COMP 1000: Think Like a Computer: The PreTeXt Interactive Edition

A statement is an instruction that the Python interpreter can execute. You have mostly only seen the assignment statement so far. Some other kinds of statements that you’ll see in future chapters are while statements, for statements, if statements, and import statements. (There are other kinds, too!)

An expression is a combination of literals, variable names, operators, and calls to functions. Expressions need to be evaluated. The result of evaluating an expression is a value or object.

If you ask Python to print an expression, the interpreter evaluates the expression and displays the result.

In this example len is a built-in Python function that returns the number of characters in a string.

The evaluation of an expression produces a value, which is why expressions can appear on the right hand side of assignment statements. A literal all by itself is a simple expression, and so is a variable.

In a program, anywhere that a literal value (a string or a number) is acceptable, a more complicated expression is also acceptable. Here are all the kinds of expressions we’ve seen so far:

Notice that operator expressions (like + and *) have sub-expressions before and after the operator. Each of these can themselves be simple or complex expressions. In that way, you can build up to having pretty complicated expressions.

Similarly, when calling a function, instead of putting a literal inside the parentheses, a complex expression can be placed inside the parentheses. (Again, we provide some hidden code that defines the functions square and sub).

It is important to start learning to read code that contains complex expressions. The Python interpreter examines any line of code and parses it into components. For example, if it sees an = symbol, it will try to treat the whole line as an assignment statement. It will expect to see a valid variable name to the left of the =, and will parse everything to the right of the = as an expression. It will try to figure out whether the right side is a literal, a variable name, an operator expression, or a function call expression. If it’s an operator expression, it will further try to parse the sub-expressions before and after the operator. And so on. You should learn to parse lines of code in the same way.

In order to evaluate an operator expression, the Python interpreter first completely evaluates the expression before the operator, then the one after, then combines the two resulting values using the operator. In order to evaluate a function call expression, the interpreter evaluates the expression before the parentheses (i.e., it looks up the name of the function). Then it tries to evaluate each of the expressions inside the parentheses. There may be more than one, separated by commas. The values of those expressions are passed as inputs to the function when the function is called.

If a function call expression is a sub-expression of some more complicated expression, as square(x) is in sub(square(y), square(x)), then the return value from square(x) is passed as an input to the sub function. This is one of the tricky things that you will have to get used to working out when you read (or write) code. In this example, the square function is called (twice) before the sub function is called, even though the sub function comes first when reading the code from left to right. In the following example we will use the notation of -add- to indicate that Python has looked up the name add and determined that it is a function object.

To start giving you some practice in reading and understanding complicated expressions, try doing the Parsons problem below. Be careful not to indent any of the lines of code; that’s something that will come later in the course.

look up the variable print to get the function object
---
look up the variable name to get "Amy"
---
look up the variable num_apples to get 7
---
look up the variable num_pears to get 2
---
look up the variable num_apples again to get 7
---
look up the variable num_pears again to get 2
---
add 7 and 2 to get 9
---
run the print function passing it the following values to print out: "Amy", "has", 7, "apples and", 2, "pears, which means they have", 9, "pieces of fruit"

Note that we look up (but don’t execute) functions as soon as we come across them. We can’t execute functions until we have evaluated all the expressions that form inputs for that function. In the example above, we can’t actually call the print function until we have determined the values of num_apples, num_pears and (num_apples + num_pears). Once we know all of these things, we have the complete information to send to the print function, so that’s when we execute it.

look up the variable square to get the function object
---
look up the variable x to get 2
---
look up the variable sub to get the function object
---
look up the variable square, again, to get the function object
---
look up the variable y to get 3
---
run the square function on input 3, returning the value 9
---
look up the variable x, again, to get 2
---
multiply 2 * 2 to get 4
---
run the sub function, passing inputs 9 and 4, returning the value 5
---
add 2 and 5 to get 7
---
run the square function, again, on input 7, returning the value 49

The following turtle example uses assignment statements, where the result of calling turtle functions is stored in variables and then used later.

On line 4 in this turtle example, the Python interpreter sees the = sign and knows this is an assignment statement. It needs to evaluate the expression on the right and assign the result to the variable called right_side. On the right is a turtle function (window_width()), a division operator and the literal value 2. So, the Python interpreter first notices the division operator and knows that a division operator is necessary, and then looks at the two operands to see if they are expressions that need to be evaluated. The literal 2 does not need further evaluation, but the turtle function window_width() needs evaluation. So, it calls that function, which is in the turtle module. That function returns the width of the window or canvas. Whatever that value is now gets divided by 2. The result of that calculation gets stored in right_side. If you are wondering why we divide the width by 2, it is because in Python’s turtle graphics, the origin (0,0) is located at the center of the window. So, the x coordinate of the right edge of the canvas is width/2 and the x coordinate of the left edge is -(width/2). On line 8, the turtle is moved to the top-right corner, causing a line to be drawn from the center of the window to that corner. This shows how we can use assignments with functions and operators to define where the turtle draws. We’ll learn a bit more about the coordinates of the turtle window in a later chapter.