Lists respond to the + and * operators much like strings; they mean concatenation and repetition here too, except that the result is a new list, not a string.

In fact, lists respond to all of the general sequence operations we used on strings in the prior chapter.

• concatenation
• repetition
• membership testing
• slicing
• indexing

CONCATENATION

Concatenation, in the context of programming, is the operation of joining two strings together. The term"concatenation" literally means to merge two things together.

In Python, there are a few ways to concatenate – or combine - strings. The new string that is created is referred to as a string object. Obviously, this is because everything in Python is an object – which is why Python is an objected-oriented language.

In order to merge two strings into a single object, you may use the “+” operator. When writing code, that would look like this:

str1 = “Hello”

str2 = “World”

str1 + str2

The final line in this code is the concatenation, and when the interpreter executes it a new string will be created.

One thing to note is that Python cannot concatenate a string and integer. These are considered two separate types of objects. So, if you want to merge the two, you will need to convert the integer to a string.

REPETITION

We are accustomed to using the * symbol to represent multiplication, but when the operand on the left side of the * is a list, it becomes the repetition operator. The repetition operator makes multiple copies of a list and joins them all together. Lists can be created using the repetition operator, *. For example,

numbers = [0] * 5

print numbers

This will give the output:

[0, 0, 0, 0, 0]

[0] is a list with one element, 0. The repetition operator makes 5 copies of this list and joins them all together into a single list. Another example using multiple elements in the list.

numbers = [0, 1, 2] * 3

print numbers

This will give the output:

[0, 1, 2, 0, 1, 2, 0, 1, 2]

Note that Python creates shallow copies of the lists in this. So changing objects at one place will change them at all places they are repeated. If you don't want this behaviour, don't use repetition operator to create lists.

MEMBERSHIP TESTING

The membership operators in Python are used to test whether a value is found within a sequence. For example, you can use the membership operators to test for the presence of a substring in a string. Two membership operators exist in Python:

• in – evaluates to True if the value in the left operand appears in the sequence found in the right operand. For example, ‘Hello’ in ‘Hello world’ returns True because the substring Hello is present in the string Hello world!.
• not in – evaluates to True if the value in the left operand doesn’t appear in the sequence found in the right operand. For example, ‘house’ in ‘Hello world’ returns True because the substring house is not present in the string Hello world!.

Here are a couple of examples:

>>> 'Hello' in 'Hello world!'

True

>>> 

>>> 'Hello' in 'Hello world!'

True

>>> 'house' in 'Hello world!'

False

>>> 'hello' in 'Hello world!'

False // note that Python is case sensitive

>>> 'orld' in 'Hello world!'

True

>>> 'Hello' not in 'Hello world!'

False

>>> 'car' not in 'Hello world!'

True

>>> 'house' not in 'Hello world!'

True