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David Beazley 2016-09-13 16:33:06 -05:00
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@ -60,130 +60,193 @@ described by the following list of token tuples::
('ID','t'), ('RPAREN',')' ]
The SLY ``Lexer`` class is used to do this. Here is a sample of a simple
lexer::
lexer that tokenizes the above text::
# ------------------------------------------------------------
# calclex.py
#
# Lexer for a simple expression evaluator for
# numbers and +,-,*,/
# ------------------------------------------------------------
from sly import Lexer
class CalcLexer(Lexer):
# List of token names. This is always required
tokens = (
# Set of token names. This is always required
tokens = {
'ID',
'NUMBER',
'PLUS',
'MINUS',
'TIMES',
'DIVIDE',
'ASSIGN',
'LPAREN',
'RPAREN',
)
}
# String containing ignored characters (spaces and tabs)
# String containing ignored characters between tokens
ignore = ' \t'
# Regular expression rules for simple tokens
# Regular expression rules for tokens
ID = r'[a-zA-Z_][a-zA-Z0-9_]*'
NUMBER = r'\d+'
PLUS = r'\+'
MINUS = r'-'
TIMES = r'\*'
DIVIDE = r'/'
ASSIGN = r'='
LPAREN = r'\('
RPAREN = r'\)'
if __name__ == '__main__':
data = 'x = 3 + 42 * (s - t)'
lexer = CalcLexer()
for tok in lexer.tokenize(data):
print('type=%r, value=%r' % (tok.type, tok.value))
When executed, the example will produce the following output::
type='ID', value='x'
type='ASSIGN', value='='
type='NUMBER', value='3'
type='PLUS', value='+'
type='NUMBER', value='42'
type='TIMES', value='*'
type='LPAREN', value='('
type='ID', value='s'
type='MINUS', value='-'
type='ID', value='t'
type='RPAREN', value=')'
A lexer only has one public method ``tokenize()``. This is a generator
function that produces a stream of ``Token`` instances.
The ``type`` and ``value`` attributes of ``Token`` contain the
token type name and value respectively.
The tokens set
^^^^^^^^^^^^^^^
Lexers must specify a ``tokens`` set that defines all of the possible
token type names that can be produced by the lexer. This is always
required and is used to perform a variety of validation checks.
In the example, the following code specified the token names::
class CalcLexer(Lexer):
...
# Set of token names. This is always required
tokens = {
'ID',
'NUMBER',
'PLUS',
'MINUS',
'TIMES',
'DIVIDE',
'ASSIGN',
'LPAREN',
'RPAREN',
}
...
Specification of token match patterns
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Tokens are specified by writing a regular expression rule compatible
with the ``re`` module. The name of each rule must match one of the
names of the tokens provided in the ``tokens`` set. For example::
PLUS = r'\+'
MINUS = r'-'
Regular expression patterns are compiled using the ``re.VERBOSE`` flag
which can be used to help readability. However,
unescaped whitespace is ignored and comments are allowed in this mode.
If your pattern involves whitespace, make sure you use ``\s``. If you
need to match the ``#`` character, use ``[#]`` or ``\#``.
Tokens are matched in the same order that patterns are listed in the
``Lexer`` class. Longer tokens always need to be specified before
short tokens. For example, if you wanted to have separate tokens for
``=`` and ``==``, you need to make sure that ``==`` is listed first. For
example::
class MyLexer(Lexer):
tokens = {'ASSIGN', 'EQ', ...}
...
EQ = r'==' # MUST APPEAR FIRST! (LONGER)
ASSIGN = r'='
Discarded text
^^^^^^^^^^^^^^
The special ``ignore`` specification is reserved for single characters
that should be completely ignored between tokens in the input stream.
Usually this is used to skip over whitespace and other non-essential
characters. The characters given in ``ignore`` are not ignored when
such characters are part of other regular expression patterns. For
example, if you had a rule to capture quoted text, that pattern can
include the ignored characters (which will be captured in the normal
way). The main purpose of ``ignore`` is to ignore whitespace and
other padding between the tokens that you actually want to parse.
You can also discard more specialized text patterns by writing special
regular expression rules with a name that includes the prefix
``ignore_``. For example, this lexer includes rules to ignore
comments and newlines::
# calclex.py
from sly import Lexer
class CalcLexer(Lexer):
# Set of token names. This is always required
tokens = {
'NUMBER',
'ID',
'PLUS',
'MINUS',
'TIMES',
'DIVIDE',
'ASSIGN',
'LPAREN',
'RPAREN',
}
# String containing ignored characters (between tokens)
ignore = ' \t'
# Regular expression rules for tokens
PLUS = r'\+'
MINUS = r'-'
TIMES = r'\*'
DIVIDE = r'/'
LPAREN = r'\('
RPAREN = r'\)'
ASSIGN = r'='
NUMBER = r'\d+'
ID = r'[a-zA-Z_][a-zA-Z0-9_]*'
# A regular expression rule with some action code
@_(r'\d+')
def NUMBER(self, t):
t.value = int(t.value)
return t
# Define a rule so we can track line numbers
@_(r'\n+')
def newline(self, t):
self.lineno += len(t.value)
# Error handling rule (skips ahead one character)
def error(self, value):
print("Line %d: Illegal character '%s'" %
(self.lineno, value[0]))
self.index += 1
# Ignored text
ignore_comment = r'\#.*'
ignore_newline = r'\n+'
if __name__ == '__main__':
data = '''
3 + 4 * 10
+ -20 * ^ 2
'''
data = '''x = 3 + 42
* (s # This is a comment
- t)'''
lexer = CalcLexer()
for tok in lexer.tokenize(data):
print(tok)
print('type=%r, value=%r' % (tok.type, tok.value))
When executed, the example will produce the following output::
Token(NUMBER, 3, 2, 14)
Token(PLUS, '+', 2, 16)
Token(NUMBER, 4, 2, 18)
Token(TIMES, '*', 2, 20)
Token(NUMBER, 10, 2, 22)
Token(PLUS, '+', 3, 40)
Token(MINUS, '-', 3, 42)
Token(NUMBER, 20, 3, 43)
Token(TIMES, '*', 3, 46)
Line 3: Illegal character '^'
Token(NUMBER, 2, 3, 50)
Adding Match Actions
^^^^^^^^^^^^^^^^^^^^
A lexer only has one public method ``tokenize()``. This is a generator
function that produces a stream of ``Token`` instances.
The ``type`` and ``value`` attributes of ``Token`` contain the
token type name and value respectively. The ``lineno`` and ``index``
attributes contain the line number and position in the input text
where the token appears.
The tokens list
^^^^^^^^^^^^^^^
Lexers must specify a ``tokens`` attribute that defines all of the possible token
type names that can be produced by the lexer. This list is always required
and is used to perform a variety of validation checks.
In the example, the following code specified the token names::
class CalcLexer(Lexer):
...
# List of token names. This is always required
tokens = (
'NUMBER',
'PLUS',
'MINUS',
'TIMES',
'DIVIDE',
'LPAREN',
'RPAREN',
)
...
Specification of tokens
^^^^^^^^^^^^^^^^^^^^^^^
Tokens are specified by writing a regular expression rule compatible
with Python's ``re`` module. This is done by writing definitions that
match one of the names of the tokens provided in the ``tokens``
attribute. For example::
PLUS = r'\+'
MINUS = r'-'
Sometimes you want to perform an action when a token is matched. For example,
maybe you want to convert a numeric value or look up a symbol. To do
this, write your action as a method and give the associated regular
When certain tokens are matched, you may want to trigger some kind of
action that performs extra processing. For example, converting
a numeric value or looking up language keywords. One way to do this
is to write your action as a method and give the associated regular
expression using the ``@_()`` decorator like this::
@_(r'\d+')
def NUMBER(self, t):
t.value = int(t.value)
t.value = int(t.value) # Convert to a numeric value
return t
The method always takes a single argument which is an instance of
@ -191,69 +254,23 @@ The method always takes a single argument which is an instance of
(e.g., ``'NUMBER'``). The function can change the token type and
value as it sees appropriate. When finished, the resulting token
object should be returned as a result. If no value is returned by the
function, the token is simply discarded and the next token read.
function, the token is discarded and the next token read.
Internally, the ``Lexer`` class uses the ``re`` module to do its
pattern matching. Patterns are compiled using the ``re.VERBOSE`` flag
which can be used to help readability. However, be aware that
unescaped whitespace is ignored and comments are allowed in this mode.
If your pattern involves whitespace, make sure you use ``\s``. If you
need to match the ``#`` character, use ``[#]``.
Instead of using the ``@_()`` decorator, you can also write a method
that matches the same name as a token previously specified as a
string. For example::
Controlling Match Order
^^^^^^^^^^^^^^^^^^^^^^^
Tokens are matched in the same order as patterns are listed in the
``Lexer`` class. Be aware that longer tokens may need to be specified
before short tokens. For example, if you wanted to have separate
tokens for "=" and "==", you need to make sure that "==" is listed
first. For example::
class MyLexer(Lexer):
tokens = ('ASSIGN', 'EQUALTO', ...)
ID = r'[a-zA-Z_][a-zA-Z0-9_]*'
...
EQUALTO = r'==' # MUST APPEAR FIRST!
ASSIGN = r'='
To handle reserved words, you should write a single rule to match an
identifier and do a special name lookup in a function like this::
class MyLexer(Lexer):
reserved = { 'if', 'then', 'else', 'while' }
tokens = ['LPAREN','RPAREN',...,'ID'] + [ w.upper() for w in reserved ]
@_(r'[a-zA-Z_][a-zA-Z_0-9]*')
def ID(self, t):
# Check to see if the name is a reserved word
# If so, change its type.
if t.value in self.reserved:
reserved = { 'if', 'else', 'while', 'for' }
if t.value in reserved:
t.type = t.value.upper()
return t
Note: You should avoid writing individual rules for reserved words.
For example, suppose you wrote rules like this::
FOR = r'for'
PRINT = r'print'
In this case, the rules will be triggered for identifiers that include
those words as a prefix such as "forget" or "printed".
This is probably not what you want.
Discarded text
^^^^^^^^^^^^^^
To discard text, such as a comment, simply define a token rule that returns no value. For example::
@_(r'\#.*')
def COMMENT(self, t):
pass
# No return value. Token discarded
Alternatively, you can include the prefix ``ignore_`` in a token
declaration to force a token to be ignored. For example::
ignore_COMMENT = r'\#.*'
This is potentially useful trick for debugging a lexer. You can temporarily
attach a method to fire when a token is seen and take it away later without
changing any existing part of the lexer class.
Line numbers and position tracking
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@ -261,12 +278,12 @@ Line numbers and position tracking
By default, lexers know nothing about line numbers. This is because
they don't know anything about what constitutes a "line" of input
(e.g., the newline character or even if the input is textual data).
To update this information, you need to write a special rule. In the
example, the ``newline()`` rule shows how to do this::
To update this information, you need to add a special rule for newlines.
Promote the ``ignore_newline`` token to a method like this::
# Define a rule so we can track line numbers
@_(r'\n+')
def newline(self, t):
def ignore_newline(self, t):
self.lineno += len(t.value)
Within the rule, the lineno attribute of the lexer is updated. After
@ -293,34 +310,15 @@ Since column information is often only useful in the context of error
handling, calculating the column position can be performed when needed
as opposed to including it on each token.
Ignored characters
^^^^^^^^^^^^^^^^^^
The special ``ignore`` specification is reserved for characters that
should be completely ignored in the input stream. Usually this is
used to skip over whitespace and other non-essential characters.
Although it is possible to define a regular expression rule for
whitespace in a manner similar to ``newline()``, the use of ``ignore``
provides substantially better lexing performance because it is handled
as a special case and is checked in a much more efficient manner than
the normal regular expression rules.
The characters given in ``ignore`` are not ignored when such
characters are part of other regular expression patterns. For
example, if you had a rule to capture quoted text, that pattern can
include the ignored characters (which will be captured in the normal
way). The main purpose of ``ignore`` is to ignore whitespace and
other padding between the tokens that you actually want to parse.
Literal characters
^^^^^^^^^^^^^^^^^^
Literal characters can be specified by defining a variable
Literal characters can be specified by defining a set
``literals`` in the class. For example::
class MyLexer(Lexer):
...
literals = [ '+','-','*','/' ]
literals = { '+','-','*','/' }
...
A literal character is simply a single character that is returned "as
@ -337,30 +335,32 @@ appropriately. For example::
class MyLexer(Lexer):
literals = [ '{', '}' ]
literals = { '{', '}' }
def __init__(self):
self.indentation_level = 0
self.nesting_level = 0
@_(r'\{')
def lbrace(self, t):
t.type = '{' # Set token type to the expected literal
self.indentation_level += 1
self.nesting_level += 1
return t
@_(r'\}')
def rbrace(t):
t.type = '}' # Set token type to the expected literal
self.indentation_level -=1
self.nesting_level -=1
return t
Error handling
^^^^^^^^^^^^^^
The ``error()`` method is used to handle lexing errors that occur when
illegal characters are detected. The error method receives a string
containing all remaining untokenized text. A typical handler might
look at this text and skip ahead in some manner. For example::
If a bad character is encountered while lexing, tokenizing will stop.
However, you can add an ``error()`` method to handle lexing errors
that occur when illegal characters are detected. The error method
receives a string containing all remaining untokenized text. A
typical handler might look at this text and skip ahead in some manner.
For example::
class MyLexer(Lexer):
...
@ -375,28 +375,120 @@ parser is often a hard problem. An error handler might scan ahead
to a logical synchronization point such as a semicolon, a blank line,
or similar landmark.
Maintaining extra state
A More Complete Example
^^^^^^^^^^^^^^^^^^^^^^^
In your lexer, you may want to maintain a variety of other state
information. This might include mode settings, symbol tables, and
other details. As an example, suppose that you wanted to keep track
of how many NUMBER tokens had been encountered. You can do this by
adding an ``__init__()`` method and adding more attributes. For
example::
Here is a more complete example that puts many of these concepts
into practice::
class MyLexer(Lexer):
def __init__(self):
self.num_count = 0
# calclex.py
from sly import Lexer
class CalcLexer(Lexer):
reserved_words = { 'WHILE', 'PRINT' }
# Set of token names. This is always required
tokens = {
'NUMBER',
'ID',
'PLUS',
'MINUS',
'TIMES',
'DIVIDE',
'ASSIGN',
'EQ',
'LT',
'LE',
'GT',
'GE',
'NE',
*reserved_words,
}
literals = { '(', ')', '{', '}', ';' }
# String containing ignored characters
ignore = ' \t'
# Regular expression rules for tokens
PLUS = r'\+'
MINUS = r'-'
TIMES = r'\*'
DIVIDE = r'/'
EQ = r'=='
ASSIGN = r'='
LE = r'<='
LT = r'<'
GE = r'>='
GT = r'>'
NE = r'!='
@_(r'\d+')
def NUMBER(self,t):
self.num_count += 1
def NUMBER(self, t):
t.value = int(t.value)
return t
Please note that lexers already use the ``lineno`` and ``position``
attributes during parsing.
@_(r'[a-zA-Z_][a-zA-Z0-9_]*')
def ID(self, t):
if t.value.upper() in self.reserved_words:
t.type = t.value.upper()
return t
ignore_comment = r'\#.*'
# Line number tracking
@_(r'\n+')
def ignore_newline(self, t):
self.lineno += t.value.count('\n')
def error(self, value):
print('Line %d: Bad character %r' % (self.lineno, value[0]))
self.index += 1
if __name__ == '__main__':
data = '''
# Counting
x = 0;
while (x < 10) {
print x:
x = x + 1;
}
'''
lexer = CalcLexer()
for tok in lexer.tokenize(data):
print(tok)
If you run this code, you'll get output that looks like this::
Token(ID, 'x', 3, 12)
Token(ASSIGN, '=', 3, 14)
Token(NUMBER, 0, 3, 16)
Token(;, ';', 3, 17)
Token(WHILE, 'while', 4, 19)
Token((, '(', 4, 25)
Token(ID, 'x', 4, 26)
Token(LT, '<', 4, 28)
Token(NUMBER, 10, 4, 30)
Token(), ')', 4, 32)
Token({, '{', 4, 34)
Token(PRINT, 'print', 5, 40)
Token(ID, 'x', 5, 46)
Line 5: Bad character ':'
Token(ID, 'x', 6, 53)
Token(ASSIGN, '=', 6, 55)
Token(ID, 'x', 6, 57)
Token(PLUS, '+', 6, 59)
Token(NUMBER, 1, 6, 61)
Token(;, ';', 6, 62)
Token(}, '}', 7, 64)
Study this example closely. It might take a bit to digest, but all of the
essential parts of writing a lexer are there. Tokens have to be specified
with regular expression rules. You can optionally attach actions that
execute when certain patterns are encountered. Certain features such as
character literals might make it easier to lex certain kinds of text.
You can also add error handling.
Writing a Parser
----------------