Python Tutorial Part 1 > Variables, expressions and statements
Variables, expressions and statements
Values and types
A value is one of the fundamental things'like a letter or a number'that a
program manipulates. The values we have seen so far are 2 (the result when we
added 1 + 1), 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.
The print statement also works for integers.
>>> print 4
If you are not sure what type a value has, the interpreter can tell you.
>>> type('Hello, World!')
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.
What about values like '17' and '3.2'? They look like numbers, but they are
in quotation marks like strings.
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 a legal expression:
>>> 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 list of three integers, which it prints consecutively. 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.
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.
The assignment statement creates new variables and gives them values:
>>> message = "What's up, Doc?"
>>> n = 17
>>> pi = 3.14159
This example makes three assignments. The first assigns the string "What's up,
Doc?" to a new variable named message. The second gives the integer 17 to n,
and the third gives the floating-point number 3.14159 to pi.
Notice that the first statement uses double quotes to enclose the string. In general,
single and double quotes do the same thing, but if the string contains a single quote
(or an apostrophe, which is the same character), you have to use double quotes
to enclose it.
A common way to represent variables on paper is to write the name with an arrow
pointing to the variable's 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). This diagram shows the result of the assignment statements:
"What's up, Doc?"
The print statement also works with variables.
>>> print message
What's up, Doc?
>>> print n
>>> print pi
In each case the result is the value of the variable. Variables also have types;
again, we can ask the interpreter what they are.
The type of a variable is the type of the value it refers to.
Programmers generally choose names for their variables that are meaningful. They document what the variable is used for.
Variable names can be arbitrarily long. They can contain both letters and numbers,
but they have to begin with a letter. Although it is legal to use uppercase
letters, by convention we don't. If you do, remember that case matters. Bruce and bruce are different variables.
The underscore character ( ) can appear in a name. It is often used in names with multiple words, such as my name or price of tea in china.
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 = 'Computer Science 101'
SyntaxError: invalid syntax
76trombones is illegal because it does not begin with a letter. more$ is illegal
because it contains an illegal character, the dollar sign. But what's wrong with
It turns out that class is one of the Python keywords. Keywords define the
language's rules and structure, and they cannot be used as variable names.
Python has twenty-nine keywords:
and def exec if not return
assert del finally import or try
break elif for in pass while
class else from is print yield
continue except global lambda raise
You might want to keep this list handy. If the interpreter complains about one of
your variable names and you don't know why, see if it is on this list.
A statement is an instruction that the Python interpreter can execute. We have
seen two kinds of statements: print and assignment.
When you type a statement on the command line, Python executes it and displays
the result, if there is one. The result of a print statement is a value. Assignment
statements don't produce a result.
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
x = 2
produces the output
Again, the assignment statement produces no output.
An expression is a combination of values, variables, and operators. If you type
an expression on the command line, the interpreter evaluates it and displays the
>>> 1 + 1
Although expressions contain values, variables, and operators, not every expression
contains all of these elements. A value all by itself is considered an expression,
and so is a variable.
Confusingly, evaluating an expression is not quite the same thing as printing a
>>> message = 'Hello, World!'
>>> print message
When the Python interpreter displays the value of an expression, it uses the same
format you would use to enter a value. In the case of strings, that means that it
includes the quotation marks. But if you use a print statement, Python displays
the contents of the string without the quotation marks.
In a script, an expression all by itself is a legal statement, but it doesn't do
anything. The script
1 + 1
produces no output at all. How would you change the script to display the values
of these four expressions?
Operators and operands
Operators are special symbols that represent computations like addition and
multiplication. The values the operator uses are called operands.
The following are all legal Python expressions whose meaning is more or less clear:
20+32 hour-1 hour*60+minute minute/60 5**2 (5+9)*(15-7)
The symbols +, -, and /, and the use of parenthesis for grouping, mean in Python
what they mean in mathematics. The asterisk (*) is the symbol for multiplication,
and ** is the symbol for exponentiation.
When a variable name appears in the place of an operand, it is replaced with its
value before the operation is performed.
Addition, subtraction, multiplication, and exponentiation all do what you expect,
but you might be surprised by division. The following operation has an unexpected
>>> minute = 59
The value of minute is 59, and in conventional arithmetic 59 divided by 60 is
0.98333, not 0. The reason for the discrepancy is that Python is performing
When both of the operands are integers, the result must also be an integer, and
by convention, integer division always rounds down, even in cases like this where
the next integer is very close.
A possible solution to this problem is to calculate a percentage rather than a
Again the result is rounded down, but at least now the answer is approximately
correct. Another alternative is to use floating-point division, which we get to in
Order of operations
When more than one operator appears in an expression, the order of evaluation
depends on the rules of precedence. Python follows the same precedence rules
for its mathematical operators that mathematics does. The acronym PEMDAS
is a useful way to remember the order of operations:
' Parentheses 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 though it doesn't change the result.
' Exponentiation has the next highest precedence, so 2**1+1 is 3 and not 4,
and 3*1**3 is 3 and not 27.
' Multiplication and Division have the same precedence, which is higher than
Addition and Subtraction, which also have the same precedence. So 2*3-1
yields 5 rather than 4, and 2/3-1 is -1, not 1 (remember that in integer
' Operators with the same precedence are evaluated from left to right. So
in the expression minute*100/60, the multiplication happens first, yielding
5900/60, which in turn yields 98. If the operations had been evaluated from
right to left, the result would have been 59*1, which is 59, which is wrong.
Operations on strings
In general, you cannot perform mathematical operations on strings, even if the
strings look like numbers. The following are illegal (assuming that message has
message-1 'Hello'/123 message*'Hello' '15'+2
Interestingly, the + operator does work with strings, although it does not do
exactly what you might expect. For strings, the + operator represents concate-
nation, which means joining the two operands by linking them end-to-end. For
fruit = 'banana'
bakedGood = ' nut bread'
print fruit + bakedGood
The output of this program is banana nut bread. The space before the word
nut is part of the string, and is necessary to produce the space between the
The * operator also works on strings; it performs repetition. For example, 'Fun'*3
is 'FunFunFun'. One of the operands has to be a string; the other has to be an
On one hand, this interpretation of + and * makes sense by analogy with addition
and multiplication. Just as 4*3 is equivalent to 4+4+4, we expect 'Fun'*3 to
be the same as 'Fun'+'Fun'+'Fun', 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
and multiplication have that string concatenation and repetition do not?
So far, we have looked at the elements of a program'variables, expressions, and
statements'in isolation, without talking about how to combine them.
One of the most useful features of programming languages is their ability to take
small building blocks and compose them. For example, we know how to add
numbers and we know how to print; it turns out we can do both at the same time:
>>> print 17 + 3
In reality, the addition has to happen before the printing, so the actions aren't
actually happening at the same time. The point is that any expression involving
numbers, strings, and variables can be used inside a print statement. You've
already seen an example of this:
print 'Number of minutes since midnight: ', hour*60+minute
You can also put arbitrary expressions on the right-hand side of an assignment
percentage = (minute * 100) / 60
This ability may not seem impressive now, but you will see other examples where
composition makes it possible to express complex computations neatly and concisely.
Warning: There are limits on where you can use certain expressions. For example,
the left-hand side of an assignment statement has to be a variable name, not an
expression. So, the following is illegal: minute+1 = hour.
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.
20 Variables, expressions and statements
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 are marked 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 # caution: integer division
Everything from the # to the end of the line is ignored'it has no effect on the
program. The message is intended for the programmer or for future programmers
who might use this code. In this case, it reminds the reader about the eversurprising
behavior of integer division.
This sort of comment is less necessary if you use the integer division operation,
//. It has the same effect as the division operator1, but it signals that the effect
percentage = (minute * 100) // 60
The integer division operator is like a comment that says, 'I know this is integer
division, and I like it that way!'