Python Initialize List ExamplesCreate new lists with list initialization expressions. Use the list built-in and list comprehensions.
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.
List, strings
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 A We create a list with a literal list creation expression. The list has 3 strings in it upon creation.
Part B 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 C If we are trying to copy an existing list, we can use the list built-in to create a new list.
Python program that initializes lists
# 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.
Python program that initializes list with list comprehension
# 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.
range
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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Python is a very flexible language where a single task can be performed in a number of ways, for example initializing lists can be performed in many ways. However, there are subtle differences in these seemingly similar methods. Python which is popular for its simplicity and readability is equally infamous for being slow compared to C++ or Java. The ‘for’ loop is especially known to be slow whereas methods like map[] and filter[] are known to be faster because they are written in C. Knowing the better and faster way to initialize lists might give you a slight edge in competitive programming.
The following are some of the ways to initialize lists[we create lists of size 1000 and initialize with zeros] in Python.
Using a for loop and append[]
We create an empty an list and run a for loop for n times using the append[] method to add elements to the list.
Using a while loop with a counter variable
This is similar to the above method. However we use while loop instead.