Python string interpretation and parse

Question

I'm trying to learn how to interpret and parse a string in python. I want to make a "string command" (don't know if is the right expression). But to explain better I will take an example: I want a command like in SQL, where there is a string with keywords that will make a process do what is asking for. Like this: cursor.execute("UPDATE Cars SET Price=? WHERE Id=?", (50000, 1)). But I want to create a format for my project like this (it is not necessary to be with sql): mydef("U={Cars[Price=50000], Id=1}")

Syntax table: <command>={<table>[<value name>=<value (int/str/float/bool)>], <id>=<value to id>}
Where command is: U=update, C=create, S=select, I=insert, D=delete

Well, I really want to learn how can I do it in Python. If is possible.

You probably wanna read up on lexing and parsing first. Check out their Wikipedia articles. — Johan
– Johan, Commented Aug 10, 2012 at 21:43

PaulMcG · Accepted Answer · 2012-08-10 22:27:39Z

1

Pyparsing is a simple pure-Python, small-footprint, liberally-licensed module for creating parsers like the one you describe. Here are a couple of presentations I gave at PyCon'06 (updated for the Texas Python UnConference, 2008), one an intro to pyparsing itself, and one a demo of using pyparsing for parsing and executing a simple command language (a text adventure game).

Intro to Pyparsing - http://www.ptmcg.com/geo/python/confs/TxUnconf2008Pyparsing.html

A Simple Adventure Game Command Parser - http://www.ptmcg.com/geo/python/confs/pyCon2006_pres2.html

Both presentations are written using S5, so if you mouse into the lower right hand corner, you'll see << and >> buttons, a Ø button to see the entire presentation as a single printable web page, and a combo box to jump to a particular page.

You can find out more about pyparsing at http://pyparsing.wikispaces.com.

answered Aug 10, 2012 at 22:27

PaulMcG

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2 Comments

Jingo Patrick Page Over a year ago

This pyparsing does the samething as the <string>.split but simplify the process. Correct if I'm wrong.

PaulMcG Over a year ago

Pyparsing is no longer hosted on wikispaces.com. Go to github.com/pyparsing/pyparsing

verbsintransit · Accepted Answer · 2012-08-10 21:51:07Z

0

Just to be clear, are you aware that Python2.5+ includes sqlite?

import sqlite3
conn = sqlite3.connect(dbname.db)
curs = conn.cursor()
curs.execute("""CREATE TABLE Cars (UID INTEGER PRIMARY KEY, \
        "Id" VARCHAR(42), \
        "Price" VARCHAR(42))""")
curs.execute("UPDATE Cars SET Price=? WHERE Id=?", (50000, 1))

Edit to add: I didn't actually test this; you'll at least need an insert statement to make this work.

answered Aug 10, 2012 at 21:51

verbsintransit

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1 Comment

Jingo Patrick Page Over a year ago

I know, but I really need this way of database process.

Jingo Patrick Page · Accepted Answer · 2012-08-11 20:59:01Z

I did this code, I don't know if this will work. Just want the opinion.

>>> s = '<command>={<table>[<value name>=<value>], <id>=<value id>}'
>>> s1 = s.split('=', 1)
>>> s2 = s1[1].split(',', 1)
>>> s2 = s1[1].replace('{', '').replace('}', '').split(',', 1)
>>> s3 = s2[0].replace(']', '').split('[')
>>> s4 = s3[1].split('=')
>>> s1
['<command>', '{<table>[<value name>=<value>], <id>=<value id>}']
>>> s2
['<table>[<value name>=<value>]', ' <id>=<value id>']
>>> s3
['<table>', '<value name>=<value>']
>>> s4
['<value name>', '<value>']
>>> s5 = s2[1].split('=')

to split the entire command and get the args:

<command>={<table>[<value name>=<value>],<id>=<value id>}
["<command>", "{<table>[<value name>=<value>],<id>=<value id>}"]
["<table>[<value name>=<value>]", "<id>=<value id>"]
["<table>", "<value name>=<value>"]
["<value name>", "<value>"]
["<id>", "<value id>"]

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