In Python, data types (like int, float) both represent a value, but also have some built-in attributes/functions/etc:
In [1]: a = 1.2
In [2]: a
Out[2]: 1.2
In [3]: a.is_integer()
Out[3]: False
Is it possible to reproduce this behavior within Python, e.g. define a class:
class Scalar:
def __init__(self, value)
self.value = value
# other code ....
s = Scalar(1.2)
where I could have s return 1.2 (instead of typing s.value), and do things like a = s -> a = 1.2? The closest I can get to this behavior is adding something like:
def __getitem__(self, key=None):
return self.value
and using a = s[()], but that doesn't look very good.
where I could have s return 1.2 (instead of typing s.value)
In the console? Then implement the __repr__ method.
a = s -> a = 1.2
To avoid having to use a = s.value, you can implement __call__ and call the object:
>>> class Scalar:
... def __init__(self, value):
... self.value = value
... def __repr__(self):
... return str(self.value)
... def __call__(self):
... return self.value
...
>>> s = Scalar(1.2)
>>> s
1.2
>>> a = s()
>>> a
1.2
Check the documentation about the data model on emulating numeric types.
For example:
class Scalar:
def __init__(self, value):
self.value = value
def __repr__(self):
return str(self.value)
def __call__(self):
return self.value
def __add__(self, other):
return Scalar(self.value + other.value)
def __lt__(self, other):
return self.value < other.value
def ___le__(self, other):
return self.value <= other.value
def __eq__(self, other):
return self.value == other.value
def __ne__(self, other):
return self.value != other.value
def __gt__(self, other):
return self.value > other.value
def __ge__(self, other):
return self.value >= other.value
Can be used like this:
>>> s1 = Scalar(1.2)
>>> s2 = Scalar(2.1)
>>> s1 + s2
3.3
>>> s1 < s2
True
>>> s1 > s2
False
>>> s1 != s2
True
>>> s1 <= s2
True
>>> s1 >= s2
False
There are also the __int__ and __float__ magic methods, which you can implement and use like this (this is more semantically correct):
>>> a = int(s)
>>> a = float(s)
As far as I know, that's not possible for your a = s example. You would have to change the behavior of =, the assignment operator. The assignment operator doesn't really do anything to the object on the right, it just copies a reference to it (in the case of an object, at least).
In general, it is possible to change the behavior of built in operators for your custom classes using operator overloading, but Python doesn't provide this sort of option for assignment (=) because of how different it is from operators like addition (+) and even equality (==).
Related
I have a class in Python which is little more than the primitive values, like int or float, see below
class Entry:
def __init__(self, value, timestamp):
self.value = value
self.timestamp = timestamp
def __str__(self):
return"[v= {}, ts= {}]".format(self.value, self.timestamp)
def __hash__(self):
return hash(self.timestamp)
def __eq__(self, other):
return self.timestamp == other.timestamp
def __le__(self, other):
return self.timestamp <= other.timestamp
def __lt__(self, other):
return self.timestamp < other.timestamp
def __ge__(self, other):
return self.timestamp >= other.timestamp
def __gt__(self, other):
return self.timestamp > other.timestamp
def __copy__(self):
new_entry = Entry(deepcopy(self.value), self.timestamp)
print("hi")
return new_entry
e1 = Entry("some name", 10)
e2 = e1
e2.timestamp = 20
print(e1)
I want it to behave just like the primitive types as well. So when an assignment occurs, like above, the value is deep-copied, so I don't have to think about doing it manually everywhere I do assigment like this.
As you can see, I tried overriding the __copy__ method. Unfortunely that method isn't called here. Is there another method to override? I'm pretty sure this can be accomplished in C++. Can it be done in Python too?
You can't override the = assignment operator in Python, because it isn't a "copy" operator. Instead it binds an object to a value. You can, however, use the copy module, as described here: https://docs.python.org/3/library/copy.html.
I'm trying to create an Enum subclass whose values use their definition order as their natural sort order, like in the example below:
#functools.total_ordering
class SelectionType(enum.Enum):
character = 'character'
word = 'word'
sentence = 'sentence'
paragraph = 'paragraph'
def __le__(self, other):
if not isinstance(other, SelectionType):
return NotImplemented
return self._positions[self] < self._positions[other]
SelectionType._positions = {x: i for i, x in enumerate(SelectionType)}
Is there a more direct way to get the position of an enum value in its definition order or otherwise a better way to do this?
If this is a pattern you need often, or if the values are important and cannot be replaced by numbers, make a custom Enum you can inherit from:
import enum
class ByDefinitionOrderEnum(enum.Enum):
def __init__(self, *args):
try:
# attempt to initialize other parents in the hierarchy
super().__init__(*args)
except TypeError:
# ignore -- there are no other parents
pass
ordered = len(self.__class__.__members__) + 1
self._order = ordered
def __ge__(self, other):
if self.__class__ is other.__class__:
return self._order >= other._order
return NotImplemented
def __gt__(self, other):
if self.__class__ is other.__class__:
return self._order > other._order
return NotImplemented
def __le__(self, other):
if self.__class__ is other.__class__:
return self._order <= other._order
return NotImplemented
def __lt__(self, other):
if self.__class__ is other.__class__:
return self._order < other._order
return NotImplemented
This allows you to keep any other value instead, while still sorting according to definition order.
class SelectionType(ByDefinitionOrderEnum):
character = 'character'
word = 'word'
sentence = 'sentence'
paragraph = 'paragraph'
and in use:
>>> SelectionType.word < SelectionType.sentence
True
>>> SelectionType.word.value < SelectionType.sentence.value
False
You could encode the positions as values. Use .name to get the name.
class SelectionType(enum.Enum):
character = 1
word = 2
sentence = 3
paragraph = 4
# copy the OrderedEnum recipe from https://docs.python.org/3/library/enum.html#orderedenum
def __lt__(self, other):
if self.__class__ is other.__class__:
return self.value < other.value
return NotImplemented
>>> SelectionType.word.name
'word'
>>> SelectionType.word < SelectionType.sentence
True
On Python 3.6+ you could use enum.auto() to avoid hard-coding the positions.
class SelectionType(enum.Enum):
character = enum.auto()
word = enum.auto()
sentence = enum.auto()
paragraph = enum.auto()
I'm just trying to make an Enum in Python 3 by reference of the official Python docs https://docs.python.org/3.4/library/enum.html and specifically 8.13.13.2 and 8.13.13.4 examples.
My target is having an Enum which I can iterate, compare and also having three separate attributes. But I keep finding this error:
AttributeError: can't set attribute
It seems an error in __init__() constructor.
Code:
I tried firstly with one only class like this:
class Hand(Enum):
FIVE_OF_KIND = (6,'FIVE_OF_KIND',[5])
FOUR_OF_KIND = (5,'FOUR_OF_KIND',[4,1])
FULL_HOUSE = (4,'FULL_HOUSE',[3,2])
THREE_OF_KIND = (3,'THREE_OF_KIND',[3,1,1])
DOUBLE_PAIR = (2,'DOUBLE_PAIR',[2,2,1])
PAIR = (1,'PAIR',[2,1,1,1])
NOTHING = (0,'NOTHING',[1,1,1,1,1])
def __init__(self, val, name, struct):
self.val = val
self.name = name
self.struct = struct
def __ge__(self, other):
if self.__class__ is other.__class__:
return self.value >= other.value
return NotImplemented
def __gt__(self, other):
if self.__class__ is other.__class__:
return self.value > other.value
return NotImplemented
def __le__(self, other):
if self.__class__ is other.__class__:
return self.value <= other.value
return NotImplemented
def __lt__(self, other):
if self.__class__ is other.__class__:
return self.value < other.value
return NotImplemented
and secondly with two classes like this:
class OrderedEnum(Enum):
def __ge__(self, other):
if self.__class__ is other.__class__:
return self.value >= other.value
return NotImplemented
def __gt__(self, other):
if self.__class__ is other.__class__:
return self.value > other.value
return NotImplemented
def __le__(self, other):
if self.__class__ is other.__class__:
return self.value <= other.value
return NotImplemented
def __lt__(self, other):
if self.__class__ is other.__class__:
return self.value < other.value
return NotImplemented
class Hand(OrderedEnum):
FIVE_OF_KIND = (6,'FIVE_OF_KIND',[5])
FOUR_OF_KIND = (5,'FOUR_OF_KIND',[4,1])
FULL_HOUSE = (4,'FULL_HOUSE',[3,2])
THREE_OF_KIND = (3,'THREE_OF_KIND',[3,1,1])
DOUBLE_PAIR = (2,'DOUBLE_PAIR',[2,2,1])
PAIR = (1,'PAIR',[2,1,1,1])
NOTHING = (0,'NOTHING',[1,1,1,1,1])
def __init__(self, val, name, struct):
self.val = val
self.name = name
self.struct = struct
Enum objects already have a name attribute (for example, see 8.13.13.3), and apparently you are not allowed to set it – which makes sense when you think about how an enum should behave. You can achieve what you want like this:
from enum import Enum
class OrderedEnum(Enum):
# Same as your code.
class Hand(OrderedEnum):
FIVE_OF_KIND = (6, [5])
FOUR_OF_KIND = (5, [4,1])
FULL_HOUSE = (4, [3,2])
THREE_OF_KIND = (3, [3,1,1])
DOUBLE_PAIR = (2, [2,2,1])
PAIR = (1, [2,1,1,1])
NOTHING = (0, [1,1,1,1,1])
def __init__(self, val, struct):
# No need to set self.name. It's already handled.
self.val = val
self.struct = struct
for h in Hand:
print((h.name, h.val, h.struct))
I've run into some confusing behaviour of the magic comparison methods.
Suppose we have the following class:
class MutNum(object):
def __init__ (self, val):
self.val = val
def setVal(self, newval):
self.val = newval
def __str__(self):
return str(self.val)
def __repr__(self):
return str(self.val)
# methods for comparison with a regular int or float:
def __eq__(self, other):
return self.val == other
def __gt__(self, other):
return self.val > other
def __lt__(self, other):
return self.val < other
def __ge__(self, other):
return self.__gt__(other) or self.__eq__(other)
def __le__(self, other):
return self.__lt__(other) or self.__eq__(other)
The class does what it is supposed to do, comparing a MutNum object to a regular int or float is no problem. However, and this is what I don't understand, it even compares fine when the magic methods are given two MutNum objects.
a = MutNum(42)
b = MutNum(3)
print(a > b) # True
print(a >= b) # True
print(a < b) # False
print(a <= b) # False
print(a == b) # False
Why does this work? Thanks.
It evaluates as follows (using a repr-like notation instead of referring to variables):
MutNum(42) > MutNum(3)
=> MutNum(42).__gt__(MutNum(3))
=> MutNum(42).val > MutNum(3)
=> 42 > MutNum(3)
And from there, it's just the int-MutNum comparision you already know works.
If you throw in some print's and/or sys.stderr.write's, I think you'll see what's happening. EG:
def __gt__(self, other):
sys.stderr.write('__gt__\n')
sys.stderr.write('{}\n'.format(type(other)))
sys.stderr.write('{} {}\n'.format(self.val, other))
result = self.val > other
sys.stderr.write('result {}\n'.format(result))
return result
def __lt__(self, other):
sys.stderr.write('__lt__\n')
sys.stderr.write('{}\n'.format(type(other)))
sys.stderr.write('{} {}\n'.format(self.val, other))
result = self.val < other
sys.stderr.write('result {}\n'.format(result))
return result
When you try to compare self.val (an int) to other (a MutNum), python realizes it has nothing for comparing an int to a MutNum, and reverses the order of the comparison, and compares a MutNum to an int - which is something you've defined. That is, a single > comparison is doing the > as you'd expect, but it's also doing a <.
class C(object):
def __init__(self, value):
self.value = value
def __add__(self, other):
if isinstance(other, C):
return self.value + other.value
if isinstance(other, Number):
return self.value + other
raise Exception("error")
c = C(123)
print c + c
print c + 2
print 2 + c
obviously, the first two print statements will work and the third one fails because int.add() cannot deal with a class C instance.
246
125
print 2 + c
TypeError: unsupported operand type(s) for +: 'int' and 'C'
Is there a way to get around this, so 2+c will cause C.add() to be called?
You need to add __radd__ as well to handle the reverse case:
def __radd__(self, other):
if isinstance(other, C):
return other.value + self.value
if isinstance(other, Number):
return other + self.value
return NotImplemented
and note that you should not raise an exception; return the NotImplemented singleton instead. That way the other object could still try to support __add__ or __radd__ for your object and would be given a chance to implement addition too.
When you try to add two types a and b, Python first tries to call a.__add__(b); if that call returns NotImplemented, b.__radd__(a) is attempted instead.
Demo:
>>> from numbers import Number
>>> class C(object):
... def __init__(self, value):
... self.value = value
... def __add__(self, other):
... print '__add__ called'
... if isinstance(other, C):
... return self.value + other.value
... if isinstance(other, Number):
... return self.value + other
... return NotImplemented
... def __radd__(self, other):
... print '__radd__ called'
... if isinstance(other, C):
... return other.value + self.value
... if isinstance(other, Number):
... return other + self.value
... return NotImplemented
...
>>> c = C(123)
>>> c + c
__add__ called
246
>>> c + 2
__add__ called
125
>>> 2 .__add__(c)
NotImplemented
>>> 2 + c
__radd__ called
125
You need to implement __radd__ on the class.
def __radd__(self, other):
return self.value + other
This gets called automatically, since the int class will raise a NotImplemented error