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Python - List Methods

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• Python - List Methods
  • Constuctor
    • list(iterable)
  • Manipulate Element
    • append(elmnt)
    • insert(pos, elmnt)
    • pop(pos)
    • remove(elmnt)
    • clear()
  • copy(): Copy List
  • Find element
    • count(value): Count element
    • index(elmnt)
  • Concatenating
    • + Operator
    • extend(iterable)
  • Sort
    • sort(reverse, key)
    • reverse()

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Constuctor

list(iterable)

• creates a list object.

• Parameter

  • iterable: Optional. A sequence, collection or an iterator object

print("\n--------- list() --------\n")

# must be iterable
# x_list = list(1)  # TypeError: 'int' object is not iterable
# x_list = list(1, 2)  # TypeError: set expected at most 1 argument, got 2

# empty set
x_list = list()
print(x_list)    # []

# from tuple
x_list = list(('apple', 'banana', 'cherry'))
print(x_list)    # ['apple', 'banana', 'cherry']

# from list
x_list = list(['apple', 'banana', 'cherry'])
print(x_list)    # ['apple', 'banana', 'cherry']

# from set
x_list = list({'apple', 'banana', 'cherry'})
print(x_list)    # ['apple', 'banana', 'cherry'], order is random

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Manipulate Element

append(elmnt)

• appends an element to the end of the list.

• Parameter

  • elmnt: Required. An element of any type (string, number, object etc.)

print("\n-------- append(elmnt) --------\n")

xlist = ["apple", "banana", "cherry"]
xlist.append("watermelon")
print(xlist)    # ['apple', 'banana', 'cherry', 'watermelon']

xlist = ["apple", "banana", "cherry"]
xlist.append(["watermelon", "ff"])   # can append a list
print(xlist)    # ['apple', 'banana', 'cherry', ['watermelon', 'ff']]

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insert(pos, elmnt)

• inserts the specified value at the specified position.

• Parameter

  • pos: Required. A number specifying in which position to insert the value
  • elmnt: Required. An element of any type (string, number, object etc.)

print("\n-------- list.insert(pos, elmnt) --------\n")

x_list = ['apple', 'banana', 'cherry']

x_list.insert(1, "orange")
print(x_list)   # ['apple', 'orange', 'banana', 'cherry']

x_list.insert(100, "mango")
print(x_list)   # ['apple', 'orange', 'banana', 'cherry', 'mango']

# x_list.insert(1, "orange", "mango") # TypeError: insert expected 2 arguments, got 3

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pop(pos)

• removes the element at the specified position.

• Parameter

  • pos: Optional. A number specifying the position of the element you want to remove, default value is -1, which returns the last item

• Return

  • the removed value.

print("\n-------- list.pop(pos) --------\n")

x_list = ['apple', 'banana', 'cherry', 'orange', 'mango']

print(x_list.pop())  # mango
print(x_list)   # ['apple', 'banana', 'cherry', 'orange']

print(x_list.pop(0))  # apple
print(x_list)   # ['banana', 'cherry', 'orange']

# print(x_list.pop(100))  # IndexError: pop index out of range

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remove(elmnt)

• removes the first occurrence of the element with the specified value.

• Parameter

  • elmnt: Required. Any type (string, number, list etc.) The element you want to remove

print("\n-------- list.remove(elmnt) --------\n")

x_list = ['apple', 'banana', 'cherry', 'orange']

print(x_list.remove("apple"))   # None
print(x_list)  # ['banana', 'cherry', 'orange', 'mango']

# print(x_list.remove("mango"))   # ValueError: list.remove(x): x not in list
# print(x_list.remove())   # TypeError: list.remove() takes exactly one argument (0 given)

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clear()

• removes all the elements from a list.

print("\n-------- list.clear() --------\n")

fruits = ["apple", "banana", "cherry"]
fruits.clear()
print(fruits)  # []

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copy(): Copy List

• returns a copy of the specified list.

print("\n-------- list.copy() --------\n")

fruits = ["apple", "banana", "cherry"]
x = fruits.copy()
print(x)  # ['apple', 'banana', 'cherry']

# deep copy
x[0] = 'mango'
print(fruits)   # ['apple', 'banana', 'cherry']
print(x)  # ['mango', 'banana', 'cherry']

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Find element

count(value): Count element

• returns the number of elements with the specified value.

• Parameter

  • value: Required. Any type (string, number, list, tuple, etc.). The value to search for.

print("\n-------- list.count(value) --------\n")

x_list = ["apple", "banana", "cherry", "banana"]

print(x_list.count("mango"))  # 0
print(x_list.count("cherry"))  # 1
print(x_list.count("banana"))  # 2

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index(elmnt)

• returns the position at the first occurrence of the specified value.

• Parameter

  • elmnt: Required. Any type (string, number, list, etc.). The element to search for

print("\n-------- list.index(elmnt) --------\n")

x_list = ['apple', 'banana', 'cherry', 'cherry']

print(x_list.index("banana"))   # 1
print(x_list.index("cherry"))   # 2
# print(x_list.index("mango"))   # ValueError: 'mango' is not in list

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Concatenating

+ Operator

x_list = ['Ford', 'Mitsubishi']
y_list = ['BMW', 'VW']
x_list = x_list + y_list

print(x_list)  # ['Ford', 'Mitsubishi', 'BMW', 'VW']

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extend(iterable)

• adds the specified list elements (or any iterable) to the end of the current list.

• Parameter

  • iterable: Required. Any iterable (list, set, tuple, etc.)

print("\n-------- list.extend(iterable) --------\n")

x_list = ['apple', 'banana', 'cherry']
y_list = (1, 4, 5, 9)

print(x_list.extend(y_list))  # None
print(x_list)  # ['apple', 'banana', 'cherry', 1, 4, 5, 9]

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Sort

sort(reverse, key)

• sorts the list ascending by default.

• Parameter

  • reverse: Optional. reverse=True will sort the list descending. Default is reverse=False.
  • key: Optional. A function to specify the sorting criteria(s)

print("\n-------- list.sort(reverse=True|False, key=myFunc) --------\n")

x_list = ['Ford', 'Mitsubishi', 'BMW', 'VW']

x_list.sort()
print(x_list)   # ['BMW', 'Ford', 'Mitsubishi', 'VW']

x_list.sort(reverse=True)
print(x_list)   # ['VW', 'Mitsubishi', 'Ford', 'BMW']

• Using Function

x_list = ['Ford', 'Mitsubishi', 'BMW', 'VW']


def sortby_len(e):
    return len(e)


x_list.sort(key=sortby_len)
print(x_list)   # ['VW', 'BMW', 'Ford', 'Mitsubishi']


def sortby_name(e):
    return e['year']


y_list = [
    {'car': 'Ford', 'year': 2005},
    {'car': 'Mitsubishi', 'year': 2000},
    {'car': 'BMW', 'year': 2019},
    {'car': 'VW', 'year': 2011}
]

y_list.sort(key=sortby_name)
print(y_list)
# [{'car': 'Mitsubishi', 'year': 2000}, {'car': 'Ford', 'year': 2005}, {'car': 'VW', 'year': 2011}, {'car': 'BMW', 'year': 2019}]

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reverse()

• reverses the sorting order of the elements.

print("\n-------- list.reverse() --------\n")

x_list = ['apple', 'banana', 'cherry', 'orange']

x_list.reverse()
print(x_list)   # ['orange', 'cherry', 'banana', 'apple']

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