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python

Python List Functions

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.

sort() 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.

Example

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]

type() function

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

Example

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

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

type(fam)
list

Now, let's look at fam2.

fam2 = [["liz", 1.73],
        ["emma", 1.68],
        ["mom", 1.71],
        ["dad", 1.89]]
fam2
[['liz', 1.73], ['emma', 1.68], ['mom', 1.71], ['dad', 1.89]]

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

type(fam2)
list

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

append() method

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

Example

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)

When you run the above code, it produces the following result:

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

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.

Example

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

When you run the above code, it produces the following result:

[1, 2, 3, 4, 5]

index() method

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

Example

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]
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.

print(prices[1])
237.81

max() function

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

Example

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)

When you run the above code, it produces the following result:

159.54

min() function

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

Example

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

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.

Example

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))

When you run the above code, it produces the following result:

stock_price_1 length is 1
stock_price_2 length is 3

list() function

The list() function takes an iterable construct and turns it into a list.

Syntax

list([iterable])

Example

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))

When you run the above code, it produces the following result:

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

cmp() function

For the cmp() function, it takes two values and compares them against one another. It will then return a negative, zero, or positive value based on what was inputted.

Example

In the example below, we have two stock prices, and we will compare the integer values to see which one is larger:

stock_price_1 = [50.23]
stock_price_2 = [75.14]

print(cmp(stock_price_1, stock_price_2))
print(cmp(stock_price_1, stock_price_1))
print(cmp(stock_price_2, stock_price_1))

When you run the above code, it produces the following result:

-1
0
1

The results show that the stock_price_2 list is larger than the stock_price_1 list. You can use the cmp() function on any type of list, such as strings. Note that by default, if one list is longer than the other, it will be considered to be larger.

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.