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Creating, retrieving, updating, and deleting records

From: Python 3 Essential Training

Video: Creating, retrieving, updating, and deleting records

The basic four functions of a database are considered create, retrieve, update and delete, which conveniently spells the word CRUD. In the Exercise Files, you'll notice file called sqlite3-crud.py. We're going to use that as a starting point here. So we'll just make a working copy of it and we'll call it sqlite3-crud-working.py. If you like, you can name it something simpler and we'll open that up and you'll notice that this is a rather complete file.

Creating, retrieving, updating, and deleting records

The basic four functions of a database are considered create, retrieve, update and delete, which conveniently spells the word CRUD. In the Exercise Files, you'll notice file called sqlite3-crud.py. We're going to use that as a starting point here. So we'll just make a working copy of it and we'll call it sqlite3-crud-working.py. If you like, you can name it something simpler and we'll open that up and you'll notice that this is a rather complete file.

Rather than type all this stuff, I just give it to you and we'll take a tour of it and we'll see how it works. You'll notice that at the top of the file here in imports sqlite3 and down here we have the main function. Normally I would put this at the top, but really the object of our exercise here is those functions at the top. So, we'll get to those in a moment and you'll notice a few little print statements here as comments. So, we create the table and it's called test and this is the same as in the creating a database lesson.

You'll notice that we also have our row factory in here and that's also described in the creating a database lesson, and then we create the rows and we're using this insert function that's defined above. I don't call it create because I think of create as being creating the table or creating the database. I call it insert. CRUD doesn't work as well if there is an I instead of the C, but this works for me to call it insert and actually it matches the corresponding SQL also, which is insert. And then we retrieve a row based on a key. There's the key and there is the key, we update rows and changing their values and we delete rows and if we run this, go ahead and run it, it creates a table, it creates rows and then it shows them to you and it retrieves rows and it retrieves them based on those keys and there is the dictionary objects and it updates rows, and there it changed these two, and it deletes rows and this is all that's left after it's deleted them.

So, this just shows us that it's working and now we can look at exactly how all this stuff works. When you're building a specific application and your specific application has specific database tables and specific schemas, a lot of times it's convenient to create specific functionality in your program that deals with those objects and normally you're going to do it in an object-oriented method and we'll look at that in another lesson in this chapter, but for now, here's how you can do it in a simple way.

I would not write production code this way. There's no error checking. Using functions I'm passing the database handle around. The way I would create actual applications is going to be in more object-oriented manner. We'll look at that in another lesson. For now, our purpose here is just to look at how can we do these individual tasks in very simple way. And so, for the insert function, I simply pass it a dictionary and the dictionary has the data that's going to be inserted and then up here we can use this little outline feature in the Eclipse workflow and I look at the insert function. And you'll see it takes that row which is a dictionary object and it passes the individual elements into the placeholders in the SQLite3 execute method.

So, the execute method takes as its first argument a string of SQL and in that argument you can put in placeholders and there's actually a couple of ways to do that. I'm showing you the simple way here with the question mark and these are positional and then it takes a list as the second argument and so this has to be a list or a tuple. In this case a tuple, and so it need to be as one argument which is why we're passing it a tuple with parentheses around it so that it's grouped as one argument and then that has positional ordered parameters. The first one is going to correspond with the first question mark and the second one is going to correspond with the second question mark and each of these is simply dereferencing the dictionary object that's passed into the function.

So, this very simple. in the main, all we need to do is we call it like this, insert and the first parameter here is DB, which is the database object, so that the insert function can access the database and the second parameter is this dictionary object and obviously, in your code, you could create that separately. It could be derived from all sorts of sources and for our purposes, we're simply creating the dictionary object on the fly here. And so we call that four times and that inserts the data and we'll see that right here at the top of our results, Create rows.

Notice that I have this little display rows function and that looks like this. It basically executes a select and it prints out the results from the dictionary object, because we're using the row factory. So, that's our insert function. Our retrieve function is like this. It simply passes a key and it gets a row object in return and so it prints that out as a dictionary and so that looks like this here in our results. These are the dictionary objects that are returned by those two calls to retrieve.

So, retrieve is really just this part here and when we look at it up in our code, it takes a key and I'm calling that key t1, I could just name it key if I want and it calls execute with this SQL, which is basically select an entire row from the test table where t1 equals question mark, and then it passes this one positional argument out. Now this has to be a list or a tuple, and so I create a tuple here and remember it's the comma that creates a tuple. It is not the parentheses. The parentheses are just for grouping.

So, if you want a tuple with one element, it needs to be like this parentheses and then the object and then a comma and then the closed parentheses and this is a case where I need a tuple with just one object. It calls db.execute with this SQL and that returns a cursor and then I use the cursor with the fetchone method in the cursor object from SQLite3, and that'll simply fetch one result because that's what I'm asking for with this particular retrieve. Obviously, if you want a method that retrieves more than one object, you can do that and we'll look at an example of that in the database object lesson in this chapter.

Now, returning to our main function, we update rows and it's really very similar. We're passing a dictionary to an update function and the update function simply has an execute statement with the SQL here, update test, set this value, set that value and it passes it a tuple with those elements from the dictionary that we're passing here. You see the pattern here? Delete works the same way. It gets a key and it has the SQL. It says delete from test where t1 equals question mark and then it's this one element tuple and it calls commit.

We have to call commit every time we do anything that can change the database. So, for update, we call commit, for delete, we call commit and also for insert we're calling commit and so there is our retrieve. There is our update and after update, we display the rows and after delete, we display the rows and there is our result, update, change those two, and there they are changing the two up here and Delete rows, we're deleting two rows, we delete one and we delete three with these keys and all we have left is two and four.

So, those are the major four functions of a database. Insert, retrieve, update and delete and you can see with SQLite and Python, it's incredibly simple. It's really just passing the SQL and getting the right parameters into the SQL and it just works.

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This video is part of

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Python 3 Essential Training

87 video lessons · 39434 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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