if Statements

The Python if statement is typical of if statements in most procedural languages. It takes the form of an if test, followed by one or more optional elif (“else if”) tests and a final optional else block. The tests and the else part each have an associated block of nested statements, indented under a header line. When the if statement runs, Python executes the block of code associated with the first test that evaluates to true, or the else block if all tests prove false. The general form of an if statement looks like this:

if test1:                 # if test
    statements1           # Associated block
elif test2:               # Optional elifs
    statements2
else:                     # Optional else
    statements3

To demonstrate, let’s look at a few simple examples of the if statement at work. All parts are optional, except the initial if test and its associated statements. Thus, in the simplest case, the other parts are omitted:

>>> if 1:
...     print('true')
...
true

Notice how the prompt changes to ... for continuation lines when you’re typing interactively in the basic interface used here; in IDLE, you’ll simply drop down to an indented line instead (hit Backspace to back up). A blank line (which you can get by pressing Enter twice) terminates and runs the entire statement. Remember that 1 is Boolean true (as we’ll see later, the word True is its equivalent), so this statement’s test always succeeds. To handle a false result, code the else:

>>> if not 1:
...     print('true')
... else:
...     print('false')
...
false

Now here’s an example of a more complex if statement, with all its optional parts present:

>>> x = 'killer rabbit'
>>> if x == 'roger':
...     print("shave and a haircut")
... elif x == 'bugs':
...     print("what's up doc?")
... else:
...     print('Run away! Run away!')
...
Run away! Run away!

This multiline statement extends from the if line through the block nested under the else. When it’s run, Python executes the statements nested under the first test that is true, or the else part if all tests are false (in this example, they are). In practice, both the elif and else parts may be omitted, and there may be more than one statement nested in each section. Note that the words if, elif, and else are associated by the fact that they line up vertically, with the same indentation.

If you’ve used languages like C or Pascal, you might be interested to know that there is no switch or case statement in Python that selects an action based on a variable’s value. Instead, you usually code multiway branching as a series of if/elif tests, as in the prior example, and occasionally by indexing dictionaries or searching lists. Because dictionaries and lists can be built at runtime dynamically, they are sometimes more flexible than hardcoded if logic in your script:

>>> choice = 'ham'
>>> print({'spam':  1.25,         # A dictionary-based 'switch'
...        'ham':   1.99,         # Use has_key or get for default
...        'eggs':  0.99,
...        'bacon': 1.10}[choice])
1.99

Although it may take a few moments for this to sink in the first time you see it, this dictionary is a multiway branch—indexing on the key choice branches to one of a set of values, much like a switch in C. An almost equivalent but more verbose Python if statement might look like the following:

>>> if choice == 'spam':          # The equivalent if statement
...     print(1.25)
... elif choice == 'ham':
...     print(1.99)
... elif choice == 'eggs':
...     print(0.99)
... elif choice == 'bacon':
...     print(1.10)
... else:
...     print('Bad choice')
...
1.99

Though it’s perhaps more readable, the potential downside of an if like this is that, short of constructing it as a string and running it with tools like the prior chapter’s eval or exec, you cannot construct it at runtime as easily as a dictionary. In more dynamic programs, data structures offer added flexibility.

>>> branch = {'spam': 1.25,
...           'ham':  1.99,
...           'eggs': 0.99}

>>> print(branch.get('spam', 'Bad choice'))
1.25
>>> print(branch.get('bacon', 'Bad choice'))
Bad choice

>>> choice = 'bacon'
>>> if choice in branch:
...     print(branch[choice])
... else:
...     print('Bad choice')
...
Bad choice

>>> try:
...     print(branch[choice])
... except KeyError:
...     print('Bad choice')
...
Bad choice

def function(): ...
def default(): ...

branch = {'spam': lambda: ...,             # A table of callable function objects
          'ham':  function,
          'eggs': lambda: ...}

branch.get(choice, default)()