getattrib, Setattrib, hasattrib and delattrib in python

# `getattr(object, name[, default])` Function in Python

The `getattr(object, name[, default])` function returns the value of a named attribute of an object, where `name` must be a string. If the object has an attribute with the specified `name`, then the value of that attribute is returned. On the other hand, if the object does not have an attribute with `name`, then the value of `default` is returned, or `AttributeError` is raised if `default` is not provided.

```python

>>> t = ('This', 'is', 'a', 'tuple')

>>> t.index('is')

1

>>> getattr(t, 'index')

<built-in method index of tuple object at 0x10c15e680>

>>> getattr(t, 'index')('is')

1

```

when the attribute is not defined,

```python

>>> getattr(t, 'len')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'tuple' object has no attribute 'len'
>>> getattr(t, 'len', t.count)
<built-in method count of tuple object at 0x10c15e680>

```

Besides "normal" objects like tuples, lists, and class instances, getattr also accepts modules as arguments. Since modules are also objects in Python, the attributes of modules can be retrieved just like any attribute in an object.
>>> import uuid
>>> getattr(uuid, 'UUID')
<class 'uuid.UUID'>
>>> type(getattr(uuid, 'UUID'))
<type 'type'>
>>> isinstance(getattr(uuid, 'UUID'), type)
True
>>> callable(getattr(uuid, 'UUID'))
True
>>> getattr(uuid, 'UUID')('12345678123456781234567812345678')
UUID('12345678-1234-5678-1234-567812345678')

*****

# check for existence of an attribute
# hasattr  #uses pythonic "Look Before You Leap" (LBYL) code style

>>> hasattr('abc', 'upper')
True
>>> hasattr('abc', 'lower')
True
>>> hasattr('abc', 'convert')
False

Using try-except way   # "Easier to Ask for Forgiveness than Permission" (EAFP) code style

>>> try:
...     'abc'.upper()
... except AttributeError:
...     print("abc does not have attribute 'upper'")
...
'ABC'
>>> try:
...     'abc'.convert()
... except AttributeError:
...     print("abc does not have attribute 'convert'")
...
abc does not have attribute 'convert'

**********

hasattr vs __dict__

>>> class A(object):
...   foo = 1
...
>>> class B(A):
...   pass
...
>>> b = B()
>>> hasattr(b, 'foo')
True
>>> 'foo' in b.__dict__
False

#setattrib --Assigns a value to the object’s attribute given its name.

EX: setattr(x, ‘foobar’, 123) is equivalent to x.foobar = 123

Example 1

>>> class Foo:
...     def __init__(self, x):
...         self.x = x
...
>>> f = Foo(10)
>>> f.x
10
>>> setattr(f, 'x', 20)
>>> f.x
20
>>> setattr(f, 'y', 10)
>>> f.y
10
>>> f.y = 100
>>> f.y
100

you can dynamically add a function as a method to a class

>>> def b(self):
            print 'bar'

class Foo:
    pass
f = Foo()
print dir(f) #[‘__doc__’, ‘__module__’]
setattr(Foo, ‘bar’, b)
print dir(f) #[‘__doc__’, ‘__module__’, ‘bar’]
f.bar() #bar

you can setattr() on an instance of a class that
inherits from "object", but you can't on an instance of "object"
itself

>>> o = object()
>>> setattr(o, "x", 1000)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute 'x'

>>> class Object(object):pass
...
>>> o = Object()
>>> setattr(o, "x", 1000)
>>> o.x
1000

If __setattr__() wants to assign to an instance attribute,
 it should not simply execute self.name = value — this would
 cause a recursive call to itself. Instead, it should insert
 the value in the dictionary of instance attributes,
 e.g., self.__dict__[name] = value.
 For new-style classes, rather than accessing the instance dictionary,
 it should call the base class method with the same name,
 for example, object.__setattr__(self, name, value).

 ****for blog****
The __slots__ declaration takes a sequence of instance variables and
 reserves just enough space in each instance to hold a value for each
 variable. Space is saved because __dict__ is not created for each instance.

 When there is '__slots__', there won't be '__dict__' and '__weakref__'


delattr will delete the attribute

Example:

class Box:
    pass

box = Box()

# Create a width attribute.
setattr(box, "width", 15)

# The attribute exists.
if hasattr(box, "width"):
    print(True)

# Delete the width attribute.
delattr(box, "width")

# Width no longer exists.
if not hasattr(box, "width"):
    print(False)