How to chain attribute lookups that might return None in Python?

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My problem is a general one, how to chain a series of attribute lookups when one of the intermediate ones might return None, but since I ran into this problem t

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栀梦

2021-01-07 17:28

This is how I handled it with inspiration from @TAS and Is there a Python library (or pattern) like Ruby's andand?

class Andand(object):
    def __init__(self, item=None):
        self.item = item

    def __getattr__(self, name):
        try:
            item = getattr(self.item, name)
            return item if name is 'item' else Andand(item)
        except AttributeError:
            return Andand()     

    def __call__(self):
        return self.item


title = Andand(soup).head.title.string()

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梦如初夏

2021-01-07 17:36

Here is another potential technique, which hides the assignment of the intermediate value in a method call. First we define a class to hold the intermediate value:

class DataHolder(object):
    def __init__(self, value = None):
            self.v = value

    def g(self):
            return self.v

    def s(self, value):
            self.v = value
            return value

x = DataHolder(None)

Then we get use it to store the result of each link in the chain of calls:

import bs4;

for html in ('<html><head></head><body></body></html>',
             '<html><head><title>Foo</title></head><body></body></html>'):
    soup = bs4.BeautifulSoup(html)
    print x.s(soup.head) and x.s(x.g().title) and x.s(x.g().string)
    # or
    print x.s(soup.head) and x.s(x.v.title) and x.v.string

I don't consider this a good solution, but I'm including it here for completeness.

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野的像风

2021-01-07 17:39

You might be able to use reduce for this:

>>> class Foo(object): pass
... 
>>> a = Foo()
>>> a.foo = Foo()
>>> a.foo.bar = Foo()
>>> a.foo.bar.baz = Foo()
>>> a.foo.bar.baz.qux = Foo()
>>> 
>>> reduce(lambda x,y:getattr(x,y,''),['foo','bar','baz','qux'],a)
<__main__.Foo object at 0xec2f0>
>>> reduce(lambda x,y:getattr(x,y,''),['foo','bar','baz','qux','quince'],a)
''

In python3.x, I think that reduce is moved to functools though :(

I suppose you could also do this with a simpler function:

def attr_getter(item,attributes)
    for a in attributes:
        try:
            item = getattr(item,a)
        except AttributeError:
            return None #or whatever on error
    return item

Finally, I suppose the nicest way to do this is something like:

try:
   title = foo.bar.baz.qux
except AttributeError:
   title = None

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栀梦

2021-01-07 17:39
The most straightforward way is to wrap in a try...except block.
```
try:
    title = soup.head.title.string
except AttributeError:
    print "Title doesn't exist!"
```
There's really no reason to test at each level when removing each test would raise the same exception in the failure case. I would consider this idiomatic in Python.
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走了就别回头了

2021-01-07 17:41
My best shoot to handle middle-way null attributes like this is to use pydash as sample code on repl.it here
```
import pydash
title = pydash.get(soup, 'head.title.string', None)
```
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予麋鹿

2021-01-07 17:48

One solution would be to wrap the outer object inside a Proxy that handles None values for you. See below for a beginning implementation.

import unittest

class SafeProxy(object):

    def __init__(self, instance):
        self.__dict__["instance"] = instance

    def __eq__(self, other):
        return self.instance==other

    def __call__(self, *args, **kwargs):
        return self.instance(*args, **kwargs)

    # TODO: Implement other special members

    def __getattr__(self, name):
        if hasattr(self.__dict__["instance"], name):
            return SafeProxy(getattr(self.instance, name))

        if name=="val":
            return lambda: self.instance

        return SafeProxy(None)

    def __setattr__(self, name, value):
        setattr(self.instance, name, value)


# Simple stub for creating objects for testing
class Dynamic(object):
    def __init__(self, **kwargs):
        for name, value in kwargs.iteritems():
            self.__setattr__(name, value)

    def __setattr__(self, name, value):
        self.__dict__[name] = value


class Test(unittest.TestCase):

    def test_nestedObject(self):
        inner = Dynamic(value="value")
        middle = Dynamic(child=inner)
        outer = Dynamic(child=middle)
        wrapper = SafeProxy(outer)
        self.assertEqual("value", wrapper.child.child.value)
        self.assertEqual(None, wrapper.child.child.child.value)

    def test_NoneObject(self):
        self.assertEqual(None, SafeProxy(None))

    def test_stringOperations(self):
        s = SafeProxy("string")
        self.assertEqual("String", s.title())
        self.assertEqual(type(""), type(s.val()))
        self.assertEqual()

if __name__=="__main__":
    unittest.main()

NOTE: I am personally not sure wether I would use this in an actual project, but it makes an interesting experiment and I put it here to get people thoughts on this.

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