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Using pyparsing

Administrator — Wed, 24 Oct 2007 14:37:10 +0000

The day after I hyped the Getting Started with PyParsing book, I got to use it. Here’s the script :

  from pyparsing import SkipTo, Suppress, CaselessLiteral
  import glob
 
  # Example to match
  # delete from TABLENAME
  # where       CUSTOMER_ID = 'INTERNAL';
 
  table_name = SkipTo("where")
  where_clause = SkipTo(';')
 
  delete_stmt = Suppress("delete") +  Suppress("from") + table_name
     + Suppress("where") + where_clause + Suppress(";")
 
  for filename in glob.glob('*.sql'):
      f = open(filename)
      print '-- ', filename, ':'
      lines=f.read()
      for tokens, start,end in delete_stmt.scanString(lines):
          print 'select * from ' + tokens[0] + ' where ' + tokens[1] + ';'
      f.close()

This loops over all the files called *.sql in the current directory. Then it checks if there are any matches to the delete_stmt variable. So it has to match the literal “delete” and then the literal “from”. These are not put in the output, because they are in a Suppress() object.

After that we select everything up to ‘where’ as the table_name. Then the literal “where” has to be present. Everything up to the semicolon is then read into the where_clause variable.

Lastly the table name and where clause are used to create a select statement.

PS : The code was changed so that this

  from pyparsing import *

became :

  from pyparsing import SkipTo, Suppress, CaselessLiteral

This way we don’t pollute the current namespace.

PyParsing

Administrator — Mon, 22 Oct 2007 16:51:53 +0000

I just bought and read the Getting Started with Pyparsing PDF book. And it’s good. PyParsing is a way of building a parser using Python code. You should think Yacc/Lex, but readable. It can be used to parse text, and it can also handle HTML.

This is the example from the PyParsing website :

  from pyparsing import Word, alphas
  greet = Word( alphas ) + "," + Word( alphas ) + "!" # <-- grammar defined here
  hello = "Hello, World!"
  print hello, "->", greet.parseString( hello )

OnLamp.com had a good article called Building Recursive Descent Parsers with Python, which is a good starting point. But the book is better.

Books

Administrator — Fri, 19 Oct 2007 09:00:19 +0000

This is a book I want to read :

The ‘Finding things’ chapter, written by Tim Bray, was on the O’Reilly site before, but seems to have gone.