def parse(cls, w):
"""
This is the usual way to instantiate an interface expression
from a string.
The most basic interface expression is the empty expression,
to which all other expressions are super. It is represented
as an empty union set using square brackets.
>>> Expr.parse('[]')
[]
Parsing an empty string will just return the integer 0
instead of an expression object.
>>> nothing = Expr.parse('')
>>> nothing
0
>>> nothing.__class__.__name__
'int'
"""
if isinstance(w, Expr): return w
if re.match(r'^\s*(?:0)?\s*$', w): return 0
expr = cls.get(w)
if expr is None:
tree = Rule.expr(w)
expr = cls.from_tree(tree)
cls.set(w, expr)
return expr
def render(w, g):
'''
Render expression w to graph g.
'''
try:
tree = Rule.expr(w)
except Exception, e:
g.add_node('invalid', label='%s' % e)
return
