2.1: Values and Types
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A value is one of the basic things a program works with, like a letter or a number. The values we have seen so far are
2, and "Hello, World!"
These values belong to different types:
2 is an integer, and "Hello, World!" is a string, so called because it contains a "string" of letters. You (and the interpreter) can identify strings because they are enclosed in quotation marks.
python command to start the interpreter.
python >>> print(4) 4
If you are not sure what type a value has, the interpreter can tell you.
>>> type('Hello, World!') <class 'str'> >>> type(17) <class 'int'>
Not surprisingly, strings belong to the type
str and integers belong to the type
int. Less obviously, numbers with a decimal point belong to a type called
float, because these numbers are represented in a format called floating point.
>>> type(3.2) <class 'float'>
What about values like "17" and "3.2"? They look like numbers, but they are in quotation marks like strings.
>>> type('17') <class 'str'> >>> type('3.2') <class 'str'>
When you type a large integer, you might be tempted to use commas between groups of three digits, as in
1,000,000. This is not a legal integer in Python, but it is legal:
>>> print(1,000,000) 1 0 0
Well, that's not what we expected at all! Python interprets
1,000,000 as a comma-separated sequence of integers, which it prints with spaces between.
This is the first example we have seen of a semantic error: the code runs without producing an error message, but it doesn't do the "right" thing.