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IMPORTANT NOTE (2018-12-22): PLY is no longer be released in any
package-installable format. If you want to use the latest version, you
need to COPY the contents of the ply/ directory into your own project
and use it. Although PLY is no longer distributed as a package, it is
maintained as a mature library. No new major features are planned, but
issues reported for bugs are still welcome. Any changes to the
software will be noted here.
Version 2022_01_02
------------------
12/12/21 PLY is no longer being developed in public. Instead
periodic versions will be posted. In general, there has
been an effort at cleanup and modernization.
Here is a short summary of major changes:
- PLY no longer writes table files or cached data. If you want this,
it's your responsibility to serialize the parser tables. Use pickle.
- Elimination of side-effects and global variables (generally).
- Elimination of numerous optional features in an effort to
simplify the API.
- More use of modern Python features including iterators/generators,
keyword-only arguments, f-strings, etc.
- Dropped support for Python 2.x
01/26/20 PLY no longer writes cached table files. Honestly, the use of
the cached files made more sense when I was developing PLY on
my 200Mhz PC in 2001. It's not as much as an issue now. For small
to medium sized grammars, PLY should be almost instantaneous.
If you're working with a large grammar, you can arrange
to pickle the associated grammar instance yourself if need be.
The removal of table files eliminated a large number of optional
arguments to yacc() concerning the names and packages of these files.
01/26/20 PLY no longer supports Python 2.x.
01/01/20 Added an install.py script to make it easy to install PLY into
virtual environment if you just want to play with it.
01/01/20 Some project reorganization. Moved the preprocessor and ctokens
file to the examples directory. Bumped the version number.
01/19/19 Some improvements to the preprocessor module contributed by
Rob Reilink. Issue #195 fixes the evaluation of expressions
such as #if a != b. Issue #196 fixes some some issues
related to the lack of a specified Unicode encoding on reading
text files.
12/22/18 Incorporated some minor changes to eliminate warning messages
related to open() calls.
Version 3.11
---------------------
02/15/18 beazley
Fixed some minor bugs related to re flags and token order.
Github pull requests #151 and #153.
02/15/18 beazley
Added a set_lexpos() method to grammar symbols. Github issue #148.
04/13/17 beazley
Mostly minor bug fixes and small code cleanups.
Version 3.10
---------------------
01/31/17: beazley
Changed grammar signature computation to not involve hashing
functions. Parts are just combined into a big string.
10/07/16: beazley
Fixed Issue #101: Incorrect shift-reduce conflict resolution with
precedence specifier.
PLY was incorrectly resolving shift-reduce conflicts in certain
cases. For example, in the example/calc/calc.py example, you
could trigger it doing this:
calc > -3 - 4
1 (correct answer should be -7)
calc >
Issue and suggested patch contributed by https://github.com/RomaVis
Version 3.9
---------------------
08/30/16: beazley
Exposed the parser state number as the parser.state attribute
in productions and error functions. For example:
def p_somerule(p):
'''
rule : A B C
'''
print('State:', p.parser.state)
May address issue #65 (publish current state in error callback).
08/30/16: beazley
Fixed Issue #88. Python3 compatibility with ply/cpp.
08/30/16: beazley
Fixed Issue #93. Ply can crash if SyntaxError is raised inside
a production. Not actually sure if the original implementation
worked as documented at all. Yacc has been modified to follow
the spec as outlined in the CHANGES noted for 11/27/07 below.
08/30/16: beazley
Fixed Issue #97. Failure with code validation when the original
source files aren't present. Validation step now ignores
the missing file.
08/30/16: beazley
Minor fixes to version numbers.
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 https://github.com/dabeaz/ply/issues/63
Version 3.6
---------------------
04/25/15: beazley
If PLY is unable to create the 'parser.out' or 'parsetab.py' 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 'parsetab.py' and 'lextab.py' 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 __init__.py 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.
*** POTENTIAL INCOMPATIBILITY ***
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
accepted).
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):
r'\{'
# Some action
t.type = '{'
return t
def t_rbrace(t):
r'\}'
# Some action
t.type = '}'
return t
04/19/15: beazley
Import of the 'parsetab.py' 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 parsetab.py 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 parsetab.py
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 'calc.py',
then the parsetab.py 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 parsetab.py 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/cpp.py (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/cpp.py.
Reported by Alexei Pososin.
10/15/12: beazley
Fixed token names in ply/ctokens.py 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
this:
def p_error(p):
...
parser.errok()
parser = yacc.yacc()
*** POTENTIAL INCOMPATIBILITY *** The original global functions now issue a
DeprecationWarning.
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
spans:
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 cpp.py compatible with Python 3. Note: This
is an experimental file not currently used by the rest of PLY.
02/17/11: beazley
Fixed setup.py 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
mdsherry.
08/25/09: beazley
Fixed a bug related to regular expression compilation flags not being
properly stored in lextab.py 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:
yacc(picklefile="parsetab.p")
This option can be used if the normal parsetab.py file is
extremely large. For example, on jython, it is impossible
to read parsing tables if the parsetab.py exceeds a certain
threshold.
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
indices.
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
harmless.
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="foo.bar.parsetab",outputdir="foo/bar")
yacc would create a file "foo.bar.parsetab.py" in the given directory.
Now, it simply generates a file "parsetab.py" 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 foo.bar.spam.parsetab 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.open(filename,os.O_RDONLY),
os.path.getsize(filename),
access=mmap.ACCESS_READ)
lexer.input(data)
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):
r'\d+'
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
resets.
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 setup.py 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 lex.py 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:
yacc.parse(data,tracking=True)
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.
*** POTENTIAL INCOMPATIBILITY ***
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
example:
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]
t.lexer.skip(1)
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):
r'//.*'
pass
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 http://www.gnu.org/software/flex/manual/html_chapter/flex_11.html.
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 = (
('foo','exclusive'),
('bar','inclusive')
)
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/ylex.py 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 yacc.py 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
judgement.
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):
r'\n'
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.
*** POTENTIAL INCOMPATIBILITY ***
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, lex.py 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.