Programming Python, 3rd Edition

Step 4: Adding Console Interaction

So far, our database program consists of class instances stored in a shelve file, as coded in the preceding section. It’s sufficient as a storage medium, but it requires us to run scripts from the command line or type code interactively in order to view or process its content. Improving on this is straightforward: simply code more general programs that interact with users, either from a console window or from a full-blown graphical interface.

A Console Shelve Interface

Let’s start with something simple. The most basic kind of interface we can code would allow users to type keys and values in a console window in order to process the database (instead of writing Python program code). Example 2-21, for instance, implements a simple interactive loop that allows a user to query multiple record objects in the shelve by key.

Example 2-21. PP3E\Preview\peopleinteract_query.py

# interactive queries
import shelve
fieldnames = ('name', 'age', 'job', 'pay')
maxfield   = max(len(f) for f in fieldnames)
db = shelve.open('class-shelve')

while True:
    key = raw_input('\nKey? => ')       # key or empty line, exc at eof
    if not key: break
    try:
        record = db[key]                # fetch by key, show in console
    except:
        print 'No such key "%s"!' % key
    else:
        for field in fieldnames:
            print field.ljust(maxfield), '=>', getattr(record, field)

This script uses getattr to fetch an object’s attribute when given its name string, and the ljust left-justify method of strings to align outputs (maxfield, derived ...