In this lesson, we are going to transfer our knowledge on operations we can do with strings to new sequence-based data types, namely lists and tuples. Both are, like the name list indicates, an enumeration of elements.
Let’s look at an example for each of them:
- To create a list, you can simply enclose the elements in square brackets ([]). You can also create an empty list without any elements using empty brackets, [].
myFirstList = [a, 3, “red”] - A tuple (pronounced like it rhymes either with “supple” or with “quadruple”) does not require brackets, but may have a round brackets and it is going to be shown in round brackets when you print it.
myFirstTuple = a, 3, “red” To create one with a single element, you have to include a final comma (to distinguish it from being a “normal”, non-sequential variable).
Overview of sequence-based data types: lists, tuples, strings
As the sequence-based data types can seem quite similar, it’s useful to compare them.
To break a string into individual letters and convert it into a list or a tuple, you can use the list or the tuple function, respectively.
Particularities of operations with lists and tuples
The 4 general operations mentioned in the lesson on strings also work for lists and tuples: You can access elements with the bracket operator, you can use the slice operator and you can do a traversal. However, there are some peculiarities.
For the relational operators there are two things to keep in mind:
- With lists or tuples, bigger (>) and smaller (<) are considered as “bigger or equal” (>=) and “smaller or equal” (<=), respectively.
- Subsequent elements in a position are not considered, i.e. if one list has a 1 as an element and the other list has 10 in the position, the result would be True, because Python considers the 1 as the first element of this position and ignores the subsequent 0.
useful methods for lists:
list methods:
- lst.append() to add elements to the end of lst
- lst.extend(lst2) to append all of the elements of lst2 to lst
- lst.sort() arranges the elements of lst from low to high or in alphabetical order
Most list methods are void; they modify the list and return None. Thus, they are not valid for tuples, which cannot be modified.
string methods that involve lists:
- string_name.split([delimiter]): converts a string into a list of words, the optional argument called a delimiter specifies which characters to use as word boundaries (default: blank space)
- delimiter.join(list_name): takes a list of strings and concatenates the elements based on the delimiter
more useful functions
- sum(lst) to add up the elements of a list (only possible if of the same type)
- sorted(seq): takes a sequence and returns a new list with the same elements in sorted order
- reversed(seq): takes a sequence and returns an iterator that traverses the list in reverse order
3 ways to delete elements from a list
- lst.pop(index): deletes the element in the position of the index from the list and returns it → if you don’t indicate an index, the default is the last element
- del lst[index]: deletes the element in the position of the index (or a slice of indices) from the list
- lst.remove(element): deletes the element from the list and returns None
Which sequence to choose: lists vs. tuples?
As lists are mutable, they are more common than tuples. However, in some cases, it’s better to use tuples, for instance, because …
- … tuple assignment makes it possible to swap the values of two variables without using a temporary variable or to use the split method to separate a sequence and assign the parts to a tuple
a, b = b, a - … a function can only return one value, but it can be a tuple, so basically, it’s like returning multiple values
Example exercise
Create a program that asks the user for a certain amount of numbers (floating point). Store those numbers in a list. Show to the user the total, average and standard deviation of those numbers.
At the end, convert your list into a tuple.
If you want to learn more, I recommend once again Allen B. Downey’s Think Python, whose chapters on lists and tuples also inspired this post.
#mastery17
Learning with K26 