1. SQL – Python and Databases (Continued)

2. Tokens
You can parse a query into tokens (words/units) and afterwards decide what they mean. Take CSE450 (Translation of Programming Languages, aka Compilers) if you want to learn more.
" CREATE TABLE myTable(name TEXT,age INTEGER) ; "
["CREATE", "TABLE", "myTable", "(", "name", "TEXT", ",", "age", "INTEGER", ")", ";"]

3. TEXT
Be sure not to break up TEXT values into different tokens (this changes the whitespace in them)
"INSERT INTO myTable VALUES ('My name');"
WRONG: ["INSERT", "INTO", "myTable", "VALUES", "(", "'", "My", "name", "'", ")", ";"]
RIGHT: ["INSERT", "INTO", "myTable", "VALUES", "(",
"'My name'", ")", ";"]

4. Now what? Once you have a list of tokens, you can process them.
All statements end with a semicolon
Check that the last token is a semicolon (raise an error if it isn't)
Remove it from the list of tokens
Some tokens should be combined to a single value

5. Integer
Integers can have a negative sign in front of them, so if you have the tokens:
[..., ",", "-", "23782", ","...]
You should convert it to:
[..., ",", "-23782", ",", ...]

6. REAL
REAL values can have a negative sign, optional 0 or other integer, a decimal point, then another integer
For project 2, all REAL values will have a decimal point and no exponential portion
So,
["-", "53", ".", "89273"]
Becomes,
[" "]

7. Conversion
The queries are always just strings, but to store data in a database, you need to convert the token to a python object.
"'Howdy!'" becomes "Howdy!" (the single quotes are not part of the string)
"-4" becomes -4 (you can use the int function)
" " becomes (you can use the float function)
"NULL" becomes None

8. Where should you store your database?
Possible locations:
In the connection object
Problem: what if you connect to the same database multiple times
In a file
Problem: you don't know how to write and read multiple relations to a file
We'll learn how later
In a module-level (global) variable
This is what I recommend while we are only making transient databases

9. Possible Scaffold for project 2
_ALL_DATABASES = {} def connect(filename): return Connection(filename) class Connection(): def __init__(filename): #connect or create database def execute(query): #your_code_here that works with a Database instance class Database(): ... class Table(): ... class Row(): ...

10. Why is _ALL_DATABASES a dictionary?
Because it can expand as rows are added
Because it can hold a mapping of attributes to values
Because it can hold a mapping of filenames to databases
Because it can hold a mapping of table names to table instances