Skip to content
Find file
Fetching contributors…
Cannot retrieve contributors at this time
1344 lines (1029 sloc) 53.1 KB
Version 3.8
10/02/15: beazley
Fixed issues related to Python 3.5. Patch contributed by Barry Warsaw.
Version 3.7
08/25/15: beazley
Fixed problems when reading table files from pickled data.
05/07/15: beazley
Fixed regression in handling of table modules if specified as module
objects. See
Version 3.6
04/25/15: beazley
If PLY is unable to create the 'parser.out' or '' files due
to permission issues, it now just issues a warning message and
continues to operate. This could happen if a module using PLY
is installed in a funny way where tables have to be regenerated, but
for whatever reason, the user doesn't have write permission on
the directory where PLY wants to put them.
04/24/15: beazley
Fixed some issues related to use of packages and table file
modules. Just to emphasize, PLY now generates its special
files such as '' and '' in the *SAME*
directory as the source file that uses lex() and yacc().
If for some reason, you want to change the name of the table
module, use the tabmodule and lextab options:
lexer = lex.lex(lextab='spamlextab')
parser = yacc.yacc(tabmodule='spamparsetab')
If you specify a simple name as shown, the module will still be
created in the same directory as the file invoking lex() or yacc().
If you want the table files to be placed into a different package,
then give a fully qualified package name. For example:
lexer = lex.lex(lextab='pkgname.files.lextab')
parser = yacc.yacc(tabmodule='pkgname.files.parsetab')
For this to work, 'pkgname.files' must already exist as a valid
Python package (i.e., the directories must already exist and be
set up with the proper files, etc.).
Version 3.5
04/21/15: beazley
Added support for defaulted_states in the parser. A
defaulted_state is a state where the only legal action is a
reduction of a single grammar rule across all valid input
tokens. For such states, the rule is reduced and the
reading of the next lookahead token is delayed until it is
actually needed at a later point in time.
This delay in consuming the next lookahead token is a
potentially important feature in advanced parsing
applications that require tight interaction between the
lexer and the parser. For example, a grammar rule change
modify the lexer state upon reduction and have such changes
take effect before the next input token is read.
One potential danger of defaulted_states is that syntax
errors might be deferred to a a later point of processing
than where they were detected in past versions of PLY.
Thus, it's possible that your error handling could change
slightly on the same inputs. defaulted_states do not change
the overall parsing of the input (i.e., the same grammar is
If for some reason, you need to disable defaulted states,
you can do this:
parser = yacc.yacc()
parser.defaulted_states = {}
04/21/15: beazley
Fixed debug logging in the parser. It wasn't properly reporting goto states
on grammar rule reductions.
04/20/15: beazley
Added actions to be defined to character literals (Issue #32). For example:
literals = [ '{', '}' ]
def t_lbrace(t):
# Some action
t.type = '{'
return t
def t_rbrace(t):
# Some action
t.type = '}'
return t
04/19/15: beazley
Import of the '' file is now constrained to only consider the
directory specified by the outputdir argument to yacc(). If not supplied,
the import will only consider the directory in which the grammar is defined.
This should greatly reduce problems with the wrong file being
imported by mistake. For example, if it's found somewhere else on the path
by accident.
*** POTENTIAL INCOMPATIBILITY *** It's possible that this might break some
packaging/deployment setup if PLY was instructed to place its
in a different location. You'll have to specify a proper outputdir= argument
to yacc() to fix this if needed.
04/19/15: beazley
Changed default output directory to be the same as that in which the
yacc grammar is defined. If your grammar is in a file '',
then the and parser.out files should be generated in the
same directory as that file. The destination directory can be changed
using the outputdir= argument to yacc().
04/19/15: beazley
Changed the file signature slightly so that the parsetab won't
regenerate if created on a different major version of Python (ie., a
parsetab created on Python 2 will work with Python 3).
04/16/15: beazley
Fixed Issue #44 call_errorfunc() should return the result of errorfunc()
04/16/15: beazley
Support for versions of Python <2.7 is officially dropped. PLY may work, but
the unit tests requires Python 2.7 or newer.
04/16/15: beazley
Fixed bug related to calling yacc(start=...). PLY wasn't regenerating the
table file correctly for this case.
04/16/15: beazley
Added skipped tests for PyPy and Java. Related to use of Python's -O option.
05/29/13: beazley
Added filter to make unit tests pass under 'python -3'.
Reported by Neil Muller.
05/29/13: beazley
Fixed CPP_INTEGER regex in ply/ (Issue 21).
Reported by @vbraun.
05/29/13: beazley
Fixed yacc validation bugs when from __future__ import unicode_literals
is being used. Reported by Kenn Knowles.
05/29/13: beazley
Added support for Travis-CI. Contributed by Kenn Knowles.
05/29/13: beazley
Added a .gitignore file. Suggested by Kenn Knowles.
05/29/13: beazley
Fixed validation problems for source files that include a
different source code encoding specifier. Fix relies on
the inspect module. Should work on Python 2.6 and newer.
Not sure about older versions of Python.
Contributed by Michael Droettboom
05/21/13: beazley
Fixed unit tests for yacc to eliminate random failures due to dict hash value
randomization in Python 3.3
Reported by Arfrever
10/15/12: beazley
Fixed comment whitespace processing bugs in ply/
Reported by Alexei Pososin.
10/15/12: beazley
Fixed token names in ply/ to match rule names.
Reported by Alexei Pososin.
04/26/12: beazley
Changes to functions available in panic mode error recover. In previous versions
of PLY, the following global functions were available for use in the p_error() rule:
yacc.errok() # Reset error state
yacc.token() # Get the next token
yacc.restart() # Reset the parsing stack
The use of global variables was problematic for code involving multiple parsers
and frankly was a poor design overall. These functions have been moved to methods
of the parser instance created by the yacc() function. You should write code like
def p_error(p):
parser = yacc.yacc()
*** POTENTIAL INCOMPATIBILITY *** The original global functions now issue a
04/19/12: beazley
Fixed some problems with line and position tracking and the use of error
symbols. If you have a grammar rule involving an error rule like this:
def p_assignment_bad(p):
'''assignment : location EQUALS error SEMI'''
You can now do line and position tracking on the error token. For example:
def p_assignment_bad(p):
'''assignment : location EQUALS error SEMI'''
start_line = p.lineno(3)
start_pos = p.lexpos(3)
If the trackng=True option is supplied to parse(), you can additionally get
def p_assignment_bad(p):
'''assignment : location EQUALS error SEMI'''
start_line, end_line = p.linespan(3)
start_pos, end_pos = p.lexspan(3)
Note that error handling is still a hairy thing in PLY. This won't work
unless your lexer is providing accurate information. Please report bugs.
Suggested by a bug reported by Davis Herring.
04/18/12: beazley
Change to doc string handling in lex module. Regex patterns are now first
pulled from a function's .regex attribute. If that doesn't exist, then
.doc is checked as a fallback. The @TOKEN decorator now sets the .regex
attribute of a function instead of its doc string.
Changed suggested by Kristoffer Ellersgaard Koch.
04/18/12: beazley
Fixed issue #1: Fixed _tabversion. It should use __tabversion__ instead of __version__
Reported by Daniele Tricoli
04/18/12: beazley
Fixed issue #8: Literals empty list causes IndexError
Reported by Walter Nissen.
04/18/12: beazley
Fixed issue #12: Typo in code snippet in documentation
Reported by florianschanda.
04/18/12: beazley
Fixed issue #10: Correctly escape t_XOREQUAL pattern.
Reported by Andy Kittner.
Version 3.4
02/17/11: beazley
Minor patch to make compatible with Python 3. Note: This
is an experimental file not currently used by the rest of PLY.
02/17/11: beazley
Fixed trove classifiers to properly list PLY as
Python 3 compatible.
01/02/11: beazley
Migration of repository to github.
Version 3.3
08/25/09: beazley
Fixed issue 15 related to the set_lineno() method in yacc. Reported by
08/25/09: beazley
Fixed a bug related to regular expression compilation flags not being
properly stored in files created by the lexer when running
in optimize mode. Reported by Bruce Frederiksen.
Version 3.2
03/24/09: beazley
Added an extra check to not print duplicated warning messages
about reduce/reduce conflicts.
03/24/09: beazley
Switched PLY over to a BSD-license.
03/23/09: beazley
Performance optimization. Discovered a few places to make
speedups in LR table generation.
03/23/09: beazley
New warning message. PLY now warns about rules never
reduced due to reduce/reduce conflicts. Suggested by
Bruce Frederiksen.
03/23/09: beazley
Some clean-up of warning messages related to reduce/reduce errors.
03/23/09: beazley
Added a new picklefile option to yacc() to write the parsing
tables to a filename using the pickle module. Here is how
it works:
This option can be used if the normal file is
extremely large. For example, on jython, it is impossible
to read parsing tables if the exceeds a certain
The filename supplied to the picklefile option is opened
relative to the current working directory of the Python
interpreter. If you need to refer to the file elsewhere,
you will need to supply an absolute or relative path.
For maximum portability, the pickle file is written
using protocol 0.
03/13/09: beazley
Fixed a bug in parser.out generation where the rule numbers
where off by one.
03/13/09: beazley
Fixed a string formatting bug with one of the error messages.
Reported by Richard Reitmeyer
Version 3.1
02/28/09: beazley
Fixed broken start argument to yacc(). PLY-3.0 broke this
feature by accident.
02/28/09: beazley
Fixed debugging output. yacc() no longer reports shift/reduce
or reduce/reduce conflicts if debugging is turned off. This
restores similar behavior in PLY-2.5. Reported by Andrew Waters.
Version 3.0
02/03/09: beazley
Fixed missing lexer attribute on certain tokens when
invoking the parser p_error() function. Reported by
Bart Whiteley.
02/02/09: beazley
The lex() command now does all error-reporting and diagonistics
using the logging module interface. Pass in a Logger object
using the errorlog parameter to specify a different logger.
02/02/09: beazley
Refactored ply.lex to use a more object-oriented and organized
approach to collecting lexer information.
02/01/09: beazley
Removed the nowarn option from lex(). All output is controlled
by passing in a logger object. Just pass in a logger with a high
level setting to suppress output. This argument was never
documented to begin with so hopefully no one was relying upon it.
02/01/09: beazley
Discovered and removed a dead if-statement in the lexer. This
resulted in a 6-7% speedup in lexing when I tested it.
01/13/09: beazley
Minor change to the procedure for signalling a syntax error in a
production rule. A normal SyntaxError exception should be raised
instead of yacc.SyntaxError.
01/13/09: beazley
Added a new method p.set_lineno(n,lineno) that can be used to set the
line number of symbol n in grammar rules. This simplifies manual
tracking of line numbers.
01/11/09: beazley
Vastly improved debugging support for yacc.parse(). Instead of passing
debug as an integer, you can supply a Logging object (see the logging
module). Messages will be generated at the ERROR, INFO, and DEBUG
logging levels, each level providing progressively more information.
The debugging trace also shows states, grammar rule, values passed
into grammar rules, and the result of each reduction.
01/09/09: beazley
The yacc() command now does all error-reporting and diagnostics using
the interface of the logging module. Use the errorlog parameter to
specify a logging object for error messages. Use the debuglog parameter
to specify a logging object for the 'parser.out' output.
01/09/09: beazley
*HUGE* refactoring of the the ply.yacc() implementation. The high-level
user interface is backwards compatible, but the internals are completely
reorganized into classes. No more global variables. The internals
are also more extensible. For example, you can use the classes to
construct a LALR(1) parser in an entirely different manner than
what is currently the case. Documentation is forthcoming.
01/07/09: beazley
Various cleanup and refactoring of yacc internals.
01/06/09: beazley
Fixed a bug with precedence assignment. yacc was assigning the precedence
each rule based on the left-most token, when in fact, it should have been
using the right-most token. Reported by Bruce Frederiksen.
11/27/08: beazley
Numerous changes to support Python 3.0 including removal of deprecated
statements (e.g., has_key) and the additional of compatibility code
to emulate features from Python 2 that have been removed, but which
are needed. Fixed the unit testing suite to work with Python 3.0.
The code should be backwards compatible with Python 2.
11/26/08: beazley
Loosened the rules on what kind of objects can be passed in as the
"module" parameter to lex() and yacc(). Previously, you could only use
a module or an instance. Now, PLY just uses dir() to get a list of
symbols on whatever the object is without regard for its type.
11/26/08: beazley
Changed all except: statements to be compatible with Python2.x/3.x syntax.
11/26/08: beazley
Changed all raise Exception, value statements to raise Exception(value) for
forward compatibility.
11/26/08: beazley
Removed all print statements from lex and yacc, using sys.stdout and sys.stderr
directly. Preparation for Python 3.0 support.
11/04/08: beazley
Fixed a bug with referring to symbols on the the parsing stack using negative
05/29/08: beazley
Completely revamped the testing system to use the unittest module for everything.
Added additional tests to cover new errors/warnings.
Version 2.5
05/28/08: beazley
Fixed a bug with writing lex-tables in optimized mode and start states.
Reported by Kevin Henry.
Version 2.4
05/04/08: beazley
A version number is now embedded in the table file signature so that
yacc can more gracefully accomodate changes to the output format
in the future.
05/04/08: beazley
Removed undocumented .pushback() method on grammar productions. I'm
not sure this ever worked and can't recall ever using it. Might have
been an abandoned idea that never really got fleshed out. This
feature was never described or tested so removing it is hopefully
05/04/08: beazley
Added extra error checking to yacc() to detect precedence rules defined
for undefined terminal symbols. This allows yacc() to detect a potential
problem that can be really tricky to debug if no warning message or error
message is generated about it.
05/04/08: beazley
lex() now has an outputdir that can specify the output directory for
tables when running in optimize mode. For example:
lexer = lex.lex(optimize=True, lextab="ltab", outputdir="foo/bar")
The behavior of specifying a table module and output directory are
more aligned with the behavior of yacc().
05/04/08: beazley
[Issue 9]
Fixed filename bug in when specifying the modulename in lex() and yacc().
If you specified options such as the following:
parser = yacc.yacc(tabmodule="",outputdir="foo/bar")
yacc would create a file "" in the given directory.
Now, it simply generates a file "" in that directory.
Bug reported by cptbinho.
05/04/08: beazley
Slight modification to lex() and yacc() to allow their table files
to be loaded from a previously loaded module. This might make
it easier to load the parsing tables from a complicated package
structure. For example:
import as parsetab
parser = yacc.yacc(tabmodule=parsetab)
Note: lex and yacc will never regenerate the table file if used
in the form---you will get a warning message instead.
This idea suggested by Brian Clapper.
04/28/08: beazley
Fixed a big with p_error() functions being picked up correctly
when running in yacc(optimize=1) mode. Patch contributed by
Bart Whiteley.
02/28/08: beazley
Fixed a bug with 'nonassoc' precedence rules. Basically the
non-precedence was being ignored and not producing the correct
run-time behavior in the parser.
02/16/08: beazley
Slight relaxation of what the input() method to a lexer will
accept as a string. Instead of testing the input to see
if the input is a string or unicode string, it checks to see
if the input object looks like it contains string data.
This change makes it possible to pass string-like objects
in as input. For example, the object returned by mmap.
import mmap, os
data = mmap.mmap(,os.O_RDONLY),
11/29/07: beazley
Modification of ply.lex to allow token functions to aliased.
This is subtle, but it makes it easier to create libraries and
to reuse token specifications. For example, suppose you defined
a function like this:
def number(t):
t.value = int(t.value)
return t
This change would allow you to define a token rule as follows:
t_NUMBER = number
In this case, the token type will be set to 'NUMBER' and use
the associated number() function to process tokens.
11/28/07: beazley
Slight modification to lex and yacc to grab symbols from both
the local and global dictionaries of the caller. This
modification allows lexers and parsers to be defined using
inner functions and closures.
11/28/07: beazley
Performance optimization: The lexer.lexmatch and t.lexer
attributes are no longer set for lexer tokens that are not
defined by functions. The only normal use of these attributes
would be in lexer rules that need to perform some kind of
special processing. Thus, it doesn't make any sense to set
them on every token.
*** POTENTIAL INCOMPATIBILITY *** This might break code
that is mucking around with internal lexer state in some
sort of magical way.
11/27/07: beazley
Added the ability to put the parser into error-handling mode
from within a normal production. To do this, simply raise
a yacc.SyntaxError exception like this:
def p_some_production(p):
'some_production : prod1 prod2'
raise yacc.SyntaxError # Signal an error
A number of things happen after this occurs:
- The last symbol shifted onto the symbol stack is discarded
and parser state backed up to what it was before the
the rule reduction.
- The current lookahead symbol is saved and replaced by
the 'error' symbol.
- The parser enters error recovery mode where it tries
to either reduce the 'error' rule or it starts
discarding items off of the stack until the parser
When an error is manually set, the parser does *not* call
the p_error() function (if any is defined).
*** NEW FEATURE *** Suggested on the mailing list
11/27/07: beazley
Fixed structure bug in examples/ansic. Reported by Dion Blazakis.
11/27/07: beazley
Fixed a bug in the lexer related to start conditions and ignored
token rules. If a rule was defined that changed state, but
returned no token, the lexer could be left in an inconsistent
state. Reported by
11/27/07: beazley
Modified to support Python Eggs. Patch contributed by
Simon Cross.
11/09/07: beazely
Fixed a bug in error handling in yacc. If a syntax error occurred and the
parser rolled the entire parse stack back, the parser would be left in in
inconsistent state that would cause it to trigger incorrect actions on
subsequent input. Reported by Ton Biegstraaten, Justin King, and others.
11/09/07: beazley
Fixed a bug when passing empty input strings to yacc.parse(). This
would result in an error message about "No input given". Reported
by Andrew Dalke.
Version 2.3
02/20/07: beazley
Fixed a bug with character literals if the literal '.' appeared as the
last symbol of a grammar rule. Reported by Ales Smrcka.
02/19/07: beazley
Warning messages are now redirected to stderr instead of being printed
to standard output.
02/19/07: beazley
Added a warning message to if it detects a literal backslash
character inside the t_ignore declaration. This is to help
problems that might occur if someone accidentally defines t_ignore
as a Python raw string. For example:
t_ignore = r' \t'
The idea for this is from an email I received from David Cimimi who
reported bizarre behavior in lexing as a result of defining t_ignore
as a raw string by accident.
02/18/07: beazley
Performance improvements. Made some changes to the internal
table organization and LR parser to improve parsing performance.
02/18/07: beazley
Automatic tracking of line number and position information must now be
enabled by a special flag to parse(). For example:
In many applications, it's just not that important to have the
parser automatically track all line numbers. By making this an
optional feature, it allows the parser to run significantly faster
(more than a 20% speed increase in many cases). Note: positional
information is always available for raw tokens---this change only
applies to positional information associated with nonterminal
grammar symbols.
02/18/07: beazley
Yacc no longer supports extended slices of grammar productions.
However, it does support regular slices. For example:
def p_foo(p):
'''foo: a b c d e'''
p[0] = p[1:3]
This change is a performance improvement to the parser--it streamlines
normal access to the grammar values since slices are now handled in
a __getslice__() method as opposed to __getitem__().
02/12/07: beazley
Fixed a bug in the handling of token names when combined with
start conditions. Bug reported by Todd O'Bryan.
Version 2.2
11/01/06: beazley
Added lexpos() and lexspan() methods to grammar symbols. These
mirror the same functionality of lineno() and linespan(). For
def p_expr(p):
'expr : expr PLUS expr'
p.lexpos(1) # Lexing position of left-hand-expression
p.lexpos(1) # Lexing position of PLUS
start,end = p.lexspan(3) # Lexing range of right hand expression
11/01/06: beazley
Minor change to error handling. The recommended way to skip characters
in the input is to use t.lexer.skip() as shown here:
def t_error(t):
print "Illegal character '%s'" % t.value[0]
The old approach of just using t.skip(1) will still work, but won't
be documented.
10/31/06: beazley
Discarded tokens can now be specified as simple strings instead of
functions. To do this, simply include the text "ignore_" in the
token declaration. For example:
t_ignore_cppcomment = r'//.*'
Previously, this had to be done with a function. For example:
def t_ignore_cppcomment(t):
If start conditions/states are being used, state names should appear
before the "ignore_" text.
10/19/06: beazley
The Lex module now provides support for flex-style start conditions
as described at
Please refer to this document to understand this change note. Refer to
the PLY documentation for PLY-specific explanation of how this works.
To use start conditions, you first need to declare a set of states in
your lexer file:
states = (
This serves the same role as the %s and %x specifiers in flex.
One a state has been declared, tokens for that state can be
declared by defining rules of the form t_state_TOK. For example:
t_PLUS = '\+' # Rule defined in INITIAL state
t_foo_NUM = '\d+' # Rule defined in foo state
t_bar_NUM = '\d+' # Rule defined in bar state
t_foo_bar_NUM = '\d+' # Rule defined in both foo and bar
t_ANY_NUM = '\d+' # Rule defined in all states
In addition to defining tokens for each state, the t_ignore and t_error
specifications can be customized for specific states. For example:
t_foo_ignore = " " # Ignored characters for foo state
def t_bar_error(t):
# Handle errors in bar state
With token rules, the following methods can be used to change states
def t_TOKNAME(t):
t.lexer.begin('foo') # Begin state 'foo'
t.lexer.push_state('foo') # Begin state 'foo', push old state
# onto a stack
t.lexer.pop_state() # Restore previous state
t.lexer.current_state() # Returns name of current state
These methods mirror the BEGIN(), yy_push_state(), yy_pop_state(), and
yy_top_state() functions in flex.
The use of start states can be used as one way to write sub-lexers.
For example, the lexer or parser might instruct the lexer to start
generating a different set of tokens depending on the context.
example/yply/ shows the use of start states to grab C/C++
code fragments out of traditional yacc specification files.
*** NEW FEATURE *** Suggested by Daniel Larraz with whom I also
discussed various aspects of the design.
10/19/06: beazley
Minor change to the way in which was reporting shift/reduce
conflicts. Although the underlying LALR(1) algorithm was correct,
PLY was under-reporting the number of conflicts compared to yacc/bison
when precedence rules were in effect. This change should make PLY
report the same number of conflicts as yacc.
10/19/06: beazley
Modified yacc so that grammar rules could also include the '-'
character. For example:
def p_expr_list(p):
'expression-list : expression-list expression'
Suggested by Oldrich Jedlicka.
10/18/06: beazley
Attribute lexer.lexmatch added so that token rules can access the re
match object that was generated. For example:
def t_FOO(t):
r'some regex'
m = t.lexer.lexmatch
# Do something with m
This may be useful if you want to access named groups specified within
the regex for a specific token. Suggested by Oldrich Jedlicka.
10/16/06: beazley
Changed the error message that results if an illegal character
is encountered and no default error function is defined in lex.
The exception is now more informative about the actual cause of
the error.
Version 2.1
10/02/06: beazley
The last Lexer object built by lex() can be found in lex.lexer.
The last Parser object built by yacc() can be found in yacc.parser.
10/02/06: beazley
New example added: examples/yply
This example uses PLY to convert Unix-yacc specification files to
PLY programs with the same grammar. This may be useful if you
want to convert a grammar from bison/yacc to use with PLY.
10/02/06: beazley
Added support for a start symbol to be specified in the yacc
input file itself. Just do this:
start = 'name'
where 'name' matches some grammar rule. For example:
def p_name(p):
'name : A B C'
This mirrors the functionality of the yacc %start specifier.
09/30/06: beazley
Some new examples added.:
examples/GardenSnake : A simple indentation based language similar
to Python. Shows how you might handle
whitespace. Contributed by Andrew Dalke.
examples/BASIC : An implementation of 1964 Dartmouth BASIC.
Contributed by Dave against his better
09/28/06: beazley
Minor patch to allow named groups to be used in lex regular
expression rules. For example:
t_QSTRING = r'''(?P<quote>['"]).*?(?P=quote)'''
Patch submitted by Adam Ring.
09/28/06: beazley
LALR(1) is now the default parsing method. To use SLR, use
yacc.yacc(method="SLR"). Note: there is no performance impact
on parsing when using LALR(1) instead of SLR. However, constructing
the parsing tables will take a little longer.
09/26/06: beazley
Change to line number tracking. To modify line numbers, modify
the line number of the lexer itself. For example:
def t_NEWLINE(t):
t.lexer.lineno += 1
This modification is both cleanup and a performance optimization.
In past versions, lex was monitoring every token for changes in
the line number. This extra processing is unnecessary for a vast
majority of tokens. Thus, this new approach cleans it up a bit.
You will need to change code in your lexer that updates the line
number. For example, "t.lineno += 1" becomes "t.lexer.lineno += 1"
09/26/06: beazley
Added the lexing position to tokens as an attribute lexpos. This
is the raw index into the input text at which a token appears.
This information can be used to compute column numbers and other
details (e.g., scan backwards from lexpos to the first newline
to get a column position).
09/25/06: beazley
Changed the name of the __copy__() method on the Lexer class
to clone(). This is used to clone a Lexer object (e.g., if
you're running different lexers at the same time).
09/21/06: beazley
Limitations related to the use of the re module have been eliminated.
Several users reported problems with regular expressions exceeding
more than 100 named groups. To solve this, is now capable
of automatically splitting its master regular regular expression into
smaller expressions as needed. This should, in theory, make it
possible to specify an arbitrarily large number of tokens.
09/21/06: beazley
Improved error checking in Rules that match the empty string
are now rejected (otherwise they cause the lexer to enter an infinite
loop). An extra check for rules containing '#' has also been added.
Since lex compiles regular expressions in verbose mode, '#' is interpreted
as a regex comment, it is critical to use '\#' instead.
09/18/06: beazley
Added a @TOKEN decorator function to that can be used to
define token rules where the documentation string might be computed
in some way.
digit = r'([0-9])'
nondigit = r'([_A-Za-z])'
identifier = r'(' + nondigit + r'(' + digit + r'|' + nondigit + r')*)'
from ply.lex import TOKEN
def t_ID(t):
# Do whatever
The @TOKEN decorator merely sets the documentation string of the
associated token function as needed for lex to work.
Note: An alternative solution is the following:
def t_ID(t):
# Do whatever
t_ID.__doc__ = identifier
Note: Decorators require the use of Python 2.4 or later. If compatibility
with old versions is needed, use the latter solution.
The need for this feature was suggested by Cem Karan.
09/14/06: beazley
Support for single-character literal tokens has been added to yacc.
These literals must be enclosed in quotes. For example:
def p_expr(p):
"expr : expr '+' expr"
def p_expr(p):
'expr : expr "-" expr'
In addition to this, it is necessary to tell the lexer module about
literal characters. This is done by defining the variable 'literals'
as a list of characters. This should be defined in the module that
invokes the lex.lex() function. For example:
literals = ['+','-','*','/','(',')','=']
or simply
literals = '+=*/()='
It is important to note that literals can only be a single character.
When the lexer fails to match a token using its normal regular expression
rules, it will check the current character against the literal list.
If found, it will be returned with a token type set to match the literal
character. Otherwise, an illegal character will be signalled.
09/14/06: beazley
Modified PLY to install itself as a proper Python package called 'ply'.
This will make it a little more friendly to other modules. This
changes the usage of PLY only slightly. Just do this to import the
import ply.lex as lex
import ply.yacc as yacc
Alternatively, you can do this:
from ply import *
Which imports both the lex and yacc modules.
Change suggested by Lee June.
09/13/06: beazley
Changed the handling of negative indices when used in production rules.
A negative production index now accesses already parsed symbols on the
parsing stack. For example,
def p_foo(p):
"foo: A B C D"
print p[1] # Value of 'A' symbol
print p[2] # Value of 'B' symbol
print p[-1] # Value of whatever symbol appears before A
# on the parsing stack.
p[0] = some_val # Sets the value of the 'foo' grammer symbol
This behavior makes it easier to work with embedded actions within the
parsing rules. For example, in C-yacc, it is possible to write code like
bar: A { printf("seen an A = %d\n", $1); } B { do_stuff; }
In this example, the printf() code executes immediately after A has been
parsed. Within the embedded action code, $1 refers to the A symbol on
the stack.
To perform this equivalent action in PLY, you need to write a pair
of rules like this:
def p_bar(p):
"bar : A seen_A B"
def p_seen_A(p):
"seen_A :"
print "seen an A =", p[-1]
The second rule "seen_A" is merely a empty production which should be
reduced as soon as A is parsed in the "bar" rule above. The use
of the negative index p[-1] is used to access whatever symbol appeared
before the seen_A symbol.
This feature also makes it possible to support inherited attributes.
For example:
def p_decl(p):
"decl : scope name"
def p_scope(p):
"""scope : GLOBAL
| LOCAL"""
p[0] = p[1]
def p_name(p):
"name : ID"
if p[-1] == "GLOBAL":
# ...
else if p[-1] == "LOCAL":
In this case, the name rule is inheriting an attribute from the
scope declaration that precedes it.
If you are currently using negative indices within existing grammar rules,
your code will break. This should be extremely rare if non-existent in
most cases. The argument to various grammar rules is not usually not
processed in the same way as a list of items.
Version 2.0
09/07/06: beazley
Major cleanup and refactoring of the LR table generation code. Both SLR
and LALR(1) table generation is now performed by the same code base with
only minor extensions for extra LALR(1) processing.
09/07/06: beazley
Completely reimplemented the entire LALR(1) parsing engine to use the
DeRemer and Pennello algorithm for calculating lookahead sets. This
significantly improves the performance of generating LALR(1) tables
and has the added feature of actually working correctly! If you
experienced weird behavior with LALR(1) in prior releases, this should
hopefully resolve all of those problems. Many thanks to
Andrew Waters and Markus Schoepflin for submitting bug reports
and helping me test out the revised LALR(1) support.
Version 1.8
08/02/06: beazley
Fixed a problem related to the handling of default actions in LALR(1)
parsing. If you experienced subtle and/or bizarre behavior when trying
to use the LALR(1) engine, this may correct those problems. Patch
contributed by Russ Cox. Note: This patch has been superceded by
revisions for LALR(1) parsing in Ply-2.0.
08/02/06: beazley
Added support for slicing of productions in yacc.
Patch contributed by Patrick Mezard.
Version 1.7
03/02/06: beazley
Fixed infinite recursion problem ReduceToTerminals() function that
would sometimes come up in LALR(1) table generation. Reported by
Markus Schoepflin.
03/01/06: beazley
Added "reflags" argument to lex(). For example:
This can be used to specify optional flags to the re.compile() function
used inside the lexer. This may be necessary for special situations such
as processing Unicode (e.g., if you want escapes like \w and \b to consult
the Unicode character property database). The need for this suggested by
Andreas Jung.
03/01/06: beazley
Fixed a bug with an uninitialized variable on repeated instantiations of parser
objects when the write_tables=0 argument was used. Reported by Michael Brown.
03/01/06: beazley
Modified to accept Unicode strings both as the regular expressions for
tokens and as input. Hopefully this is the only change needed for Unicode support.
Patch contributed by Johan Dahl.
03/01/06: beazley
Modified the class-based interface to work with new-style or old-style classes.
Patch contributed by Michael Brown (although I tweaked it slightly so it would work
with older versions of Python).
Version 1.6
05/27/05: beazley
Incorporated patch contributed by Christopher Stawarz to fix an extremely
devious bug in LALR(1) parser generation. This patch should fix problems
numerous people reported with LALR parsing.
05/27/05: beazley
Fixed problem with copy constructor. Reported by Dave Aitel, Aaron Lav,
and Thad Austin.
05/27/05: beazley
Added outputdir option to yacc() to control output directory. Contributed
by Christopher Stawarz.
05/27/05: beazley
Added test script to run tests using the Python unittest module.
Contributed by Miki Tebeka.
Version 1.5
05/26/04: beazley
Major enhancement. LALR(1) parsing support is now working.
This feature was implemented by Elias Ioup (
and optimized by David Beazley. To use LALR(1) parsing do
the following:
Computing LALR(1) parsing tables takes about twice as long as
the default SLR method. However, LALR(1) allows you to handle
more complex grammars. For example, the ANSI C grammar
(in example/ansic) has 13 shift-reduce conflicts with SLR, but
only has 1 shift-reduce conflict with LALR(1).
05/20/04: beazley
Added a __len__ method to parser production lists. Can
be used in parser rules like this:
def p_somerule(p):
"""a : B C D
| E F"
if (len(p) == 3):
# Must have been first rule
elif (len(p) == 2):
# Must be second rule
Suggested by Joshua Gerth and others.
Version 1.4
04/23/04: beazley
Incorporated a variety of patches contributed by Eric Raymond.
These include:
0. Cleans up some comments so they don't wrap on an 80-column display.
1. Directs compiler errors to stderr where they belong.
2. Implements and documents automatic line counting when \n is ignored.
3. Changes the way progress messages are dumped when debugging is on.
The new format is both less verbose and conveys more information than
the old, including shift and reduce actions.
04/23/04: beazley
Added a Python file to simply installation. Contributed
by Adam Kerrison.
04/23/04: beazley
Added patches contributed by Adam Kerrison.
- Some output is now only shown when debugging is enabled. This
means that PLY will be completely silent when not in debugging mode.
- An optional parameter "write_tables" can be passed to yacc() to
control whether or not parsing tables are written. By default,
it is true, but it can be turned off if you don't want the yacc
table file. Note: disabling this will cause yacc() to regenerate
the parsing table each time.
04/23/04: beazley
Added patches contributed by David McNab. This patch addes two
- The parser can be supplied as a class instead of a module.
For an example of this, see the example/classcalc directory.
- Debugging output can be directed to a filename of the user's
choice. Use
Version 1.3
12/10/02: jmdyck
Various minor adjustments to the code that Dave checked in today.
Updated test/yacc_{inf,unused}.exp to reflect today's changes.
12/10/02: beazley
Incorporated a variety of minor bug fixes to empty production
handling and infinite recursion checking. Contributed by
Michael Dyck.
12/10/02: beazley
Removed bogus recover() method call in yacc.restart()
Version 1.2
11/27/02: beazley
Lexer and parser objects are now available as an attribute
of tokens and slices respectively. For example:
def t_NUMBER(t):
print t.lexer
def p_expr_plus(t):
'expr: expr PLUS expr'
print t.lexer
print t.parser
This can be used for state management (if needed).
10/31/02: beazley
Modified to work with Python optimize mode. To make
this work, you need to use
Furthermore, you need to first run Python in normal mode
to generate the necessary files. After that,
you can use python -O or python -OO.
Note: optimized mode turns off a lot of error checking.
Only use when you are sure that your grammar is working.
Make sure is up to date!
10/30/02: beazley
Added cloning of Lexer objects. For example:
import copy
l = lex.lex()
lc = copy.copy(l)
l.input("Some text")
lc.input("Some other text")
This might be useful if the same "lexer" is meant to
be used in different contexts---or if multiple lexers
are running concurrently.
10/30/02: beazley
Fixed subtle bug with first set computation and empty productions.
Patch submitted by Michael Dyck.
10/30/02: beazley
Fixed error messages to use "filename:line: message" instead
of "filename:line. message". This makes error reporting more
friendly to emacs. Patch submitted by François Pinard.
10/30/02: beazley
Improvements to parser.out file. Terminals and nonterminals
are sorted instead of being printed in random order.
Patch submitted by François Pinard.
10/30/02: beazley
Improvements to parser.out file output. Rules are now printed
in a way that's easier to understand. Contributed by Russ Cox.
10/30/02: beazley
Added 'nonassoc' associativity support. This can be used
to disable the chaining of operators like a < b < c.
To use, simply specify 'nonassoc' in the precedence table
precedence = (
('nonassoc', 'LESSTHAN', 'GREATERTHAN'), # Nonassociative operators
('left', 'PLUS', 'MINUS'),
('left', 'TIMES', 'DIVIDE'),
('right', 'UMINUS'), # Unary minus operator
Patch contributed by Russ Cox.
10/30/02: beazley
Modified the lexer to provide optional support for Python -O and -OO
modes. To make this work, Python *first* needs to be run in
unoptimized mode. This reads the lexing information and creates a
file "". Then, run lex like this:
# module
Once the lextab file has been created, subsequent calls to
lex.lex() will read data from the lextab file instead of using
introspection. In optimized mode (-O, -OO) everything should
work normally despite the loss of doc strings.
To change the name of the file '' use the following:
(this creates a file
Version 1.1 October 25, 2001
10/25/01: beazley
Modified the table generator to produce much more compact data.
This should greatly reduce the size of the[c] file.
Caveat: the tables still need to be constructed so a little more
work is done in parsetab on import.
10/25/01: beazley
There may be a possible bug in the cycle detector that reports errors
about infinite recursion. I'm having a little trouble tracking it
down, but if you get this problem, you can disable the cycle
detector as follows:
yacc.yacc(check_recursion = 0)
10/25/01: beazley
Fixed a bug in that sometimes caused illegal characters to be
reported incorrectly. Reported by Sverre Jørgensen.
7/8/01 : beazley
Added a reference to the underlying lexer object when tokens are handled by
functions. The lexer is available as the 'lexer' attribute. This
was added to provide better lexing support for languages such as Fortran
where certain types of tokens can't be conveniently expressed as regular
expressions (and where the tokenizing function may want to perform a
little backtracking). Suggested by Pearu Peterson.
6/20/01 : beazley
Modified yacc() function so that an optional starting symbol can be specified.
For example:
Normally yacc always treats the first production rule as the starting symbol.
However, if you are debugging your grammar it may be useful to specify
an alternative starting symbol. Idea suggested by Rich Salz.
Version 1.0 June 18, 2001
Initial public offering
Jump to Line
Something went wrong with that request. Please try again.