Python parser

A simple parser for python made using lex yacc / GNU's flex bison.

The stable version currently supports only certain python expressions as stated below. The code for the same is in python-expression-parser directory. A similarly made python for-loop parser can also be found in the python-loop-parser directory.

Screenshots

How to make

• The following packages are required
  • gnu's lex yacc sudo apt-get install bison flex
  • make utility sudo apt-get install build-essential
• go to source folder cd python-expression-parser
• make clean to remove old stuff lying around
• make to build : this will generate the usual files
• make run to execute or simply run ./expression-parser

How to debug

• go to source folder cd python-expression-parser
• make clean to remove old stuff lying around
• make debug to build with debugging flags. This will generate a lot of extra files. Browse them to know more about them.
• run ./debug-expression-parser

Challenges

• Program works when input in only a single line.

  • It can be made into consecutive multiple line input using BEGIN <stmt-list> END.
  • But then, when a syntax error is encountered,
    • i will have to handle the error,
    • empty the current stack
    • then recover from the error
    • and then keep the program going until statements are over

• One program run parses only one statement.

  • It can be made for infinite statements.
  • That is, continous INPUT_OUTPUT stream until ctrl+c on terminal can be achieved.
  • This can be done by removing return 0; from the success case.
  • In this case as well, the current stack needs to be emptied.

Assumptions

• each statement ends with semicolon

Cases Handled

• arithmetic expressions along with brackets

  a=5;
  a=5+24;
  a=rod;
  a=5*(4+3);

• string assignments

  var="hello";
  var='hello';

• print statements

   print 'hello';		# single quotes handled
   print "hello";		# double quotes handled

• keywords cant be identifiers

Cases not handled

• Multiple Statements on a Single Line

  var=5; var2=6; var7=23;

• multi line statements -- a single statement written in multiple lines

  var= 2+ \
      3+	\
      4

• static array initialisation not included

  var = [3,3,23];
  days = ['Monday', 'Tuesday', 'Wednesday','Thursday', 'Friday']

• single or multi line comments

• triple quote print statements - multiline strings

  print """hello""";
  print '''hello''';
  

• typecasting

  print "hello" + str(5);

• shorthand operators not handled

  var+=3;
  var+=2;

Disclaimer

• This assignment covers only simple cases.
• Only to be used as a reference point.
• This code was made in 2015. Use at your own discretion.

Reference links

• Lex Yacc Introduction

  • http://epaperpress.com/lexandyacc/intro.html
  • http://dinosaur.compilertools.net/lex/
  • http://dinosaur.compilertools.net/yacc/

• Lex yacc primer

  • http://tldp.org/HOWTO/Lex-YACC-HOWTO-7.html

• Bison manual references

  • http://www.gnu.org/software/bison/manual/html_node/Understanding.html
  • http://www.gnu.org/software/bison/manual/bison.html#Action-Features
  • http://www.gnu.org/software/bison/manual/bison.html#Algorithm
  • http://www.gnu.org/software/bison/manual/html_node/Error-Recovery.html

• Python expressions

  • http://www.tutorialspoint.com/python/python_loop_control.htm
  • http://www.tutorialspoint.com/python/python_basic_syntax.htm
  • https://docs.python.org/2/reference/expressions.html