Variables, expressions and statements

=====================================

One of the most powerful features of a programming language is the

ability to manipulate **variables**. A variable is a name that refers to

a value.

Assignment statements

---------------------

An **assignment statement** creates a new variable and gives it a value:

::

>>> message = 'And now for something completely different'

>>> n = 17

>>> pi = 3.141592653589793

This example makes three assignments. The first assigns a string to a

new variable named message; the second gives the integer 17 to n; the

third assigns the (approximate) value of :math:`\pi` to pi.

A common way to represent variables on paper is to write the name with

an arrow pointing to its value. This kind of figure is called a **state

diagram** because it shows what state each of the variables is in (think

of it as the variable’s state of mind). Figure [fig.state2] shows the

result of the previous example.

.. figure:: figs/state2.pdf

:alt: State diagram.

State diagram.

Variable names

--------------

Programmers generally choose names for their variables that are

meaningful—they document what the variable is used for.

Variable names can be as long as you like. They can contain both letters

and numbers, but they can’t begin with a number. It is legal to use

uppercase letters, but it is conventional to use only lower case for

variables names.

The underscore character, ``_``, can appear in a name. It is often used

in names with multiple words, such as ``your_name`` or

``airspeed_of_unladen_swallow``.

If you give a variable an illegal name, you get a syntax error:

::

>>> 76trombones = 'big parade'

SyntaxError: invalid syntax

>>> more@ = 1000000

SyntaxError: invalid syntax

>>> class = 'Advanced Theoretical Zymurgy'

SyntaxError: invalid syntax

76trombones is illegal because it begins with a number. more@ is illegal

because it contains an illegal character, @. But what’s wrong with

class?

It turns out that class is one of Python’s **keywords**. The interpreter

uses keywords to recognize the structure of the program, and they cannot

be used as variable names.

Python 3 has these keywords:

::

False class finally is return

None continue for lambda try

True def from nonlocal while

and del global not with

as elif if or yield

assert else import pass

break except in raise

You don’t have to memorize this list. In most development environments,

keywords are displayed in a different color; if you try to use one as a

variable name, you’ll know.

Expressions and statements

--------------------------

An **expression** is a combination of values, variables, and operators.

A value all by itself is considered an expression, and so is a variable,

so the following are all legal expressions:

::

>>> 42

42

>>> n

17

>>> n + 25

42

When you type an expression at the prompt, the interpreter **evaluates**

it, which means that it finds the value of the expression. In this

example, n has the value 17 and n + 25 has the value 42.

A **statement** is a unit of code that has an effect, like creating a

variable or displaying a value.

::

>>> n = 17

>>> print(n)

The first line is an assignment statement that gives a value to n. The

second line is a print statement that displays the value of n.

When you type a statement, the interpreter **executes** it, which means

that it does whatever the statement says. In general, statements don’t

have values.

Script mode

-----------

So far we have run Python in **interactive mode**, which means that you

interact directly with the interpreter. Interactive mode is a good way

to get started, but if you are working with more than a few lines of

code, it can be clumsy.

The alternative is to save code in a file called a **script** and then

run the interpreter in **script mode** to execute the script. By

convention, Python scripts have names that end with .py.

If you know how to create and run a script on your computer, you are

ready to go. Otherwise I recommend using PythonAnywhere again. I have

posted instructions for running in script mode at

http://tinyurl.com/thinkpython2e.

Because Python provides both modes, you can test bits of code in

interactive mode before you put them in a script. But there are

differences between interactive mode and script mode that can be

confusing.

For example, if you are using Python as a calculator, you might type

::

>>> miles = 26.2

>>> miles * 1.61

42.182

The first line assigns a value to miles, but it has no visible effect.

The second line is an expression, so the interpreter evaluates it and

displays the result. It turns out that a marathon is about 42

kilometers.

But if you type the same code into a script and run it, you get no

output at all. In script mode an expression, all by itself, has no

visible effect. Python actually evaluates the expression, but it doesn’t

display the value unless you tell it to:

::

miles = 26.2

print(miles * 1.61)

This behavior can be confusing at first.

A script usually contains a sequence of statements. If there is more

than one statement, the results appear one at a time as the statements

execute.

For example, the script

::

print(1)

x = 2

print(x)

produces the output

::

1

2

The assignment statement produces no output.

To check your understanding, type the following statements in the Python

interpreter and see what they do:

::

5

x = 5

x + 1

Now put the same statements in a script and run it. What is the output?

Modify the script by transforming each expression into a print statement

and then run it again.

Order of operations

-------------------

When an expression contains more than one operator, the order of

evaluation depends on the **order of operations**. For mathematical

operators, Python follows mathematical convention. The acronym

**PEMDAS** is a useful way to remember the rules:

- **P**\ arentheses have the highest precedence and can be used to

force an expression to evaluate in the order you want. Since

expressions in parentheses are evaluated first, 2 \* (3-1) is 4, and

(1+1)\*\*(5-2) is 8. You can also use parentheses to make an

expression easier to read, as in (minute \* 100) / 60, even if it

doesn’t change the result.

- **E**\ xponentiation has the next highest precedence, so 1 + 2\*\*3

is 9, not 27, and 2 \* 3\*\*2 is 18, not 36.

- **M**\ ultiplication and **D**\ ivision have higher precedence than

**A**\ ddition and **S**\ ubtraction. So 2\*3-1 is 5, not 4, and

6+4/2 is 8, not 5.

- Operators with the same precedence are evaluated from left to right

(except exponentiation). So in the expression degrees / 2 \* pi, the

division happens first and the result is multiplied by pi. To divide

by :math:`2 \pi`, you can use parentheses or write degrees / 2 / pi.

I don’t work very hard to remember the precedence of operators. If I

can’t tell by looking at the expression, I use parentheses to make it

obvious.

String operations

-----------------

In general, you can’t perform mathematical operations on strings, even

if the strings look like numbers, so the following are illegal:

::

'2'-'1' 'eggs'/'easy' 'third'*'a charm'

But there are two exceptions, + and .

The + operator performs **string concatenation**, which means it joins

the strings by linking them end-to-end. For example:

::

>>> first = 'throat'

>>> second = 'warbler'

>>> first + second

throatwarbler

The operator also works on strings; it performs repetition. For example,

``'Spam'*3`` is ``'SpamSpamSpam'``. If one of the values is a string,

the other has to be an integer.

This use of + and makes sense by analogy with addition and

multiplication. Just as 4\*3 is equivalent to 4+4+4, we expect

``'Spam'*3`` to be the same as ``'Spam'+'Spam'+'Spam'``, and it is. On

the other hand, there is a significant way in which string concatenation

and repetition are different from integer addition and multiplication.

Can you think of a property that addition has that string concatenation

does not?

Comments

--------

As programs get bigger and more complicated, they get more difficult to

read. Formal languages are dense, and it is often difficult to look at a

piece of code and figure out what it is doing, or why.

For this reason, it is a good idea to add notes to your programs to

explain in natural language what the program is doing. These notes are

called **comments**, and they start with the ``#`` symbol:

::

# compute the percentage of the hour that has elapsed

percentage = (minute * 100) / 60

In this case, the comment appears on a line by itself. You can also put

comments at the end of a line:

::

percentage = (minute * 100) / 60 # percentage of an hour

Everything from the # to the end of the line is ignored—it has no effect

on the execution of the program.

Comments are most useful when they document non-obvious features of the

code. It is reasonable to assume that the reader can figure out *what*

the code does; it is more useful to explain *why*.

This comment is redundant with the code and useless:

::

v = 5 # assign 5 to v

This comment contains useful information that is not in the code:

::

v = 5 # velocity in meters/second.

Good variable names can reduce the need for comments, but long names can

make complex expressions hard to read, so there is a tradeoff.

Debugging

---------

Three kinds of errors can occur in a program: syntax errors, runtime

errors, and semantic errors. It is useful to distinguish between them in

order to track them down more quickly.

Syntax error:

“Syntax” refers to the structure of a program and the rules about

that structure. For example, parentheses have to come in matching

pairs, so (1 + 2) is legal, but 8) is a **syntax error**.

If there is a syntax error anywhere in your program, Python displays

an error message and quits, and you will not be able to run the

program. During the first few weeks of your programming career, you

might spend a lot of time tracking down syntax errors. As you gain

experience, you will make fewer errors and find them faster.

Runtime error:

The second type of error is a runtime error, so called because the

error does not appear until after the program has started running.

These errors are also called **exceptions** because they usually

indicate that something exceptional (and bad) has happened.

Runtime errors are rare in the simple programs you will see in the

first few chapters, so it might be a while before you encounter one.

Semantic error:

The third type of error is “semantic”, which means related to

meaning. If there is a semantic error in your program, it will run

without generating error messages, but it will not do the right

thing. It will do something else. Specifically, it will do what you

told it to do.

Identifying semantic errors can be tricky because it requires you to

work backward by looking at the output of the program and trying to

figure out what it is doing.

.. _glossary02:

Glossary

--------

.. include:: glossary/02.txt

Exercises

---------

Repeating my advice from the previous chapter, whenever you learn a new

feature, you should try it out in interactive mode and make errors on

purpose to see what goes wrong.

- We’ve seen that n = 42 is legal. What about 42 = n?

- How about x = y = 1?

- In some languages every statement ends with a semi-colon, ;. What

happens if you put a semi-colon at the end of a Python statement?

- What if you put a period at the end of a statement?

- In math notation you can multiply :math:`x` and :math:`y` like this:

:math:`x y`. What happens if you try that in Python?

Practice using the Python interpreter as a calculator:

#. The volume of a sphere with radius :math:`r` is

:math:`\frac{4}{3} \pi r^3`. What is the volume of a sphere with

radius 5?

#. Suppose the cover price of a book is $24.95, but bookstores get a 40%

discount. Shipping costs $3 for the first copy and 75 cents for each

additional copy. What is the total wholesale cost for 60 copies?

#. If I leave my house at 6:52 am and run 1 mile at an easy pace (8:15

per mile), then 3 miles at tempo (7:12 per mile) and 1 mile at easy

pace again, what time do I get home for breakfast?