Magic Methods

Question 1

__new__(cls, *args, **kwargs)

Answer 1

Called to create a new instance of a class
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• Static method (special case that doesn’t need to be declared as such)
• Takes the class as its first argument, the remaining arguments are those passed to the construction call
• Invoked during obj creation
• Returns an instance of the class, then the init method will be called where self is this new instance
• If it doesn’t return an instance of the class, init is not called

Question 2

__init__(self, *args, **kwargs)

Answer 2

Called after instance is created by __new__
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• Called after the instance has been created, but before it is returned to the caller
• The arguments are those passed to the construction call
• Shouldn’t return anything (ie return None)
• Customizes the object

Question 3

__del__(self)

Answer 3

Called when an instance is about to be destroyed
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• It is not guaranteed to call when interpreter exits
• del x doesn’t directly call x.__del__()
  
  • the former decrements the reference count for x by one
  • the latter is only called when x’s reference count reaches zero

Question 4

__repr__(self)

Answer 4

The “official” string representation of an object
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• Called by repr() (and in nested contexts)
• The return value must be a str object
• This should look like a valid Python expression that could be used to recreate an obj with the same value
• If not possible, provide a useful, information-rich description
• Used in informal contexts when __str__ is not provided

Question 5

__str__(self)

Answer 5

The “informal” string representation of an object
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• Called by str(), format(), and print()
• The return value must be a str object
• Doesn’t expect to return a valid Python expression, can be convenient or concise representation

Question 6

__bytes__(self)

Answer 6

Called by bytes() to compute a byte-string representation of an object, returning a bytes object

Question 7

__format__(self, format_spec)

Answer 7

The “formatted” string representation of an object
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• Called by format(), formatted str literals and str.format()
• The return value must be a str object
• format_spec is a string containing description of the formatting options desired (according to mini formatting language)

Question 8

The so-called “rich comparison” methods

Answer 8

\_\_lt\_\_(self, other)
\_\_le\_\_(self, other)
\_\_eq\_\_(self, other)
\_\_ne\_\_(self, other)
\_\_gt\_\_(self, other)
\_\_ge\_\_(self, other)
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

• For successful comparisons, False and True are returned
• If it does not implement the operation for a given pair of arguments, it may return the singleton NotImplemented

Question 9

__hash__(self)

Answer 9

Called by built-in hash() and for operations on members of hashed collections like set, frozen set, and dict
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• Should return an integer
• The only required property is that objects which compare equal have the same hash value
• It is advised to mix together the hash values of the components of the object that also play a part in comparison by packing them in a tuple and hashing the tuple:

def \_\_hash\_\_(self):
    return hash((self.name, self.nick, self.color))

• If a class doesn’t define __eq__ it shouldn’t define __hash__
• If defines __eq__ but not __hash__, can’t be used in washable collections
• If a class defines mutable obj and implements an __eq__ method, should not implement __hash__ (because washable collections require immutable keys)
• User defined classes have default eq and hash, all objects compare unequal and hash returns an appropriate value where if x == y implies x is y and hash(x) == hash(y)
• Overriding eq but not hash implicitly sets hash to None
  
  • Raises TypeError when hash is attempted
• If you don’t override eq but want to suppress hash, set __hash__ = None, not raise TypeError (will be erroneous on some calls)

Question 10

__bool__(self)

Answer 10

Called to implement truth value testing
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• Should return False or True
• When undefined, __len__ is called
  
  • If it is defined, nonzero result evaluates as True
  • If neither is defined, evaluates as True

Question 11

__getattr__(self, name)

Answer 11

Called when default attribute access fails with an AttributeError (when __getattribute__ or __get__ raises the exception)
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• Should return the attribute value or raise an AttributeError exception
• If found through normal lookup, getattr is not called
• Because it is only called when attribute is undefined, can fetch other attributes without recursion

Question 12

__getattribute__(self, name)

Answer 12

Called unconditionally to implement attribute accesses for instances of the class
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• Should return the attribute value or raise an AttributeError exception
• To fetch another attribute and avoid recursion, use superclass function object.__getattribute__(self, name)

Question 13

__setattr__(self, name, value)

Answer 13

Called when an attribute assignment is attempted
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• Called instead of normal mechanism of storing to the instance dict
• To set an attribute and avoid recursion, use superclass function object.__setattr__(self, name, value)
• You can also assign it to self.__dict__[name] = value

Question 14

__delattr__(self, name)

Answer 14

Called when attribute deletion is attempted
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• Use if del name should be meaningful
• Less often implemented, but requires similar superclass or dict functionality like setattr

Question 15

__dir__(self)

Answer 15

Called when dir() is called on the object, returning a sequence
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• dir() converts the returned sequence to a sorted list.

dir([object]):
Without arguments, return the list of names in the current local scope. With an argument, attempt to return a list of valid attributes for that object.

If the object has a method named __dir__(), this method will be called and must return the list of attributes. This allows objects that implement a custom __getattr__() or __getattribute__() function to customize the way dir() reports their attributes.

Question 16

__get__(self, instance, owner=None)

Answer 16

Descriptor - only applies when an instance of the descriptor class (where this method is defined) is used as an attribute in the owner class (appears in the class dictionary for the class or its parents)

Called to get the attribute of the owner class (or an instance of the owner class)

Question 17

__set__(self, instance, value)

Answer 17

Descriptor - only applies when an instance of the descriptor class (where this method is defined) is used as an attribute in the owner class (appears in the class dictionary for the class or its parents)

Called to set the attribute on an instance of the owner class to a new value

Question 18

__delete__(self, instance)

Answer 18

Descriptor - only applies when an instance of the descriptor class (where this method is defined) is used as an attribute in the owner class (appears in the class dictionary for the class or its parents)

Called to delete the attribute on an instance of the owner class

Question 19

__set_name__(self, owner, name)

Answer 19

Descriptor - only applies when an instance of the descriptor class (where this method is defined) is used as an attribute in the owner class (appears in the class dictionary for the class or its parents)

Called at the time the owning class is created, assigning the attribute name to the descriptor
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• Called implicitly, so when adding a descriptor after initial creation, must be passed explicitly

Question 20

__slots__

Answer 20

Not a method, a class variable that allows us to explicitly declare data members (like properties) and deny the creation of \_\_dict\_\_ (space saved can be significant, as well as attribute lookup speed) and \_\_weakref\_\_
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class MyClass:
    \_\_slots\_\_ = ['name', 'age']
    def \_\_init\_\_(self, name, age):
         self.name = name
         self. age = age

Question 21

__enter__(self)

Answer 21

Enter the runtime context related to this object

Question 22

__exit__(self, exc_type, exc_value, traceback)

Answer 22

Exit the runtime context related to this object
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• The parameters describe the exception that caused the context to be exited
• If the context exits w/o exception, they’ll all be None
• Can return True to suppress the exception, otherwise exception will be processed normally
• Should not reraise the passed-in exception

Question 23

__len__(self)

Answer 23

Called to implement len()
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• Should return an integer >= 0 that is the length of the object
• If __bool__ is undefined, truthfulness is determined as False when this method returns 0, and True if it returns any other length
  
  • If this method also not implemented, defaults to True

Question 24

__getitem__(self, key)

Answer 24

Called to implement evaluation of self[key] (indexing)
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• Is used for iteration context if iter/next are undefined
• Special interpretation of negative indexes can be specified
• For sequence types, should raise IndexError for values outside the set of indexes
• For mapping types, if key is missing, should raise KeyError
• This is for indexing, as opposed to __index__ which assumes object is an integer and returns one

Question 25

__setitem__(self, key, value)

Answer 25

Called to implement assignment of self[key] (indexing) _________________________________ - Only implement if object should be mutable - Same exceptions should be raised as getitem for improper values - IndexError if sequence type and immutable or out of bounds - KeyError if mapping type and immutable

Question 26

__delitem__(self, key)

Answer 26

Called to implement deletion of self[key] (indexing)

Question 27

__iter__(self)

Answer 27

This method is called when an iterator is required for a container _________________________________ - Should return a new iterator object that can iterate over all the objects in the container - For mappings, should iterate over the keys

Question 28

__reversed__(self)

Answer 28

Called by the reversed() built-in to implement reverse iteration _________________________________ - Should return a new iterator that iterates over all the objects in the container in reverse order - If not implemented, the built in will fall back on sequence protocol (using len and getitem) - only provide if you can implement a more efficient protocol

Question 29

__contains__(self, item)

Answer 29

Called to implement membership test operators _________________________________ - Should return True if item is in self, False otherwise - For sequence objects, considers values - For mapping objects, should just consider keys - For objects that don't define contains, membership test falls back on iter and then getitem - only provide if you can implement a more efficient protocol

Question 30

__call__(self)

Answer 30

Called when the instance is called as a function