Variables and Basic Expressions

First of all, let’s exercise some basic math. In the following interaction, we first assign two variables (a and b) to integers so we can use them later in a larger expression. Variables are simply names—created by you or Python—that are used to keep track of information in your program. We’ll say more about this in the next chapter, but in Python:

 

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  • Variables are created when they are first assigned values.
  • Variables are replaced with their values when used in expressions.
  • Variables must be assigned before they can be used in expressions.
  • Variables refer to objects and are never declared ahead of time.

In other words, these assignments cause the variables a and b to spring into existence automatically:

% python
>>> a = 3                  # Name created
>>> b = 4

I’ve also used a comment here. Recall that in Python code, text after a # mark and continuing to the end of the line is considered to be a comment and is ignored. Comments are a way to write human-readable documentation for your code. Because code you type interactively is temporary, you won’t normally write comments in this context, but I’ve added them to some of this book’s examples to help explain the code.[15] In the next part of the book, we’ll meet a related feature—documentation strings—that attaches the text of your comments to objects.

Now, let’s use our new integer objects in some expressions. At this point, the values of a and b are still 3 and 4, respectively. Variables like these are replaced with their values whenever they’re used inside an expression, and the expression results are echoed back immediately when working interactively:

>>> a + 1, a – 1           # Addition (3 + 1), subtraction (3 - 1)
(4, 2)
>>> b * 3, b / 2           # Multiplication (4 * 3), division (4 / 2)
(12, 2.0)
>>> a % 2, b ** 2          # Modulus (remainder), power (4 ** 2)
(1, 16)
>>> 2 + 4.0, 2.0 ** b      # Mixed-type conversions
(6.0, 16.0)

Technically, the results being echoed back here are tuples of two values because the lines typed at the prompt contain two expressions separated by commas; that’s why the results are displayed in parentheses (more on tuples later). Note that the expressions work because the variables a and b within them have been assigned values. If you use a different variable that has never been assigned, Python reports an error rather than filling in some default value:

>>> c * 2
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'c' is not defined

You don’t need to predeclare variables in Python, but they must have been assigned at least once before you can use them. In practice, this means you have to initialize counters to zero before you can add to them, initialize lists to an empty list before you can append to them, and so on.

Here are two slightly larger expressions to illustrate operator grouping and more about conversions:

>>> b / 2 + a               # Same as ((4 / 2) + 3)
5.0
>>> print(b / (2.0 + a))    # Same as (4 / (2.0 + 3))
0.8

In the first expression, there are no parentheses, so Python automatically groups the components according to its precedence rules—because / is lower in Table 5-2 than +, it binds more tightly and so is evaluated first. The result is as if the expression had been organized with parentheses as shown in the comment to the right of the code.

Also, notice that all the numbers are integers in the first expression. Because of that, Python 2.6 performs integer division and addition and will give a result of 5, whereas Python 3.0 performs true division with remainders and gives the result shown. If you want integer division in 3.0, code this as b // 2 + a (more on division in a moment).

In the second expression, parentheses are added around the + part to force Python to evaluate it first (i.e., before the /). We also made one of the operands floating-point by adding a decimal point: 2.0. Because of the mixed types, Python converts the integer referenced by a to a floating-point value (3.0) before performing the +. If all the numbers in this expression were integers, integer division (4 / 5) would yield the truncated integer 0 in Python 2.6 but the floating-point 0.8 in Python 3.0 (again, stay tuned for division details).