introduction to parsing variable word(alphasalphanumsvar_list variable variable suppress(","var_list wrongthe problem seems to be simply matter of python syntax--we can' refer to var_list before we have defined it pyparsing offers solution to thiswe can create an "emptyparser element using forward()and then later on we can append parse elements--including itself--to it so now we can try again var_list forward(var_list <(variable variable suppress(","var_listwrongthis second version is syntactically validbut againit goes against the grain of how pyparsing works--and as part of larger parser its use could lead to parser that is very slowor that simply doesn' work (note that we must use parentheses to ensure that the whole right-hand expression is appended and not just the first part because <has higher precedence level than |that isit binds more tightly than although its use is not appropriate herethe forward(class is very useful in other contextsand we will use it in couple of the examples in the following subsections instead of using forward(in situations like thisthere are alternative coding patterns that go with the pyparsing grain here is the simplest and most literal versionvar_list variable zeroormore(suppress(","variablethis pattern is ideal for handling binary operatorsfor exampleplus_expression operand zeroormore(suppress("+"operandboth of these kinds of usage are so common that pyparsing offers convenience functions that provide suitable parser elements we will look at the operatorprecedence(function that is used to create parser elements for unarybinaryand ternary operators in the example in the last of the following subsections for delimited liststhe convenience function to use is delimitedlist()which we will show nowand which we will use in an example in the following subsectionsvar_list delimitedlist(variablethe delimitedlist(function takes parser element and an optional delimiter--we didn' need to specify the delimiter in this case because the default is commathe delimiter we happen to be using so far the discussion has been fairly abstract in the following four subsections we will create four parserseach of increasing sophisticationthat demonstrate how to make the best use of the pyparsing module the first three parsers are pyparsing versions of the handcrafted parsers we created in the previous