# PLY Internals

David M. Beazley
dave@dabeaz.com

PLY Version: 3.0

- Introduction
- Grammar Class
- Productions
- LRItems
- LRTable
- LRGeneratedTable
- LRParser
- ParserReflect
- High-level operation

## 1. Introduction

This document describes classes and functions that make up the internal operation of PLY. Using this programming interface, it is possible to manually build an parser using a different interface specification than what PLY normally uses. For example, you could build a gramar from information parsed in a completely different input format. Some of these objects may be useful for building more advanced parsing engines such as GLR.

It should be stressed that using PLY at this level is not for the faint of heart. Generally, it's assumed that you know a bit of the underlying compiler theory and how an LR parser is put together.

## 2. Grammar Class

The file `ply.yacc` defines a class `Grammar` that is used to hold and manipulate information about a grammar specification. It encapsulates the same basic information about a grammar that is put into a YACC file including the list of tokens, precedence rules, and grammar rules. Various operations are provided to perform different validations on the grammar. In addition, there are operations to compute the first and follow sets that are needed by the various table generation algorithms.

`Grammar(terminals)`

> Creates a new grammar object. `terminals` is a list of strings specifying the terminals for the grammar. An instance `g` of `Grammar` has the following methods:

`g.set_precedence(term,assoc,level)`

> Sets the precedence level and associativity for a given terminal `term`. `assoc` is one of `'right'`, `'left'`, or `'nonassoc'` and `level` is a positive integer. The higher the value of `level`, the higher the precedence. Here is an example of typical precedence settings:

>     g.set_precedence('PLUS',  'left',1)
>     g.set_precedence('MINUS', 'left',1)
>     g.set_precedence('TIMES', 'left',2)
>     g.set_precedence('DIVIDE','left',2)
>     g.set_precedence('UMINUS','left',3)

> This method must be called prior to adding any productions to the grammar with `g.add_production()`. The precedence of individual grammar rules is determined by the precedence of the right-most terminal.

`g.add_production(name,syms,func=None,file='',line=0)`

> Adds a new grammar rule. `name` is the name of the rule, `syms` is a list of symbols making up the right hand side of the rule, `func` is the function to call when reducing the rule. `file` and `line` specify the filename and line number of the rule and are used for generating error messages.

> The list of symbols in `syms` may include character literals and `%prec` specifiers. Here are some examples:

>     g.add_production('expr',['expr','PLUS','term'],func,file,line)
>     g.add_production('expr',['expr','"+"','term'],func,file,line)
>     g.add_production('expr',['MINUS','expr','%prec','UMINUS'],func,file,line)

> If any kind of error is detected, a `GrammarError` exception is raised with a message indicating the reason for the failure.

`g.set_start(start=None)`

> Sets the starting rule for the grammar. `start` is a string specifying the name of the start rule. If `start` is omitted, the first grammar rule added with `add_production()` is taken to be the starting rule. This method must always be called after all productions have been added.

`g.find_unreachable()`

> Diagnostic function. Returns a list of all unreachable non-terminals defined in the grammar. This is used to identify inactive parts of the grammar specification.

`g.infinite_cycle()`

> Diagnostic function. Returns a list of all non-terminals in the grammar that result in an infinite cycle. This condition occurs if there is no way for a grammar rule to expand to a string containing only terminal symbols.

`g.undefined_symbols()`

> Diagnostic function. Returns a list of tuples `(name, prod)` corresponding to undefined symbols in the grammar. `name` is the name of the undefined symbol and `prod` is an instance of `Production` which has information about the production rule where the undefined symbol was used.

`g.unused_terminals()`

> Diagnostic function. Returns a list of terminals that were defined, but never used in the grammar.

`g.unused_rules()`

> Diagnostic function. Returns a list of `Production` instances corresponding to production rules that were defined in the grammar, but never used anywhere. This is slightly different than `find_unreachable()`.

`g.unused_precedence()`

> Diagnostic function. Returns a list of tuples `(term, assoc)` corresponding to precedence rules that were set, but never used the grammar. `term` is the terminal name and `assoc` is the precedence associativity (e.g., `'left'`, `'right'`, or `'nonassoc'`.

`g.compute_first()`

> Compute all of the first sets for all symbols in the grammar. Returns a dictionary mapping symbol names to a list of all first symbols.

`g.compute_follow()`

> Compute all of the follow sets for all non-terminals in the grammar. The follow set is the set of all possible symbols that might follow a given non-terminal. Returns a dictionary mapping non-terminal names to a list of symbols.

`g.build_lritems()`

> Calculates all of the LR items for all productions in the grammar. This step is required before using the grammar for any kind of table generation. See the section on LR items below.

The following attributes are set by the above methods and may be useful in code that works with the grammar. All of these attributes should be assumed to be read-only. Changing their values directly will likely break the grammar.

`g.Productions`

> A list of all productions added. The first entry is reserved for a production representing the starting rule. The objects in this list are instances of the `Production` class, described shortly.

`g.Prodnames`

> A dictionary mapping the names of nonterminals to a list of all productions of that nonterminal.

`g.Terminals`

> A dictionary mapping the names of terminals to a list of the production numbers where they are used.

`g.Nonterminals`

> A dictionary mapping the names of nonterminals to a list of the production numbers where they are used.

`g.First`

> A dictionary representing the first sets for all grammar symbols. This is computed and returned by the `compute_first()` method.

`g.Follow`

> A dictionary representing the follow sets for all grammar rules. This is computed and returned by the `compute_follow()` method.

`g.Start`

> Starting symbol for the grammar. Set by the `set_start()` method.

For the purposes of debugging, a `Grammar` object supports the `__len__()` and `__getitem__()` special methods. Accessing `g[n]` returns the nth production from the grammar.

## 3. Productions

`Grammar` objects store grammar rules as instances of a `Production` class. This class has no public constructor--you should only create productions by calling `Grammar.add_production()`. The following attributes are available on a `Production` instance `p`.

`p.name`

> The name of the production. For a grammar rule such as `A : B C D`, this is `'A'`.

`p.prod`

> A tuple of symbols making up the right-hand side of the production. For a grammar rule such as `A : B C D`, this is `('B','C','D')`.

`p.number`

> Production number. An integer containing the index of the production in the grammar's `Productions` list.

`p.func`

> The name of the reduction function associated with the production. This is the function that will execute when reducing the entire grammar rule during parsing.

`p.callable`

> The callable object associated with the name in `p.func`. This is `None` unless the production has been bound using `bind()`.

`p.file`

> Filename associated with the production. Typically this is the file where the production was defined. Used for error messages.

`p.lineno`

> Line number associated with the production. Typically this is the line number in `p.file` where the production was defined. Used for error messages.

`p.prec`

> Precedence and associativity associated with the production. This is a tuple `(assoc,level)` where `assoc` is one of `'left'`,`'right'`, or `'nonassoc'` and `level` is an integer. This value is determined by the precedence of the right-most terminal symbol in the production or by use of the `%prec` specifier when adding the production.

`p.usyms`

> A list of all unique symbols found in the production.

`p.lr_items`

> A list of all LR items for this production. This attribute only has a meaningful value if the `Grammar.build_lritems()` method has been called. The items in this list are instances of `LRItem` described below.

`p.lr_next`

> The head of a linked-list representation of the LR items in `p.lr_items`. This attribute only has a meaningful value if the `Grammar.build_lritems()` method has been called. Each `LRItem` instance has a `lr_next` attribute to move to the next item. The list is terminated by `None`.

`p.bind(dict)`

> Binds the production function name in `p.func` to a callable object in `dict`. This operation is typically carried out in the last step prior to running the parsing engine and is needed since parsing tables are typically read from files which only include the function names, not the functions themselves.

`Production` objects support the `__len__()`, `__getitem__()`, and `__str__()` special methods. `len(p)` returns the number of symbols in `p.prod` and `p[n]` is the same as `p.prod[n]`.

## 4. LRItems

The construction of parsing tables in an LR-based parser generator is primarily done over a set of "LR Items". An LR item represents a stage of parsing one of the grammar rules. To compute the LR items, it is first necessary to call `Grammar.build_lritems()`. Once this step, all of the productions in the grammar will have their LR items attached to them.

Here is an interactive example that shows what LR items look like if you interactively experiment. In this example, `g` is a `Grammar` object.

>     >>> g.build_lritems()
>     >>> p = g[1]
>     >>> p
>     Production(statement -> ID = expr)
>     >>>

In the above code, `p` represents the first grammar rule. In this case, a rule `'statement -> ID = expr'`.

Now, let's look at the LR items for `p`.

>     >>> p.lr_items
>     [LRItem(statement -> . ID = expr),
>      LRItem(statement -> ID . = expr),
>      LRItem(statement -> ID = . expr),
>      LRItem(statement -> ID = expr .)]
>     >>>

In each LR item, the dot (.) represents a specific stage of parsing. In each LR item, the dot is advanced by one symbol. It is only when the dot reaches the very end that a production is successfully parsed.

An instance `lr` of `LRItem` has the following attributes that hold information related to that specific stage of parsing.

`lr.name`

> The name of the grammar rule. For example, `'statement'` in the above example.

`lr.prod`

> A tuple of symbols representing the right-hand side of the production, including the special `'.'` character. For example, `('ID','.','=','expr')`.

`lr.number`

> An integer representing the production number in the grammar.

`lr.usyms`

> A set of unique symbols in the production. Inherited from the original `Production` instance.

`lr.lr_index`

> An integer representing the position of the dot (.). You should never use `lr.prod.index()` to search for it--the result will be wrong if the grammar happens to also use (.) as a character literal.

`lr.lr_after`

> A list of all productions that can legally appear immediately to the right of the dot (.). This list contains `Production` instances. This attribute represents all of the possible branches a parse can take from the current position. For example, suppose that `lr` represents a stage immediately before an expression like this:

>     >>> lr
>     LRItem(statement -> ID = . expr)
>     >>>

> Then, the value of `lr.lr_after` might look like this, showing all productions that can legally appear next:

>     >>> lr.lr_after
>     [Production(expr -> expr PLUS expr),
>      Production(expr -> expr MINUS expr),
>      Production(expr -> expr TIMES expr),
>      Production(expr -> expr DIVIDE expr),
>      Production(expr -> MINUS expr),
>      Production(expr -> LPAREN expr RPAREN),
>      Production(expr -> NUMBER),
>      Production(expr -> ID)]
>     >>>

`lr.lr_before`

> The grammar symbol that appears immediately before the dot (.) or `None` if at the beginning of the parse.

`lr.lr_next`

> A link to the next LR item, representing the next stage of the parse. `None` if `lr` is the last LR item.

`LRItem` instances also support the `__len__()` and `__getitem__()` special methods. `len(lr)` returns the number of items in `lr.prod` including the dot (.). `lr[n]` returns `lr.prod[n]`.

It goes without saying that all of the attributes associated with LR items should be assumed to be read-only. Modifications will very likely create a small black-hole that will consume you and your code.

## 5. LRTable

The `LRTable` class is used to represent LR parsing table data. This minimally includes the production list, action table, and goto table.

`LRTable()`

> Create an empty LRTable object. This object contains only the information needed to run an LR parser.

An instance `lrtab` of `LRTable` has the following methods:

`lrtab.read_table(module)`

> Populates the LR table with information from the module specified in `module`. `module` is either a module object already loaded with `import` or the name of a Python module. If it's a string containing a module name, it is loaded and parsing data is extracted. Returns the signature value that was used when initially writing the tables. Raises a `VersionError` exception if the module was created using an incompatible version of PLY.

`lrtab.bind_callables(dict)`

> This binds all of the function names used in productions to callable objects found in the dictionary `dict`. During table generation and when reading LR tables from files, PLY only uses the names of action functions such as `'p_expr'`, `'p_statement'`, etc. In order to actually run the parser, these names have to be bound to callable objects. This method is always called prior to running a parser.

After `lrtab` has been populated, the following attributes are defined.

`lrtab.lr_method`

> The LR parsing method used (e.g., `'LALR'`)

`lrtab.lr_productions`

> The production list. If the parsing tables have been newly constructed, this will be a list of `Production` instances. If the parsing tables have been read from a file, it's a list of `MiniProduction` instances. This, together with `lr_action` and `lr_goto` contain all of the information needed by the LR parsing engine.

`lrtab.lr_action`

> The LR action dictionary that implements the underlying state machine. The keys of this dictionary are the LR states.

`lrtab.lr_goto`

> The LR goto table that contains information about grammar rule reductions.

## 6. LRGeneratedTable

The `LRGeneratedTable` class represents constructed LR parsing tables on a grammar. It is a subclass of `LRTable`.

`LRGeneratedTable(grammar, method='LALR',log=None)`

> Create the LR parsing tables on a grammar. `grammar` is an instance of `Grammar`, `method` is a string with the parsing method (`'SLR'` or `'LALR'`), and `log` is a logger object used to write debugging information. The debugging information written to `log` is the same as what appears in the `parser.out` file created by yacc. By supplying a custom logger with a different message format, it is possible to get more information (e.g., the line number in `yacc.py` used for issuing each line of output in the log). The result is an instance of `LRGeneratedTable`.

An instance `lr` of `LRGeneratedTable` has the following attributes.

`lr.grammar`

> A link to the Grammar object used to construct the parsing tables.

`lr.lr_method`

> The LR parsing method used (e.g., `'LALR'`)

`lr.lr_productions`

> A reference to `grammar.Productions`. This, together with `lr_action` and `lr_goto` contain all of the information needed by the LR parsing engine.

`lr.lr_action`

> The LR action dictionary that implements the underlying state machine. The keys of this dictionary are the LR states.

`lr.lr_goto`

> The LR goto table that contains information about grammar rule reductions.

`lr.sr_conflicts`

> A list of tuples `(state,token,resolution)` identifying all shift/reduce conflicts. `state` is the LR state number where the conflict occurred, `token` is the token causing the conflict, and `resolution` is a string describing the resolution taken. `resolution` is either `'shift'` or `'reduce'`.

`lr.rr_conflicts`

> A list of tuples `(state,rule,rejected)` identifying all reduce/reduce conflicts. `state` is the LR state number where the conflict occurred, `rule` is the production rule that was selected and `rejected` is the production rule that was rejected. Both `rule` and rejected are instances of `Production`. They can be inspected to provide the user with more information.

There are two public methods of `LRGeneratedTable`.

`lr.write_table(modulename,outputdir="",signature="")`

> Writes the LR parsing table information to a Python module. `modulename` is a string specifying the name of a module such as `"parsetab"`. `outputdir` is the name of a directory where the module should be created. `signature` is a string representing a grammar signature that's written into the output file. This can be used to detect when the data stored in a module file is out-of-sync with the the grammar specification (and that the tables need to be regenerated). If `modulename` is a string `"parsetab"`, this function creates a file called `parsetab.py`. If the module name represents a package such as `"foo.bar.parsetab"`, then only the last component, `"parsetab"` is used.

## 7. LRParser

The `LRParser` class implements the low-level LR parsing engine.

`LRParser(lrtab, error_func)`

> Create an LRParser. `lrtab` is an instance of `LRTable` containing the LR production and state tables. `error_func` is the error function to invoke in the event of a parsing error.

An instance `p` of `LRParser` has the following methods:

`p.parse(input=None,lexer=None,debug=0,tracking=0,tokenfunc=None)`

> Run the parser. `input` is a string, which if supplied is fed into the lexer using its `input()` method. `lexer` is an instance of the `Lexer` class to use for tokenizing. If not supplied, the last lexer created with the `lex` module is used. `debug` is a boolean flag that enables debugging. `tracking` is a boolean flag that tells the parser to perform additional line number tracking. `tokenfunc` is a callable function that returns the next token. If supplied, the parser will use it to get all tokens.

`p.restart()`

> Resets the parser state for a parse already in progress.

## 8. ParserReflect

The `ParserReflect` class is used to collect parser specification data from a Python module or object. This class is what collects all of the `p_rule()` functions in a PLY file, performs basic error checking, and collects all of the needed information to build a grammar. Most of the high-level PLY interface as used by the `yacc()` function is actually implemented by this class.

`ParserReflect(pdict, log=None)`

> Creates a `ParserReflect` instance. `pdict` is a dictionary containing parser specification data. This dictionary typically corresponds to the module or class dictionary of code that implements a PLY parser. `log` is a logger instance that will be used to report error messages.

An instance `p` of `ParserReflect` has the following methods:

`p.get_all()`

> Collect and store all required parsing information.

`p.validate_all()`

> Validate all of the collected parsing information. This is a seprate step from `p.get_all()` as a performance optimization. In order to increase parser start-up time, a parser can elect to only validate the parsing data when regenerating the parsing tables. The validation step tries to collect as much information as possible rather than raising an exception at the first sign of trouble. The attribute `p.error` is set if there are any validation errors. The value of this attribute is also returned.

`p.signature()`

> Compute a signature representing the contents of the collected parsing data. The signature value should change if anything in the parser specification has changed in a way that would justify parser table regeneration. This method can be called after `p.get_all()`, but before `p.validate_all()`.

The following attributes are set in the process of collecting data:

`p.start`

> The grammar start symbol, if any. Taken from `pdict['start']`.

`p.error_func`

> The error handling function or `None`. Taken from `pdict['p_error']`.

`p.tokens`

> The token list. Taken from `pdict['tokens']`.

`p.prec`

> The precedence specifier. Taken from `pdict['precedence']`.

`p.preclist`

> A parsed version of the precedence specified. A list of tuples of the form `(token,assoc,level)` where `token` is the terminal symbol, `assoc` is the associativity (e.g., `'left'`) and `level` is a numeric precedence level.

`p.grammar`

> A list of tuples `(name, rules)` representing the grammar rules. `name` is the name of a Python function or method in `pdict` that starts with `"p_"`. `rules` is a list of tuples `(filename,line,prodname,syms)` representing the grammar rules found in the documentation string of that function. `filename` and `line` contain location information that can be used for debugging. `prodname` is the name of the production. `syms` is the right-hand side of the production. If you have a function like this

>     def p_expr(p):
>         '''expr : expr PLUS expr
>                 | expr MINUS expr
>                 | expr TIMES expr
>                 | expr DIVIDE expr'''

> then the corresponding entry in `p.grammar` might look like this:

>     ('p_expr', [ ('calc.py',10,'expr', ['expr','PLUS','expr']),
>                  ('calc.py',11,'expr', ['expr','MINUS','expr']),
>                  ('calc.py',12,'expr', ['expr','TIMES','expr']),
>                  ('calc.py',13,'expr', ['expr','DIVIDE','expr'])
>                ])

`p.pfuncs`

> A sorted list of tuples `(line, file, name, doc)` representing all of the `p_` functions found. `line` and `file` give location information. `name` is the name of the function. `doc` is the documentation string. This list is sorted in ascending order by line number.

`p.files`

> A dictionary holding all of the source filenames that were encountered while collecting parser information. Only the keys of this dictionary have any meaning.

`p.error`

> An attribute that indicates whether or not any critical errors occurred in validation. If this is set, it means that that some kind of problem was detected and that no further processing should be performed.

## 9. High-level operation

Using all of the above classes requires some attention to detail. The `yacc()` function carries out a very specific sequence of operations to create a grammar. This same sequence should be emulated if you build an alternative PLY interface.

1. A `ParserReflect` object is created and raw grammar specification data is collected.
2. A `Grammar` object is created and populated with information from the specification data.
3. A `LRGenerator` object is created to run the LALR algorithm over the `Grammar` object.
4. Productions in the LRGenerator and bound to callables using the `bind_callables()` method.
5. A `LRParser` object is created from from the information in the `LRGenerator` object.