Missing Keys: if Tests

One other note about dictionaries before we move on. Although we can assign to a new key to expand a dictionary, fetching a nonexistent key is still a mistake:

>>> D
{'a': 1, 'c': 3, 'b': 2}

>>> D['e'] = 99                      # Assigning new keys grows dictionaries
>>> D
{'a': 1, 'c': 3, 'b': 2, 'e': 99}

>>> D['f']                           # Referencing a nonexistent key is an error
...error text omitted...
KeyError: 'f'

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This is what we want—it’s usually a programming error to fetch something that isn’t really there. But in some generic programs, we can’t always know what keys will be present when we write our code. How do we handle such cases and avoid errors? One trick is to test ahead of time. The dictionary in membership expression allows us to query the existence of a key and branch on the result with a Python if statement (as with the for, be sure to press Enter twice to run the if interactively here):

>>> 'f' in D
False

>>> if not 'f' in D:
       print('missing')

missing

I’ll have much more to say about the if statement and statement syntax in general later in this book, but the form we’re using here is straightforward: it consists of the word if, followed by an expression that is interpreted as a true or false result, followed by a block of code to run if the test is true. In its full form, the if statement can also have an else clause for a default case, and one or more elif (else if) clauses for other tests. It’s the main selection tool in Python, and it’s the way we code logic in our scripts.

Still, there are other ways to create dictionaries and avoid accessing nonexistent keys: the get method (a conditional index with a default); the Python 2.X has_key method (which is no longer available in 3.0); the try statement (a tool we’ll first meet in Chapter 10 that catches and recovers from exceptions altogether); and the if/else expression (essentially, an if statement squeezed onto a single line). Here are a few examples:

>>> value = D.get('x', 0)                      # Index but with a default
>>> value
0
>>> value = D['x'] if 'x' in D else 0          # if/else expression form
>>> value
0

We’ll save the details on such alternatives until a later chapter. For now, let’s move on to tuples.

 

[14] Keep in mind that the rec record we just created really could be a database record, when we employ Python’s object persistence system—an easy way to store native Python objects in files or access-by-key databases. We won’t go into details here, but watch for discussion of Python’s pickle and shelve modules later in this book.