2.11. Updating Variables¶
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 two-step process. First, evaluate the
right-hand side expression. Second, let the variable name on the
left-hand 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. The visualizer makes this very clear.
x = x + 1
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 1 is called an increment; subtracting 1 is called a decrement. Sometimes programmers also talk about bumping a variable, which means the same as incrementing it by 1.
Check your understanding
- 12
- The value of x changes in the second statement.
- -1
- In the second statement, substitute the current value of x before subtracting 1.
- 11
- Yes, this statement sets the value of x equal to the current value minus 1.
- 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.
data-11-1: What is printed when the following statements execute?
x: int
x = 12
x = x - 1
print(str(x))
- 12
- The value of x changes in the second statement.
- 9
- Each statement changes the value of x, so 9 is not the final result.
- 15
- Yes, starting with 12, subtract 3, than add 5, and finally add 1.
- 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.
data-11-2: What is printed when the following statements execute?
x: int
x = 12
x = x - 3
x = x + 5
x = x + 1
print(str(x))
data-11-3: Construct the code that will result in the value 134 being printed.mybankbalance: int
mybankbalance = 100
mybankbalance = mybankbalance + 34
print(str(mybankbalance))