Ruby’s “method_missing” in Python

Question

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Python equivalent of Ruby's 'method_missing'

Is there any technique available in Python for intercepting messages (method calls) like the method_missing technique in Ruby?

Answer 1

As others have mentioned, in Python, when you execute o.f(x), it's really a two-step operation: First, get the f attribute of o, then call it with parameter x. It's the first step that fails because there is no attribute f, and it's that step that invokes the Python magic method __getattr__.

So you have to implement __getattr__, and what it returns must be callable. Keep in mind, if you also try to get o.some_data_that_doesnt_exist, the same __getattr__ will be called, and it won't know that it's a "data" attribute vs. a "method" that being sought.

Here's an example of returning a callable:

class MyRubylikeThing(object):
    #...

    def __getattr__(self, name):
        def _missing(*args, **kwargs):
            print "A missing method was called."
            print "The object was %r, the method was %r. " % (self, name)
            print "It was called with %r and %r as arguments" % (args, kwargs)
        return _missing

r = MyRubylikeThing()
r.hello("there", "world", also="bye")

produces:

A missing method was called.
The object was <__main__.MyRubylikeThing object at 0x01FA5940>, the method was 'hello'.
It was called with ('there', 'world') and {'also': 'bye'} as arguments