Python List Functions & Methods Tutorial and Examples

In Python, you can use a list() function which creates a collection that can be manipulated for your analysis. This collection of data is called a list object.

While all methods are functions in Python, not all functions are methods. There is a key difference between functions and methods in Python. Functions take objects as inputs. Methods in contrast act on objects.

Python offers the following list functions:

• sort(): Sorts the list in ascending order.

• type(list): It returns the class type of an object.

• append(): Adds a single element to a list.

• extend(): Adds multiple elements to a list.

• index(): Returns the first appearance of the specified value.

• max(list): It returns an item from the list with max value.

• min(list): It returns an item from the list with min value.

• len(list): It gives the total length of the list.

• list(seq): Converts a tuple into a list.

• cmp(list1, list2): It compares elements of both lists list1 and list2.
• filter(fun,list): filter the list using the Python function. 

• Python List Functions & Methods

Python sort list method

The sort() method is a built-in Python method that, by default, sorts the list in ascending order. However, you can modify the order from ascending to descending by specifying the sorting criteria.

Sort a list in ascending order

Let's say you want to sort the element in prices in ascending order. You would type prices followed by a . (period) followed by the method name, i.e., sort including the parentheses.

prices = [238.11, 237.81, 238.91]
prices.sort()
print(prices)

# [237.81, 238.11, 238.91]

Python list type() function

For the type() function, it returns the class type of an object.

Check the data type of a list

Here we will see what type of both fam and fam2 are:

fam = ["liz", 1.73, "emma", 1.68, "mom", 1.71, "dad", 1.89]
print(fam)

# ['liz', 1.73, 'emma', 1.68, 'mom', 1.71, 'dad', 1.89]

Let's see what the type of the object is:

fam = ["liz", 1.73, "emma", 1.68, "mom", 1.71, "dad", 1.89]
print(type(fam))

# <class 'list'>

Now, let's look at fam2.

fam2 = [["liz", 1.73],
        ["emma", 1.68],
        ["mom", 1.71],
        ["dad", 1.89]]
print(fam2)

# [['liz', 1.73], ['emma', 1.68], ['mom', 1.71], ['dad', 1.89]]

Let's see what the type of the object is:

fam2 = [["liz", 1.73],
        ["emma", 1.68],
        ["mom", 1.71],
        ["dad", 1.89]]
print(type(fam2))

# <class 'list'>

These calls show that both fam and fam2 are, in fact, lists.

Python list append method

The append() method will add certain content you enter to the end of the elements you select.

Add a single element to a list

In this example, let's extend the string by adding "April" to the list with the method append(). Using append() will increase the length of the list by 1.

months = ['January', 'February', 'March']
months.append('April')
print(months)

# ['January', 'February', 'March', 'April']

Python list extend method

The extend() method increases the length of the list by the number of elements that are provided to the method, so if you want to add multiple elements to the list, you can use this method.

Add multiple elements to a list

x = [1, 2, 3]
x.extend([4, 5])
print(x)

# [1, 2, 3, 4, 5]

Python list index method

The index() method returns the first appearance of the specified value.

Find an element's position

In the below example, let's look at the index of February in the list months.

months = ['January', 'February', 'March']
prices = [238.11, 237.81, 238.91]

print(months.index('February'))

# 1

This method helps identify that February is located at index 1. Now we can access the corresponding price of February using this index.

months = ['January', 'February', 'March']
prices = [238.11, 237.81, 238.91]

print(prices[1])

# 237.81

Python list max function

The max() function will return the highest value of the inputted values.

Find the largest value in a list

In this example, we will look to use the max() function to find the maximum price in the list named price.

# Find the maximum price in the list price
prices = [159.54, 37.13, 71.17]
price_max = max(prices)
print(price_max)

# 159.54

Python list min function

The min() function will return the lowest value of the inputted values.

Find the smallest value in a list

In this example, you will find the month with the smallest consumer price index (CPI).

To identify the month with the smallest consumer price index, you first apply the min() function on prices to identify the min_price. Next, you can use the index method to find the index location of the min_price. Using this indexed location on months, you can identify the month with the smallest consumer price index.

months = ['January', 'February', 'March']
prices = [238.11, 237.81, 238.91]

# Identify min price
min_price = min(prices)

# Identify min price index
min_index = prices.index(min_price)

# Identify the month with min price
min_month = months[min_index]
print(min_month)

# February

Python list len function

The len() function shows the number of elements in a list. In the below example, we will look at stock price data again using integers.

Count the elements in a list

stock_price_1 = [50.23]
stock_price_2 = [75.14, 85.64, 11.28]

print('stock_price_1 length is', len(stock_price_1))
print('stock_price_2 length is', len(stock_price_2))

# stock_price_1 length is 1
# stock_price_2 length is 3

Python list() function

The list() function takes an iterable construct and turns it into a list. Here is the syntax:

list([iterable])

Convert a tuple to a list

In the below example, you will be working with stock price data. Let's print out an empty list, convert a tuple into a list, and finally, print a list as a list.

# empty list
print(list())

# tuple of stock prices
stocks = ('238.11', '237.81', '238.91')
print(list(stocks))

# list of stock prices
stocks_1 = ['238.11', '237.81', '238.91']
print(list(stocks_1))

# []
# ['238.11', '237.81', '238.91']
# ['238.11', '237.81', '238.91']

Python filter() function

For the filter() function, it takes a function and the lists and returns an iterator of filtered elements. You can either create a Python filter function or use a lambda function to get a filtered list. 

Note: you can also use loops, list comprehension, and string pattern matching to get a filter list. 

Filter list elements by a condition

First, we will create a filter function that will return boolean values. It will consist of simple logical functions. In our case, we will filter item prices greater than 350. Then, we will use filter() to apply the function on item_price list and get an iterator of filtered elements. 

To access the filtered elements, we can either extract value using a for loop or convert an iterator into the lists.  

def filter_price(price):
    if (price > 350):
        return True
    else:
        return False
 
item_price = [230, 400, 450, 350, 370]

# applying filter function
filtered_price = filter(filter_price, item_price)

print(list(filtered_price))

# [400, 450, 370]

You can also convert your code into a single line using lambda function and get filtered elements. 

item_price = [230, 400, 450, 350, 370]

# lambda function with filter()
filtered_price = filter(lambda a: a > 350, item_price)
print(list(filtered_price))

# [400, 450, 370]

To learn more about list methods and functions, please see this video from our course Introduction to Python for Finance.

This content is taken from DataCamp's Introduction to Python for Finance course by Adina Howe.