pkg_resources.py 文件源码

def evaluate(cls, nodelist):
        while len(nodelist)==2: nodelist = nodelist[1]
        kind = nodelist[0]
        name = nodelist[1]
        if kind==token.NAME:
            try:
                op = cls.values[name]
            except KeyError:
                raise SyntaxError("Unknown name %r" % name)
            return op()
        if kind==token.STRING:
            s = nodelist[1]
            if s[:1] not in "'\"" or s.startswith('"""') or s.startswith("'''") \
                    or '\\' in s:
                raise SyntaxError(
                    "Only plain strings allowed in environment markers")
            return s[1:-1]
        raise SyntaxError("Language feature not supported in environment markers")