3.6 if…else and if…elif…else Statements

The if…else statement performs different suites, based on whether a condition is True or False. The pseudocode below displays 'Passed' if the student’s grade is greater than or equal to 60; otherwise, it displays 'Failed':

If student’s grade is greater than or equal to 60
        Display 'Passed'
Else
        Display 'Failed'

In either case, the next pseudocode statement in sequence after the entire If…Else is “performed.” We indent both the If and Else suites, and by the same amount. Let’s create and initialize (that is, give a starting value to) the variable grade, then show and execute the Python if…else statement for the preceding pseudocode:

In [1]: grade = 85

In [2]: if grade >= 60:
   ...:     print('Passed')
   ...: else:
   ...:     print('Failed')
   ...:
Passed

The condition above is True, so the if suite displays 'Passed'. Note that when you press Enter after typing print('Passed'), IPython indents the next line four spaces. You must delete those four spaces so that the else: suite correctly aligns under the i in if.

The following code assigns 57 to the variable grade, then shows the if…else statement again to demonstrate that only the else suite executes when the condition is False:

In [3]: grade = 57

In [4]: if grade >= 60:
   ...:     print('Passed')
   ...: else:
   ...:     print('Failed')
   ...:
Failed

The up and down arrow keys navigate backwards and forwards through the current interactive session’s snippets. Pressing Enter re-executes the snippet that’s displayed. Let’s set grade to 99, press the up arrow key twice to recall the code from snippet [4], then press Enter to re-execute that code as snippet [6]. Every recalled snippet that you execute gets a new ID:

In [5]: grade = 99

In [6]: if grade >= 60:
   ...:     print('Passed')
   ...: else:
   ...:     print('Failed')
   ...:
Passed

if…else Statement Flowchart

The flowchart below shows the preceding if…else statement’s flow of control:

3.6-3 Full Alternative Text

Conditional Expressions

Sometimes the suites in an if…else statement assign different values to a variable, based on a condition, as in:

In [7]: grade = 87

In [8]: if grade >= 60:
   ...:     result = 'Passed'
   ...: else:
   ...:     result = 'Failed'
   ...:

We can then print or evaluate that variable:

In [9]: result
Out[9]: 'Passed'

You can write statements like snippet [8] using a concise conditional expression:

In [10]: result = ('Passed' if grade >= 60 else 'Failed')

In [11]: result
Out[11]: 'Passed'

The parentheses are not required, but they make it clear that the statement assigns the conditional expression’s value to result. First, Python evaluates the condition grade >= 60:

• If it’s True, snippet [10] assigns to result the value of the expression to the left of if, namely 'Passed'. The else part does not execute.

• If it’s False, snippet [10] assigns to result the value of the expression to the right of else, namely 'Failed'.

In interactive mode, you also can evaluate the conditional expression directly, as in:

In [12]: 'Passed' if grade >= 60 else 'Failed'
Out[12]: 'Passed'

Multiple Statements in a Suite

The following code shows two statements in the else suite of an if…else statement:

In [13]: grade = 49

In [14]: if grade >= 60:
    ...:     print('Passed')
    ...: else:
    ...:     print('Failed')
    ...:     print('You must take this course again')
    ...:
Failed
You must take this course again

In this case, grade is less than 60, so both statements in the else’s suite execute. If you do not indent the second print, then it’s not in the else’s suite. So, that statement always executes, creating strange incorrect output:

In [15]: grade = 100

In [16]: if grade >= 60:
    ...: print('Passed')
    ...: else:
    ...: print('Failed')
    ...: print('You must take this course again')
    ...:
Passed
You must take this course again

if…elif…else Statement

You can test for many cases using the if…elif…else statement. The following pseudocode displays “A” for grades greater than or equal to 90, “B” for grades in the range 80–89, “C” for grades 70–79, “D” for grades 60–69 and “F” for all other grades:

If student’s grade is greater than or equal to 90
        Display “A”
Else If student’s grade is greater than or equal to 80
        Display “B”
Else If student’s grade is greater than or equal to 70
        Display “C”
Else If student’s grade is greater than or equal to 60
        Display “D”
Else
        Display “F”

Only the action for the first True condition executes. Let’s show and execute the Python code for the preceding pseudocode. The pseudocode Else If is written with the keyword elif. Snippet [18] displays C, because grade is 77:

In [17]: grade = 77

In [18]: if grade >= 90:
    ...: print('A')
    ...: elif grade >= 80:
    ...: print('B')
    ...: elif grade >= 70:
    ...: print('C')
    ...: elif grade >= 60:
    ...: print('D')
    ...: else:
    ...: print('F')
    ...:
C

The first condition—grade >= 90—is False, so print('A') is skipped. The second condition—grade >= 80—also is False, so print('B') is skipped. The third condition—grade >= 70—is True, so print('C') executes. Then all the remaining code in the if…elif…else statement is skipped. An if…elif…else is faster than separate if statements, because condition testing stops as soon as a condition is True.

if…elif…else Statement Flowchart

The following flowchart shows the general flow through an if…elif…else statement. It shows that, after any suite executes, control immediately exits the statement. The words to the left are not part of the flowchart. We added them to show how the flowchart corresponds to the equivalent Python code.

3.6-4 Full Alternative Text

else Is Optional

The else in the if…elif…else statement is optional. Including it enables you to handle values that do not satisfy any of the conditions. When an if…elif statement without an else tests a value that does not make any of its conditions True, the program does not execute any of the statement’s suites. The next statement in sequence after the if…elif statement executes. If you specify the else, you must place it after the last elif; otherwise, a SyntaxError occurs.

Logic Errors

The incorrectly indented code segment in snippet [16] is an example of a nonfatal logic error. The code executes, but it produces incorrect results. For a fatal logic error in a script, an exception occurs (such as a ZeroDivisionError from an attempt to divide by 0), so Python displays a traceback, then terminates the script. A fatal error in interactive mode terminates only the current snippet. Then IPython waits for your next input.

Self Check

1. (True/False) A fatal logic error causes a script to produce incorrect results, then continue executing.
   Answer: False. A fatal logic error causes a script to terminate.

2. (IPython Session) Show that a SyntaxError occurs if an if…elif statement specifies an else before the last elif.
   Answer:

In [1]: grade = 80

In [2]: if grade >= 90:
   ...:     print('A')
   ...: else:
   ...:     print('Not A or B')
   ...: elif grade >= 80:
  File "<ipython-input-2-033bcba40157>", line 5
    elif grade >= 80:
       ^
SyntaxError: invalid syntax