4.14 其他方式定义词法规则
最后更新于：2022-04-01 01:09:59
上面的例子，词法分析器都是在单个的Python模块中指定的。如果你想将标记的规则放到不同的模块，使用module关键字参数。例如，你可能有一个专有的模块，包含了标记的规则：
~~~
# module: tokrules.py
# This module just contains the lexing rules
# List of token names. This is always required
tokens = (
'NUMBER',
'PLUS',
'MINUS',
'TIMES',
'DIVIDE',
'LPAREN',
'RPAREN',
)
# Regular expression rules for simple tokens
t_PLUS = r'\+'
t_MINUS = r'-'
t_TIMES = r'\*'
t_DIVIDE = r'/'
t_LPAREN = r'\('
t_RPAREN = r'\)'
# A regular expression rule with some action code
def t_NUMBER(t):
r'\d+'
t.value = int(t.value)
return t
# Define a rule so we can track line numbers
def t_newline(t):
r'\n+'
t.lexer.lineno += len(t.value)
# A string containing ignored characters (spaces and tabs)
t_ignore = ' \t'
# Error handling rule
def t_error(t):
print "Illegal character '%s'" % t.value[0]
t.lexer.skip(1)
~~~
现在，如果你想要从不同的模块中构建分析器，应该这样（在交互模式下）：
~~~
>>> import tokrules
>>> lexer = lex.lex(module=tokrules)
>>> lexer.input("3 + 4")
>>> lexer.token()
LexToken(NUMBER,3,1,1,0)
>>> lexer.token()
LexToken(PLUS,'+',1,2)
>>> lexer.token()
LexToken(NUMBER,4,1,4)
>>> lexer.token()
None
~~~
`module`选项也可以指定类型的实例，例如：
~~~
import ply.lex as lex
class MyLexer:
# List of token names. This is always required
tokens = (
'NUMBER',
'PLUS',
'MINUS',
'TIMES',
'DIVIDE',
'LPAREN',
'RPAREN',
)
# Regular expression rules for simple tokens
t_PLUS = r'\+'
t_MINUS = r'-'
t_TIMES = r'\*'
t_DIVIDE = r'/'
t_LPAREN = r'\('
t_RPAREN = r'\)'
# A regular expression rule with some action code
# Note addition of self parameter since we're in a class
def t_NUMBER(self,t):
r'\d+'
t.value = int(t.value)
return t
# Define a rule so we can track line numbers
def t_newline(self,t):
r'\n+'
t.lexer.lineno += len(t.value)
# A string containing ignored characters (spaces and tabs)
t_ignore = ' \t'
# Error handling rule
def t_error(self,t):
print "Illegal character '%s'" % t.value[0]
t.lexer.skip(1)
# Build the lexer
def build(self,**kwargs):
self.lexer = lex.lex(module=self, **kwargs)
# Test it output
def test(self,data):
self.lexer.input(data)
while True:
tok = lexer.token()
if not tok: break
print tok
# Build the lexer and try it out
m = MyLexer()
m.build() # Build the lexer
m.test("3 + 4") # Test it
~~~
当从类中定义lexer，你需要创建类的实例，而不是类本身。这是因为，lexer的方法只有被绑定（bound-methods）对象后才能使PLY正常工作。
当给lex()方法使用module选项时，PLY使用`dir()`方法，从对象中获取符号信息，因为不能直接访问对象的`__dict__`属性。（译者注：可能是因为兼容性原因，__dict__这个方法可能不存在）
最后，如果你希望保持较好的封装性，但不希望什么东西都写在类里面，lexers可以在闭包中定义，例如：
~~~
import ply.lex as lex
# List of token names. This is always required
tokens = (
'NUMBER',
'PLUS',
'MINUS',
'TIMES',
'DIVIDE',
'LPAREN',
'RPAREN',
)
def MyLexer():
# Regular expression rules for simple tokens
t_PLUS = r'\+'
t_MINUS = r'-'
t_TIMES = r'\*'
t_DIVIDE = r'/'
t_LPAREN = r'\('
t_RPAREN = r'\)'
# A regular expression rule with some action code
def t_NUMBER(t):
r'\d+'
t.value = int(t.value)
return t
# Define a rule so we can track line numbers
def t_newline(t):
r'\n+'
t.lexer.lineno += len(t.value)
# A string containing ignored characters (spaces and tabs)
t_ignore = ' \t'
# Error handling rule
def t_error(t):
print "Illegal character '%s'" % t.value[0]
t.lexer.skip(1)
# Build the lexer from my environment and return it
return lex.lex()
~~~