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.
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.
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].
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.
The LRTable class represents constructed LR parsing tables on a
grammar.
LRTable(grammar, log=None)
: Create the LR parsing tables on a grammar. grammar is an instance
of Grammar 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).
An instance lr of LRTable 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.
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.
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)
: 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.
p.restart()
: Resets the parser state for a parse already in progress.
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.
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 LRTable object is created to run the LALR algorithm over the
Grammar object.
4. Productions in the LRTable and bound to callables using the
bind_callables() method.
5. A LRParser object is created from from the information in the
LRTable object.