##Appendix 1: How to Call Magic Methods##
Some of the magic methods in Python directly map to built-in functions; in this case, how to invoke them is fairly obvious. However, in other cases, the invocation is far less obvious. This appendix is devoted to exposing non-obvious syntax that leads to magic methods getting called.
| Magic Method | When it gets invoked (example) | Explanation |
|---|---|---|
__new__(cls [,...]) |
instance = MyClass(arg1, arg2) |
__new__ is called on instance creation |
__init__(self [,...]) |
instance = MyClass(arg1, arg2) |
__init__ is called on instance creation |
__cmp__(self, other) |
self == other, self > other, etc. |
Called for any comparison |
__pos__(self) |
+self |
Unary plus sign |
__neg__(self) |
-self |
Unary minus sign |
__invert__(self) |
~self |
Bitwise inversion |
__index__(self) |
x[self] |
Conversion when object is used as index |
__nonzero__(self) |
bool(self) |
Boolean value of the object |
__getattr__(self, name) |
self.name # name doesn't exist |
Accessing nonexistent attribute |
__setattr__(self, name, val) |
self.name = val |
Assigning to an attribute |
__delattr__(self, name) |
del self.name |
Deleting an attribute |
__getattribute__(self, name) |
self.name |
Accessing any attribute |
__getitem__(self, key) |
self[key] |
Accessing an item using an index |
__setitem__(self, key, val) |
self[key] = val |
Assigning to an item using an index |
__delitem__(self, key) |
del self[key] |
Deleting an item using an index |
__iter__(self) |
for x in self |
Iteration |
__contains__(self, value) |
value in self, value not in self |
Membership tests using in |
__call__(self [,...]) |
self(args) |
"Calling" an instance |
__enter__(self) |
with self as x: |
with statement context managers |
__exit__(self, exc, val, trace) |
with self as x: |
with statement context managers |
__getstate__(self) |
pickle.dump(pkl_file, self) |
Pickling |
__setstate__(self) |
data = pickle.load(pkl_file) |
Pickling |
Hopefully, this table should have cleared up any questions you might have had about what syntax invokes which magic method.
##Appendix 2: Changes in Python 3##
Here, we document a few major places where Python 3 differs from 2.x in terms of its object model:
- Since the distinction between string and unicode has been done away with in Python 3,
__unicode__is gone and__bytes__(which behaves similarly to__str__and__unicode__in 2.7) exists for a new built-in for constructing byte arrays. - Since division defaults to true division in Python 3,
__div__is gone in Python 3 __coerce__is gone due to redundancy with other magic methods and confusing behavior__cmp__is gone due to redundancy with other magic methods__nonzero__has been renamed to__bool__