Updating VariablesΒΆ
One of the most common forms of reassignment is an update where the new value of the variable depends on the old. For example,
x = x + 1
This means get the current value of x, add one, and then update x with the new
value. The new value of x is the old value of x plus 1. Although this assignment statement may
look a bit strange, remember that executing assignment is a twostep process. First, evaluate the
righthand side expression. Second, let the variable name on the lefthand side refer to this new
resulting object. The fact that x
appears on both sides does not matter. The semantics of the assignment
statement makes sure that there is no confusion as to the result.
If you try to update a variable that doesn’t exist, you get an error because
Python evaluates the expression on the right side of the assignment operator
before it assigns the resulting value to the name on the left.
Before you can update a variable, you have to initialize it, usually with a
simple assignment. In the above example, x
was initialized to 6.
Updating a variable by adding something to it is called an increment; subtracting is called a decrement. Sometimes programmers talk about incrementing or decrementing without specifying by how much; when they do they usually mean by 1. Sometimes programmers also talk about bumping a variable, which means the same as incrementing it by 1.
Incrementing and decrementing are such common operations that programming languages often include special syntax for it. In python +=
is used for incrementing, and =
for decrementing. In some other languages, there is even a special syntax ++
and 
for incrementing or decrementing by 1. Python does not have such a special syntax. To increment x by 1 you have to write x += 1
or x = x + 1
.
Check your understanding
 (A) 12
 The value of x changes in the second statement.
 (B) 1
 In the second statement, substitute the current value of x before subtracting 1.
 (C) 11
 Yes, this statement sets the value of x equal to the current value minus 1.
 (D) Nothing. An error occurs because x can never be equal to x  1.
 Remember that variables in Python are different from variables in math in that they (temporarily) hold values, but can be reassigned.
rec554: What is printed when the following statements execute?
x = 12
x = x  1
print x
 (A) 12
 The value of x changes in the second statement.
 (B) 9
 Each statement changes the value of x, so 9 is not the final result.
 (C) 15
 Yes, starting with 12, subtract 3, than add 5, and finally add 1.
 (D) Nothing. An error occurs because x cannot be used that many times in assignment statements.
 Remember that variables in Python are different from variables in math in that they (temporarily) hold values, but can be reassigned.
rec555: What is printed when the following statements execute?
x = 12
x = x  3
x = x + 5
x = x + 1
print x
rec556: Construct the code that will result in the value 134 being printed.mybankbalance = 100
mybankbalance = mybankbalance + 34
print mybankbalance

rec557: Which of the following statements are equivalent?
 (A) x = x + y
 x is updated to be the old value of x plus the value of y.
 (B) y += x
 y is updated to be the old value of y plus the value of x.
 (C) x += x + y
 This updates x to be its old value (because of the +=) plus its old value again (because of the x on the right side) plus the value of y, so it's equivalent to x = x + x + y
 (D) x += y
 x is updated to be the old value of x plus the value of y.
 (E) x++ y
 ++ is not a syntax that means anything in python.