A Look Under the Hood: Strings Are Objects

You already know that len() can measure a string. Now we can give an honest name to the thing being measured and see why strings come with useful text operations of their own.

A value is an object with a type

In Python, every value is an object. The string "Hello" is an object, and the integer 42 is another object.

Every object has a type. A type tells Python what kind of value it has and which operations make sense for that value. The familiar type() function lets you inspect it:

Python displays <class 'str'> for the string and <class 'int'> for the integer. Read the useful parts as “string type” and “integer type.” You do not need to build your own types in this course. For now, the useful idea is simply that different kinds of objects can provide different operations.

An object provides methods

A method is a function provided by an object. String objects provide methods that work with text. Here is one:

The call message.upper() has three important parts:

• message refers to the string object;

• the dot asks for an operation provided by that object;

• upper() calls that method.

The call returns the new string "HELLO", which assignment stores in loud_message. A small dot is doing useful work here. No wand required.

Compare the familiar function form with the method form:

len(message)       # Pass the object to a built-in function
message.upper()    # Call a method provided by the object

Both are calls, so both use parentheses. The dot tells you that upper() comes from the string object.

Pause here and picture the relationship: message refers to a string object, its str type provides .upper(), and the method call returns a new string.

The diagram follows that exact call from the original variable to the returned value and the variable that receives it:

The idea to carry forward

When you soon meet name.strip(), name.lower(), and name.replace(), read them with the same model: a name refers to a string object, the dot selects one of its methods, and parentheses call it.

That is enough object vocabulary for now. The next few lessons can concentrate on what you want the text to do.