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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.
09/21/06: beazley
Improved error checking in lex.py. 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 lex.py 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
@TOKEN(identifier)
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
modules
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
this:
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"
do_stuff
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.
*** POTENTIAL INCOMPATIBILITY ***
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:
lex.lex(reflags=re.UNICODE)
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 lex.py 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 lex.py 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 rununit.py 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 (ezioup@alumni.uchicago.edu)
and optimized by David Beazley. To use LALR(1) parsing do
the following:
yacc.yacc(method="LALR")
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 setup.py 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
features:
- 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
yacc(debugfile="somefile.out")
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):
r'\d+'
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 yacc.py to work with Python optimize mode. To make
this work, you need to use
yacc.yacc(optimize=1)
Furthermore, you need to first run Python in normal mode
to generate the necessary parsetab.py 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 parsetab.py 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 "lextab.py". Then, run lex like this:
# module foo.py
...
...
lex.lex(optimize=1)
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 'lextab.py' use the following:
lex.lex(lextab="footab")
(this creates a file footab.py)
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 parsetab.py[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 lex.py 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:
yacc.yacc(start="statement")
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