jnbbender jnbbender - 5 months ago 13
Python Question

Dynamically creating a class from file in Python

I've seen these "Dynamically create a class" questions which are answered saying, "use the type() function". I'm sure I'll have to at some point but right know I'm clueless. But from what I've seen you have to already know something about the class, such as a name.

What I'm trying to do is parse an idl type of file and from that create a class which will have methods and attributes. So I have NO knowledge up front of what the class name, functions, arguments or anything will be until I parse the string.

Any ideas?



It's a bit hard to Google for, but you can search for python type(name, bases, dict) function examples to get:


An excerpt from the above, which gets to the heart of your question:

The following are basically equivalent:

def __init__(self, x):
    self.x = x

def printX(self):
    print self.x

Test = type('Test', (object,), {'__init__': __init__, 'printX': printX})


class Test(object):
    def __init__(self, x):
        self.x = x

    def printX(self):
        print self.x

There are two ways to create functions on the fly that I can think of. The usually-bad way is to write the code and reparse it (though done correctly, this can greatly increase performance). The sane way is to implement a function which interprets your IDL. This is called a higher-order function: http://effbot.org/pyfaq/how-do-you-make-a-higher-order-function-in-python.htm

An example of what you would write, if you cannot find an interpreter for your IDL (of if it's a custom IDL) is something like the link above, such as:

def makeMethod(idlCode):
    syntax = MyIDL.parse(idlCode)

    def newMethod(*args, **kw):
        if syntax.statementType == MyIDL.IF_STATEMENT:
            if secureLookup(mySyntaxTree.IF):
               return secureLookup(args[0]) 
               return secureLookup(args[1])

    return (syntax.methodName, newMethod)

There are many more elegant ways to expand this method, if you set up a mapping between constructs of your IDL and the syntax of *args and **kw, but this gives you the most flexibility and is the most straightforward and basic way I could think of.

Then you'd do pass in:

class DynamicIdlClass(object):

for idlObject in idlCode:
    methods = dict(makeMethod(clause) for clause in idlObject.clauses})
    methods['__init__'] = makeInitMethod(idlObject.initClause)
    idlObject = type('Test', (DynamicIdlClass,), methods)

    yield idlObject  # or idlObjectsList.push(idlObject), etc.