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Notes/04_Classes_objects/03_Special_methods.md
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[Contents](../Contents) \| [Previous (4.2 Inheritance)](02_Inheritance) \| [Next (4.4 Exceptions)](04_Defining_exceptions)
# 4.3 Special Methods
Various parts of Python's behavior can be customized via special or magic methods.
This section introduces that idea.
Various parts of Python's behavior can be customized via special or so-called "magic" methods.
This section introduces that idea. In addition dynamic attribute access and bound methods
are discussed.
### Introduction
Classes may define special methods. These have special meaning to the Python interpreter.
They are always preceded and followed by `__`. For example `__init__`.
Classes may define special methods. These have special meaning to the
Python interpreter. They are always preceded and followed by
`__`. For example `__init__`.
```python
class Stock(object):
@@ -49,7 +53,8 @@ for programmers.
>>>
```
Those functions, `str()` and `repr()`, use a pair of special methods in the class to get the string to be printed.
Those functions, `str()` and `repr()`, use a pair of special methods
in the class to produce the string to be displayed.
```python
class Date(object):
@@ -68,19 +73,19 @@ class Date(object):
```
*Note: The convention for `__repr__()` is to return a string that,
when fed to `eval()`., will recreate the underlying object. If this
when fed to `eval()`, will recreate the underlying object. If this
is not possible, some kind of easily readable representation is used
instead.*
### Special Methods for Mathematics
Mathematical operators are just calls to special methods.
Mathematical operators involve calls to the following methods.
```python
a + b a.__add__(b)
a - b a.__sub__(b)
a * b a.__mul__(b)
a / b a.__div__(b)
a / b a.__truediv__(b)
a // b a.__floordiv__(b)
a % b a.__mod__(b)
a << b a.__lshift__(b)
@@ -108,7 +113,7 @@ del x[a] x.__delitem__(a)
You can use them in your classes.
```python
class Sequence(object):
class Sequence:
def __len__(self):
...
def __getitem__(self,a):
@@ -204,33 +209,6 @@ x = getattr(obj, 'x', None)
### Exercise 4.9: Better output for printing objects
All Python objects have two string representations. The first
representation is created by string conversion via `str()`
(which is called by `print`). The string representation is
usually a nicely formatted version of the object meant for humans.
The second representation is a code representation of the object
created by `repr()` (or by viewing a value in the
interactive shell). The code representation typically shows you the
code that you have to type to get the object.
The two representations of an object are often different. For example,
you can see the difference by trying the following:
```python
>>> from datetime import date
>>> d = date(2017, 4, 9)
>>> print(d) # Nice output
2017-04-09
>>> print(repr(d)) # Representation output
datetime.date(2017, 4, 9)
>>> print(f'{d!r}') # Alternate way to get repr() string
datetime.date(2017, 4, 9)
>>>
```
Both kinds of string conversions can be redefined in a class if it
defines the `__str__()` and `__repr__()` methods.
Modify the `Stock` object that you defined in `stock.py`
so that the `__repr__()` method produces more useful output. For
example: