Initialize List ExamplesCreate new lists with list initialization expressions. Use the list built-in and list comprehensions.
This page was last reviewed on Mar 30, 2023.
Initialize list. With lists we store elements in linear order, one after another. Most Python programs create and use lists. A variety of syntax can be used.
string List
Shows a list
Syntax notes. We can use square brackets to create a new list—it can have 0 or more elements. We separate elements with commas. Sometimes we use multiple method calls to populate a list.
Initialization example. We create 3 separate lists in this example program. Each list ends up being separate in memory, and containing 3 color strings.
Part 1 We create a list with a literal list creation expression. The list has 3 strings in it upon creation.
Part 2 We create an empty list, and then append() 3 strings to it. Sometimes it is best to add items in separate statements.
List append
Part 3 If we are trying to copy an existing list, we can use the list built-in to create a new list.
Shows a list
# Part A: create list in single expression. colors1 = ["blue", "red", "orange"] print(colors1) # Part B: append to empty list. colors2 = [] colors2.append("blue") colors2.append("red") colors2.append("orange") print(colors2) # Part C: use list built-in with existing list. colors3 = list(colors2) print(colors3)
['blue', 'red', 'orange'] ['blue', 'red', 'orange'] ['blue', 'red', 'orange']
List comprehension. We can generate a list from a range() method call using list comprehension. This syntax can also create a list from an existing list, or other iterable.
Info The get_item function is called to populate each element from the current value of the range() call.
Result Range() returns an iterable of 0 through 4 inclusive, and get_item transforms these values into the end result.
# Return each individual list element. def get_item(value): return value * 10 # Use list comprehension to create list. items = [get_item(n) for n in range(5)] print(items)
[0, 10, 20, 30, 40]
Range method. Suppose we want to iterate through the numbers 0 through 10. We do not need to create a list to have a range—we can just call range() and loop over that.
A summary. Lists are powerful and used in most all Python programs. They can be created with expressions, through multiple append() calls, or even with comprehensions.
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Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Mar 30, 2023 (edit).
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