April 20, 2011, 3:49 a.m.
posted by bruce
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). Figure, for instance, implements a simple interactive loop that allows a user to query multiple record objects in the shelve by key.
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 from a comprehension expression, is the length of the longest field name). When run, this script goes into a loop, inputting keys from the interactive user (technically, from the standard input stream, which is usually a console window) and displaying the fetched records field by field. An empty line ends the session:
Key? => sue name => Sue Jones age => 45 job => music pay => 40000 Key? => nobody No such key "nobody"! Key? =>
Figure goes further and allows interactive updates. For an input key, it inputs values for each field and either updates an existing record or creates a new object and stores it under the key.
Notice the use of eval in this script to convert inputs (as usual, that allows any Python object type, but it means you must quote string inputs explicitly) and the use of setattr call to assign an attribute given its name string. When run, this script allows any number of records to be added and changed; to keep the current value of a record's field, press the Enter key when prompted for a new value:
Key? => tom [name]=Tom Doe new?=> [age]=55 new?=>56 [job]=mgr new?=> [pay]=65000.0 new?=>90000 Key? => nobody [name]=? new?=>'John Doh' [age]=? new?=>55 [job]=None new?=> [pay]=0 new?=>None Key? =>
This script is still fairly simplistic (e.g., errors aren't handled), but using it is much easier than manually opening and modifying the shelve at the Python interactive prompt, especially for nonprogrammers. Run the query script to check your work after an update (we could combine query and update into a single script if this becomes too cumbersome, albeit at some cost in code and user-experience complexity):
Key? => tom name => Tom Doe age => 56 job => mgr pay => 90000 Key? => nobody name => John Doh age => 55 job => None pay => None Key? =>