tree shape produced for given expressionstart pytreeclick on its parser radio buttontype the expression in the input field at the bottom rightand press "input(or your enter keythe parser class is run to generate tree from your inputand the gui displays the result depending on the operators used within an expressionsome very differently shaped trees yield the same result when evaluated try running pytree on your computer to get better feel for the parsing process ( ' like to show more example treesbut ran out of page real estate at this point in the book parsers versus python the handcoded custom parser programs we've met in this section illustrate some interesting concepts and underscore the power of python for general-purpose programming depending on your job descriptionthey may also be typical of the sort of thing you' write regularly in traditional language such as parsers are an important component in wide variety of applicationsbut in some casesthey're not as necessary as you might think let me explain why so farwe started with an expression parser and added parse tree interpreter to make the code easier to modify as isthe parser worksbut it may be slow compared to implementation if the parser is used frequentlywe could speed it up by moving parts to extension modules for instancethe scanner might be moved to initiallysince it' often called from the parser ultimatelywe might add components to the grammar that allow expressions to access application-specific variables and functions all of these steps constitute good engineering but depending on your applicationthis approach may not be the best one in python often the easiest way to evaluate input expressions in python is to let python do it for usby calling its eval built-in function in factwe can usually replace the entire expression evaluation program with this one function call the next section will show how this can be used to simplify languagebased systems in general more importantthe next section underscores core idea behind the languageif you already have an extensibleembeddablehigh-level language systemwhy invent anotherpython itself can often satisfy language-based component needs pycalca calculator program/object to wrap up this ' going to show you practical application for some of the parsing technology introduced in the preceding section this section presents pycalca python calculator program with graphical interfacesimilar to the calculator programs available on most window systems like most of the gui examples in this bookthoughpycalc offers few advantages over existing calculators because pycalc is pycalca calculator program/object