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Deconstructing a database application

From: Python 3 Essential Training

Video: Deconstructing a database application

The four basic functions of a database are create, retrieve, update and delete, often pronounced as CRUD. This is an example of an application that does those basic four functions. Of course there is a lot of functions that people perform with database, there is a lot of things that they are used for, and as you are writing code over the years you will be writing databases for a lot different purposes. But they all basically break down to those four functions. So those are the things that if you pay attention to those things, then building the more complex applications becomes a little bit easier.

Deconstructing a database application

The four basic functions of a database are create, retrieve, update and delete, often pronounced as CRUD. This is an example of an application that does those basic four functions. Of course there is a lot of functions that people perform with database, there is a lot of things that they are used for, and as you are writing code over the years you will be writing databases for a lot different purposes. But they all basically break down to those four functions. So those are the things that if you pay attention to those things, then building the more complex applications becomes a little bit easier.

So this is an application that just does that. It manages as a very simple database and it simply allows you to add records, edit records, and delete records, and view records. So those are the basic four functions create, retrieve, update and delete. What this application does as it manages a Testimonials database that I use in my web site and you will see if you look at my web site, you will see little boxes like this one over here that have testimonials. For our purposes I have replaced the testimonials with some witty little quotes that I have collected over the years and here is the database itself.

This has got 14 records in the database and if you wanted to add something and print a byline and click Add, you will see that it adds it to the database right here, and then I can edit it, if I want to. Update it and there it is updated in the database. And if I want to delete it, I can delete it.

It's all very simple to operate and very clear, and this comes from it being a very simple application and also from it being designed clearly. So, here in the Example files, under Projects and under testimonials, you will find db.py and that is that script right there. See down here,it says db.py version 1.13. So that is this script right here. So we are going to take a little tour through this script and see how it works.

At the top here you see that I imported a few libraries that start with bw. These libraries are available in the lib folder under Projects and these are libraries that I have built over the years, and there is this global namespace container. I tend to do this in all of my web scripts because it helps to keep state in otherwise stateless environment, and it helps me to keep my global name space cleaned up.

So I use a dictionary for that in Python, and I have here a call to init, which initializes these variables, again helping to keep state and to set up my various objects and to read my configuration file and things like that, and then I look for a variable a in the CGI variables. If I find it, I run something called dispatch. Otherwise I load the first main page. So what dispatch does is it simply looks for what is the action, a stands for action, what is the action that's been taken and it will dispatch the proper function for that.

If these get any bigger I actually have a library that runs a jump table that will do something like this. So I tend to do a lot of work on the web. I tend to use CGI a lot. CGI being stateless the way that it is, the web and HTTP being stateless, that helps to have things like this to manage a little bit of a state machine. The main part of the application lists these records. So this is the retrieve part of the four functions of the database.

So the first thing this does is it grabs a count of the records in the database using the countrecs method in the database library and that very quickly and efficiently returns a count all of the records and that allows us to do some math here and figure out how many we are going to display on the page and how many pages there is going to be and then we set up this little menu here which is this part here that tells us, we can jump to a specific page or we can go back and forward. So that's very useful.

Then we have our little bit of SQL and this actually does the retrieval. SELECT From ORDER BY byline and LIMIT and OFFSET. And in SQLite, limit and offset allow us to only get the first so many records and if it's not on the first page, then to skip forward to an offset first and then get that many records, and it will do this on any query. Most modern database engines have some way of doing this.

It's not a part of the SQL standard so they all tend to be a little bit different. This one, SQLite actually, uses a very similar syntax, I don't think it's exact, but a very similar syntax to how it's done in MySQL and those are the two really popular ones for web applications these days. So that's how we do the paging and we make that list at the bottom of the page. The pagebar itself with the various links is there and displaying pages is here and then we get down to the actual actions.

Adding a record to the database, it simply builds this dictionary object and passes it off to the insert CRUD function in my database library, which I explain in another movie in this chapter. Likewise, the delete function calls the delete method in the database library and the update function calls the update method in the database library. Using these CRUD methods in my normalized database library allows me to keep this code very small.

Each of these function uses exactly the same data structure. It uses this dictionary. It uses an id field that's called id, and this allows the methods in the database library to do there job. So, let's take a quick look at those methods and this is the database library, bwDB. So the insert method here, it takes the dictionary object and it actually goes through the dictionary object.

It grabs all of its keys and then uses this generator expression to create a list of all the values. And so it has got a list of the keys and it's got a list of the values. And then it uses those two lists to build the SQL using the strings format method and then it passes that SQL query off to the database. So this allows us to use a dictionary object and to build our query based on the names of the keys in that dictionary object and to use that so that we can use the same code for different applications that have different database schemas.

So, the same technique is also used for update. Where again, we build a query based on a list of keys and values. Keys and values. The other methods like delete(), they only need the id. So that makes that a lot easier. So this is a very useful technique where you can use normalized code for a number of different applications with different database schemas. And by using that normalized code and some simple coding conventions, you can keep an application like this, down to about 250 lines and still have it look and operated like something a lot bigger and a lot more complex.

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This video is part of

Image for Python 3 Essential Training
Python 3 Essential Training

87 video lessons · 38313 viewers

Bill Weinman
Author

 
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  1. 5m 14s
    1. Welcome
      1m 32s
    2. Understanding prerequisites for Python
      2m 4s
    3. Using the exercise files
      1m 38s
  2. 33m 29s
    1. Getting started with "Hello World"
      4m 43s
    2. Selecting code with conditionals
      4m 45s
    3. Repeating code with a loop
      4m 13s
    4. Reusing code with a function
      2m 43s
    5. Creating sequences with generator functions
      2m 46s
    6. Reusing code and data with a class
      4m 39s
    7. Greater reusability with inheritance and polymorphism
      7m 17s
    8. Handling errors with exceptions
      2m 23s
  3. 22m 32s
    1. Installing Python 3 and Eclipse for Windows
      11m 24s
    2. Installing Python 3 and Eclipse for Mac
      11m 8s
  4. 28m 0s
    1. Creating a main script
      3m 27s
    2. Understanding whitespace in Python
      4m 8s
    3. Commenting code
      3m 28s
    4. Assigning values
      3m 37s
    5. Selecting code and values with conditionals
      4m 46s
    6. Creating and using functions
      3m 54s
    7. Creating and using objects
      4m 40s
  5. 31m 23s
    1. Understanding variables and objects in Python
      2m 46s
    2. Distinguishing mutable and immutable objects
      2m 41s
    3. Using numbers
      3m 34s
    4. Using strings
      6m 38s
    5. Aggregating values with lists and tuples
      4m 55s
    6. Creating associative lists with dictionaries
      4m 24s
    7. Finding the type and identity of a variable
      4m 45s
    8. Specifying logical values with True and False
      1m 40s
  6. 9m 42s
    1. Selecting code with if and else conditional statements
      2m 22s
    2. Setting multiple choices with elif
      2m 14s
    3. Understanding other strategies for multiple choices
      2m 38s
    4. Using the conditional expression
      2m 28s
  7. 11m 26s
    1. Creating loops with while
      1m 27s
    2. Iterating with for
      3m 54s
    3. Enumerating iterators
      3m 22s
    4. Controlling loop flow with break, continue, and else
      2m 43s
  8. 23m 28s
    1. Performing simple arithmetic
      2m 14s
    2. Operating on bitwise values
      3m 30s
    3. Comparing values
      3m 32s
    4. Operating on Boolean values
      2m 59s
    5. Operating on parts of a container with the slice operator
      6m 52s
    6. Understanding operator precedence
      4m 21s
  9. 11m 34s
    1. Using the re module
      1m 4s
    2. Searching with regular expressions
      3m 12s
    3. Replacing with regular expressions
      3m 29s
    4. Reusing regular expressions with re.compile
      3m 49s
  10. 9m 10s
    1. Learning how exceptions work
      1m 18s
    2. Handling exceptions
      4m 15s
    3. Raising exceptions
      3m 37s
  11. 23m 1s
    1. Defining functions
      6m 23s
    2. Using lists of arguments
      2m 26s
    3. Using named function arguments
      4m 32s
    4. Returning values from functions
      1m 55s
    5. Creating a sequence with a generator function
      7m 45s
  12. 47m 29s
    1. Understanding classes and objects
      5m 12s
    2. Using methods
      6m 12s
    3. Using object data
      10m 4s
    4. Understanding inheritance
      5m 11s
    5. Applying polymorphism to classes
      7m 13s
    6. Using generators
      9m 48s
    7. Using decorators
      3m 49s
  13. 18m 54s
    1. Understanding strings as objects
      3m 25s
    2. Working with common string methods
      5m 24s
    3. Formatting strings with str.format
      5m 31s
    4. Splitting and joining strings
      2m 49s
    5. Finding and using standard string methods
      1m 45s
  14. 25m 27s
    1. Creating sequences with tuples and lists
      4m 6s
    2. Operating on sequences with built-in methods
      5m 50s
    3. Organizing data with dictionaries
      4m 56s
    4. Operating on character data with bytes and byte arrays
      10m 35s
  15. 11m 46s
    1. Opening files
      2m 4s
    2. Reading and writing text files
      4m 33s
    3. Reading and writing binary files
      5m 9s
  16. 21m 27s
    1. Creating a database with SQLite 3
      6m 56s
    2. Creating, retrieving, updating, and deleting records
      7m 31s
    3. Creating a database object
      7m 0s
  17. 18m 27s
    1. Using standard library modules
      8m 0s
    2. Finding third-party modules
      5m 47s
    3. Creating a module
      4m 40s
  18. 23m 11s
    1. Dealing with syntax errors
      8m 19s
    2. Dealing with runtime errors
      4m 0s
    3. Dealing with logical errors
      4m 22s
    4. Using unit tests
      6m 30s
  19. 19m 56s
    1. Normalizing a database interface
      6m 39s
    2. Deconstructing a database application
      8m 9s
    3. Displaying random entries from a database
      5m 8s
  20. 29s
    1. Goodbye
      29s

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