data Types

   • List items can be of any data type (string, integer, boolean, etc.).
   • A list can even contain mixed data types.
   • Examples:

     list1 = ["apple", "banana", "cherry"]
     list2 = [1, 5, 7, 9, 3]
     list3 = [True, False, False]
     list4 = ["abc", 34, True, 40, "male"]
     

7. Type of a List

   • From Python’s perspective, lists are objects of the data type 'list'.
   • Example:

     mylist = ["apple", "banana", "cherry"]
     print(type(mylist))
     

8. The list() Constructor

   • You can create a list using the list() constructor.
   • Example:

     thislist = list(("apple", "banana", "cherry"))  # note the double round brackets
     print(thislist)
     

9. Python Collections (Arrays)

   • List: Ordered and changeable. Allows duplicates.
   • Tuple: Ordered and unchangeable. Allows duplicates.
   • Set: Unordered, unchangeable*, unindexed. No duplicates.
   • Dictionary: Ordered** and changeable. No duplicates.
   • Notes:
     • Set items are unchangeable, but you can add/remove items.
     • Dictionaries are ordered as of Python 3.7.

10. Choosing a Collection Type

    • Understanding properties of different types is crucial for efficiency, security, and meaning retention.

11. Exercise Example

• What will be the result of the syntax:

  mylist = ['apple', 'banana', 'cherry']
  print(mylist[1])
  

• Answer: banana.

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 Python List Items:

Access Items by Index

• Indexed access: Use index numbers to access list items. Index starts at 0.

  thislist = ["apple", "banana", "cherry"]
  print(thislist[1])  # Output: "banana"
  

• Note: The first item has index 0.

---

Negative Indexing

• Access from the end: Negative indexing starts at -1 for the last item.

  thislist = ["apple", "banana", "cherry"]
  print(thislist[-1])  # Output: "cherry"
  

• Examples:
  • -1 = Last item
  • -2 = Second-last item

---

Range of Indexes

• Slice a range: Specify start and end indexes for a subset.

  thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
  print(thislist[2:5])  # Output: ['cherry', 'orange', 'kiwi']
  

• Important Notes:
  • Start index is included, end index is not included.
  • Start defaults to 0 if omitted.
  • End defaults to last item if omitted.

Examples:

# From the beginning to "kiwi" (not included)
print(thislist[:4])  # Output: ['apple', 'banana', 'cherry', 'orange']

# From "cherry" to the end
print(thislist[2:])  # Output: ['cherry', 'orange', 'kiwi', 'melon', 'mango']

---

Range of Negative Indexes

• Use negative indexes to slice from the end.

  thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
  print(thislist[-4:-1])  # Output: ['orange', 'kiwi', 'melon']
  

• Example: Start at -4 ("orange") and end before -1 ("mango").

---

Check if Item Exists

• Use the in keyword to check if an item is in a list.

  thislist = ["apple", "banana", "cherry"]
  if "apple" in thislist:
      print("Yes, 'apple' is in the fruits list")
  

---

Exercise

What will be the result of the following syntax?

mylist = ['apple', 'banana', 'cherry']
print(mylist[-1])

Answer: cherry.

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Changing Python List Items

1. Change Item Value

• To change a specific item, use its index:

  thislist = ["apple", "banana", "cherry"]
  thislist[1] = "blackcurrant"
  print(thislist)  # Output: ['apple', 'blackcurrant', 'cherry']
  

---

2. Change a Range of Item Values

• Replace a range with a new list:

  thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]
  thislist[1:3] = ["blackcurrant", "watermelon"]
  print(thislist)  # Output: ['apple', 'blackcurrant', 'watermelon', 'orange', 'kiwi', 'mango']
  

---

3. Insert More Items Than Replaced

• If the new list has more items than the replaced range, the extra items are inserted, shifting the remaining elements:

  thislist = ["apple", "banana", "cherry"]
  thislist[1:2] = ["blackcurrant", "watermelon"]
  print(thislist)  # Output: ['apple', 'blackcurrant', 'watermelon', 'cherry']
  

---

4. Insert Fewer Items Than Replaced

• If the new list has fewer items than the replaced range, the remaining elements shift accordingly:

  thislist = ["apple", "banana", "cherry"]
  thislist[1:3] = ["watermelon"]
  print(thislist)  # Output: ['apple', 'watermelon']
  

---

5. Insert Items Without Replacing

• Use the insert() method to add an item at a specific index without replacing existing items:

  thislist = ["apple", "banana", "cherry"]
  thislist.insert(2, "watermelon")
  print(thislist)  # Output: ['apple', 'banana', 'watermelon', 'cherry']
  

---

Exercise

What will be the result of the following syntax?

mylist = ['apple', 'banana', 'cherry']
mylist[0] = 'kiwi'
print(mylist[1])

Answer: banana.

Explanation:

• mylist[0] = 'kiwi' replaces "apple" with "kiwi."
• The list becomes ['kiwi', 'banana', 'cherry'].
• Printing mylist[1] outputs the second item, which is "banana".

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Key Points on Adding Items to Python Lists

1. Append Items

• Use the append() method to add an item to the end of the list:

  thislist = ["apple", "banana", "cherry"]
  thislist.append("orange")
  print(thislist)  # Output: ['apple', 'banana', 'cherry', 'orange']
  

---

2. Insert Items

• Use the insert() method to add an item at a specified index:

  thislist = ["apple", "banana", "cherry"]
  thislist.insert(1, "orange")
  print(thislist)  # Output: ['apple', 'orange', 'banana', 'cherry']
  

---

3. Extend List

• Use the extend() method to append elements from another list:

  thislist = ["apple", "banana", "cherry"]
  tropical = ["mango", "pineapple", "papaya"]
  thislist.extend(tropical)
  print(thislist)  # Output: ['apple', 'banana', 'cherry', 'mango', 'pineapple', 'papaya']
  

• Note: The elements of the second list are added to the end of the current list.

---

4. Add Any Iterable

• The extend() method can also append elements from any iterable object (e.g., tuples, sets, or dictionaries):

  thislist = ["apple", "banana", "cherry"]
  thistuple = ("kiwi", "orange")
  thislist.extend(thistuple)
  print(thislist)  # Output: ['apple', 'banana', 'cherry', 'kiwi', 'orange']
  

---

Exercise

What will be the result of the following syntax?

mylist = ['apple', 'banana', 'cherry']
mylist.insert(0, 'orange')
print(mylist[1])

Answer: apple.

Explanation:

• mylist.insert(0, 'orange') inserts "orange" at index 0.
• The updated list becomes ['orange', 'apple', 'banana', 'cherry'].
• Printing mylist[1] outputs the second item, which is "apple".

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Key Points on Removing Items from Python Lists

1. Remove a Specified Item

• Use the remove() method to remove the first occurrence of the specified value:

  thislist = ["apple", "banana", "cherry"]
  thislist.remove("banana")
  print(thislist)  # Output: ['apple', 'cherry']
  

• If there are duplicate items, remove() will only remove the first occurrence:

  thislist = ["apple", "banana", "cherry", "banana", "kiwi"]
  thislist.remove("banana")
  print(thislist)  # Output: ['apple', 'cherry', 'banana', 'kiwi']
  

---

2. Remove an Item by Index

• Use the pop() method to remove the item at a specific index:

  thislist = ["apple", "banana", "cherry"]
  thislist.pop(1)
  print(thislist)  # Output: ['apple', 'cherry']
  

• If no index is specified, pop() will remove the last item:

  thislist = ["apple", "banana", "cherry"]
  thislist.pop()
  print(thislist)  # Output: ['apple', 'banana']
  

---

3. Remove an Item Using del

• Use the del keyword to remove an item at a specific index:

  thislist = ["apple", "banana", "cherry"]
  del thislist[0]
  print(thislist)  # Output: ['banana', 'cherry']
  

• Use del to delete the entire list:

  thislist = ["apple", "banana", "cherry"]
  del thislist
  # List no longer exists
  

---

4. Clear the Entire List

• Use the clear() method to empty the list while keeping the list object:

  thislist = ["apple", "banana", "cherry"]
  thislist.clear()
  print(thislist)  # Output: []
  

---

Exercise

What is a list method for removing list items?

• Options:
  • pop() ✅
  • push() ❌
  • delete() ❌

Answer: pop().

Explanation:

• pop() is a built-in method for removing items from a list by index.

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Looping Through Python Lists

1. Loop Through a List Using for Loop

• You can directly loop through the items in a list:

  thislist = ["apple", "banana", "cherry"]
  for x in thislist:
      print(x)
  

  Output:

  apple
  banana
  cherry
  

---

2. Loop Through the List Using Index Numbers

• Use range() and len() to loop through items by their index:

  thislist = ["apple", "banana", "cherry"]
  for i in range(len(thislist)):
      print(thislist[i])
  

  Output:

  apple
  banana
  cherry
  

  • Here, the range [0, 1, 2] is created for the list indices.

---

3. Loop Through a List Using a while Loop

• Use the while loop with the index and the length of the list:

  thislist = ["apple", "banana", "cherry"]
  i = 0
  while i < len(thislist):
      print(thislist[i])
      i += 1
  

  Output:

  apple
  banana
  cherry
  

---

4. Loop Through a List Using List Comprehension

• Use list comprehension for a more concise looping syntax:

  thislist = ["apple", "banana", "cherry"]
  [print(x) for x in thislist]
  

  Output:

  apple
  banana
  cherry
  

---

Exercise

What is the correct syntax for looping through the items of a list?

Options:

1. for x in ['apple', 'banana', 'cherry']:
       print(x)
   ``` ✅
   

2. for x in ['apple', 'banana', 'cherry']
       print(x)
   ``` ❌ (Missing colon `:`)
   

3. foreach x in ['apple', 'banana', 'cherry']
       print(x)
   ``` ❌ (Python does not use `foreach`)
   

Answer: Option 1.

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Python List Comprehension

1. What is List Comprehension?

• Definition: A shorter syntax to create a new list based on values from an existing list.
• Example (Traditional Approach):

  fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
  newlist = []
  
  for x in fruits:
      if "a" in x:
          newlist.append(x)
  
  print(newlist)
  

  Output:

  ['apple', 'banana', 'mango']
  

• Example (Using List Comprehension):

  fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
  newlist = [x for x in fruits if "a" in x]
  print(newlist)
  

  Output:

  ['apple', 'banana', 'mango']
  

---

2. Syntax

newlist = [expression for item in iterable if condition == True]

• expression: The item or manipulated version of the item to include in the new list.
• iterable: The original list, range, tuple, etc., being iterated.
• condition: (Optional) Filters the items to be included in the new list.

---

3. Condition

• Filters the items that evaluate to True.

• Example: Exclude "apple":

  newlist = [x for x in fruits if x != "apple"]
  print(newlist)
  

  Output:

  ['banana', 'cherry', 'kiwi', 'mango']
  

• Condition is Optional: Without a condition:

  newlist = [x for x in fruits]
  print(newlist)
  

  Output:

  ['apple', 'banana', 'cherry', 'kiwi', 'mango']
  

---

4. Iterable

• Any iterable object can be used, e.g., lists, tuples, sets, etc.

• Example: Use range():

  newlist = [x for x in range(10)]
  print(newlist)
  

  Output:

  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
  

• Example with a condition (Numbers less than 5):

  newlist = [x for x in range(10) if x < 5]
  print(newlist)
  

  Output:

  [0, 1, 2, 3, 4]
  

---

5. Expression

• Manipulate each item in the iteration.

• Example: Convert to uppercase:

  newlist = [x.upper() for x in fruits]
  print(newlist)
  

  Output:

  ['APPLE', 'BANANA', 'CHERRY', 'KIWI', 'MANGO']
  

• Example: Set all values to "hello":

  newlist = ['hello' for x in fruits]
  print(newlist)
  

  Output:

  ['hello', 'hello', 'hello', 'hello', 'hello']
  

• Example: Use condition in the expression (Replace "banana" with "orange"):

  newlist = [x if x != "banana" else "orange" for x in fruits]
  print(newlist)
  

  Output:

  ['apple', 'orange', 'cherry', 'kiwi', 'mango']
  

---

Exercise

Question:
Given the code:

fruits = ['apple', 'banana', 'cherry']
newlist = [x for x in fruits if x == 'banana']

What will be the value of newlist?

Options:

1. ['apple', 'cherry']
2. ['banana'] ✅
3. True

Answer: ['banana']

****************

Key Points on Python - Sorting Lists

1. Sort List Alphanumerically (Ascending by Default)

• The sort() method sorts lists alphabetically or numerically in ascending order.

• Example (Alphabetical Sort):

  thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
  thislist.sort()
  print(thislist)
  

  Output:

  ['banana', 'kiwi', 'mango', 'orange', 'pineapple']
  

• Example (Numerical Sort):

  thislist = [100, 50, 65, 82, 23]
  thislist.sort()
  print(thislist)
  

  Output:

  [23, 50, 65, 82, 100]
  

---

2. Sort in Descending Order

• Use the keyword argument reverse=True to sort in descending order.

• Example (Descending Alphabetical Sort):

  thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
  thislist.sort(reverse=True)
  print(thislist)
  

  Output:

  ['pineapple', 'orange', 'mango', 'kiwi', 'banana']
  

• Example (Descending Numerical Sort):

  thislist = [100, 50, 65, 82, 23]
  thislist.sort(reverse=True)
  print(thislist)
  

  Output:

  [100, 82, 65, 50, 23]
  

---

3. Customize the Sort Function

• Use the key=function argument to define a custom sort function.
• Example (Sort by Distance to 50):

  def myfunc(n):
      return abs(n - 50)
  
  thislist = [100, 50, 65, 82, 23]
  thislist.sort(key=myfunc)
  print(thislist)
  

  Output:

  [50, 65, 23, 82, 100]
  

---

4. Case Sensitivity in Sorting

• By default, sort() is case-sensitive, placing uppercase letters before lowercase ones.
• Example (Case-Sensitive Sort):

  thislist = ["banana", "Orange", "Kiwi", "cherry"]
  thislist.sort()
  print(thislist)
  

  Output:

  ['Kiwi', 'Orange', 'banana', 'cherry']
  

---

5. Case-Insensitive Sorting

• Use the key=str.lower argument to perform a case-insensitive sort.
• Example:

  thislist = ["banana", "Orange", "Kiwi", "cherry"]
  thislist.sort(key=str.lower)
  print(thislist)
  

  Output:

  ['banana', 'cherry', 'Kiwi', 'Orange']
  

---

6. Reverse the Order

• The reverse() method reverses the current order of list items, regardless of their values.
• Example:

  thislist = ["banana", "Orange", "Kiwi", "cherry"]
  thislist.reverse()
  print(thislist)
  

  Output:

  ['cherry', 'Kiwi', 'Orange', 'banana']
  

---

Exercise

Question:
What is the correct syntax for sorting a list?

Options:

1. mylist.orderby(0)
2. mylist.order()
3. mylist.sort() ✅

Answer:
mylist.sort()

*****************

Key Points on Python - Copying Lists

1. Problem with Direct Assignment

• Direct assignment (list2 = list1) does not create a new list; instead, list2 becomes a reference to list1.
• Changes made to list1 will automatically reflect in list2.

---

2. Using the copy() Method

• The copy() method creates a shallow copy of the list, independent of the original list.
• Example:

  thislist = ["apple", "banana", "cherry"]
  mylist = thislist.copy()
  print(mylist)
  

  Output:

  ['apple', 'banana', 'cherry']
  

---

3. Using the list() Method

• The list() method can also create a new list from an existing one.
• Example:

  thislist = ["apple", "banana", "cherry"]
  mylist = list(thislist)
  print(mylist)
  

  Output:

  ['apple', 'banana', 'cherry']
  

---

4. Using the Slice (:) Operator

• The slice operator ([:]) creates a copy of the entire list.
• Example:

  thislist = ["apple", "banana", "cherry"]
  mylist = thislist[:]
  print(mylist)
  

  Output:

  ['apple', 'banana', 'cherry']
  

---

Exercise

Question:
What is the correct syntax for making a copy of a list?

Options:

1. list2 = list1
2. list2 = list1.copy() ✅
3. list2.copy(list1)

Answer:
list2 = list1.copy()

Here are all the points related to joining lists in Python:

1. Using the + Operator: The easiest way to join two lists is by using the + operator.

   Example:

   list1 = ["a", "b", "c"]
   list2 = [1, 2, 3]
   list3 = list1 + list2
   print(list3)
   

   Output:

   ['a', 'b', 'c', 1, 2, 3]
   

2. Using the append() Method: Another way to join two lists is by appending each item from list2 to list1 one by one.

   Example:

   list1 = ["a", "b", "c"]
   list2 = [1, 2, 3]
   for x in list2:
       list1.append(x)
   print(list1)
   

   Output:

   ['a', 'b', 'c', 1, 2, 3]
   

3. Using the extend() Method: You can also use the extend() method, which adds elements from one list to another list.

   Example:

   list1 = ["a", "b", "c"]
   list2 = [1, 2, 3]
   list1.extend(list2)
   print(list1)
   

   Output:

   ['a', 'b', 'c', 1, 2, 3]
   

---

Exercise Question:
What is a correct syntax for joining list1 and list2 into list3?

Options:

• list3 = join(list1, list2)
• list3 = list1 + list2
• list3 = [list1, list2]

Correct answer:

• list3 = list1 + list2

Here are all the points related to Python list methods:

List Methods in Python:

1. append()
   Adds an element at the end of the list.
   Example:

   list1.append(4)
   

2. clear()
   Removes all the elements from the list.
   Example:

   list1.clear()
   

3. copy()
   Returns a copy of the list.
   Example:

   list2 = list1.copy()
   

4. count()
   Returns the number of elements with the specified value.
   Example:

   list1.count(2)
   

5. extend()
   Adds the elements of a list (or any iterable) to the end of the current list.
   Example:

   list1.extend([4, 5, 6])
   

6. index()
   Returns the index of the first element with the specified value.
   Example:

   list1.index(3)
   

7. insert()
   Adds an element at the specified position.
   Example:

   list1.insert(1, 'a')
   

8. pop()
   Removes the element at the specified position and returns it.
   Example:

   list1.pop(2)
   

9. remove()
   Removes the item with the specified value.
   Example:

   list1.remove(3)
   

10. reverse()
    Reverses the order of the list.
    Example:

    list1.reverse()
    

11. sort()
    Sorts the list.
    Example:

    list1.sort()