1.6. VARIABLES AND ASSIGNMENT 15
1.6. Variables and Assignment
Each set-off line in this section should be tried in the Shell.
Try
width = 10
Nothing is displayed by the interpreter after this entry, so it is not clear anything happened. Something has
happened. This is an assignment statement, with a variable, width, on the left. A variable is a name for a
value. An assignment statement associates a variable name on the left of the equal sign with the value of an
expression calculated from the right of the equal sign. Enter
width
Once a variable is assigned a value, the variable can be used in place of that value. The response to the
expression width is the same as if its value had been entered.
The interpreter does not print a value after an assignment statement because the value of the expression
on the right is not lost. It can be recovered if you like, by entering the variable name and we did above.
Try each of the following lines:
height = 12
area = width * height
area
The equal sign is an unfortunate choice of symbol for assignment, since Python’s usage is not the mathematical
usage of the equal sign. If the symbol  had appeared on keyboards in the early 1990’s, it would probably
have been used for assignment instead of =, emphasizing the asymmetry of assignment. In mathematics an
equation is an assertion that both sides of the equal sign are already, in fact, equal. A Python assignment
statement forces the variable on the left hand side to become associated with the value of the expression on
the right side. The difference from the mathematical usage can be illustrated. Try:
10 = width
so this is not equivalent in Python to width = 10. The left hand side must be a variable, to which the
assignment is made. Try
width = width + 5
This is, of course, nonsensical as mathematics, but it makes perfectly good sense as an assignment, with the
right-hand side calculated first. Can you figure out the value that is now associated with width? Check by
entering
width
In the assignment statement, the expression on the right is evaluated first. At that point width was associated
with its original value 10, so width + 5 had the value of 10 + 5 which is 15. That value was then assigned
to the variable on the left (width again) to give it a new value. We will modify the value of variables in a
similar way routinely.
Assignment and variables work equally well with strings. Try:
first = ’Sue’
last = ’Wong’
name = first + ’ ’ + last
name
Try entering:
first = fred
Note the different form of the error message. The earlier errors in these tutorials were syntax errors: errors in
translation of the instruction. In this last case the syntax was legal, so the interpreter went on to execute the
instruction. Only then did it find the error described. There are no quotes around fred, so the interpreter
assumed fred was an identifier, but the name fred was not defined at the time the line was executed.
It is easy to forget quotes where you need them and put them around a variable name that should not
have them!
Try in the Shell: