Suppose I have the following example:
class foo:
...
def bar(self, w, x, y, z, ...):
self.w = w
self.x = x
self.y = y
self.z = z
...
I wish to reduce the n-number of attribute assignment lines in bar() to one assignment line set using a setattr() cycle through the arguments. Is there a good way to cycle through said arguments for this purpose?
I wish to retain the defined parameter names so as to limit the number of parameters passed to the function as well as the order in which they are passed. I also understand that functions can be handled like objects; so is it possible to obtain a list of the defined parameters as an attribute of the function and iterate through that?
Use locals() and you can get all the arguments (and any other local variables):
class foo:
def bar(self, w, x, y, z):
argdict = {arg: locals()[arg] for arg in ('w', 'x', 'y', 'z')}
for key, value in argdict.iteritems():
setattr(self, key, value)
...
Might be possible to do it more efficiently, and you could inline argdict if you prefer less lines to readability or find it more readable that way.
So you don't have to actually name the arguments explicitly use:
class foo:
def __init__(self, w, x, y, z):
args = locals()# gets a dictionary of all local parameters
for argName in args:
if argName!='self':
setattr(self, argName, args[argName])
The __setattr__ attribute only assigns one attribute at a time, if you want to assign multiple attribute, you can use **kwargs in your function header and for limiting the number of arguments you can simply check the length of kwargs within your function. and call the __setattr__ for each each of the arguments one by one. One good reason for this recipe is that basically assigning attribute to an object without considering anything is not a correct and desirable job, due to a lot of reasons. Thus you have to assign each attribute one at a time by considering all the required conditions.
You can also do this manually by updating the instance dictionary but you should handle the exceptions too.
In [80]: class foo:
def bar(self, **kwargs):
if len(kwargs) != 4:
raise Exception("Please enter 4 keyword argument")
for k, v in kwargs.items():
foo.__setattr__(self, k, v)
....:
In [81]: f = foo()
In [82]: f.bar(w=1, x=2, y=3, z=4)
In [83]: f.w
Out[83]: 1
In [84]: f.bar(w=1, x=2, y=3, z=4, r=5)
---------------------------------------------------------------------------
Exception Traceback (most recent call last)
<ipython-input-84-758f669d08e0> in <module>()
----> 1 f.bar(w=1, x=2, y=3, z=4, r=5)
<ipython-input-80-9e46a6a78787> in bar(self, **kwargs)
2 def bar(self, **kwargs):
3 if len(kwargs) != 4:
----> 4 raise Exception("Please enter 4 keyword argument")
5 for k, v in kwargs.items():
6 foo.__setattr__(self, k, v)
Exception: Please enter 4 keyword argument
By using __setatter__ it will take care of the exception automatically:
In [70]: f.bar(1, 2)
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-70-07d1f3c9e27f> in <module>()
----> 1 f.bar(1, 2)
<ipython-input-65-1049e26120c1> in bar(self, *args)
2 def bar(self, *args):
3 for item in args:
----> 4 foo.__setattr__(self, item, item)
5
TypeError: attribute name must be string, not 'int'
Can anyone amend namedtuple or provide an alternative class so that it works for mutable objects?
Primarily for readability, I would like something similar to namedtuple that does this:
from Camelot import namedgroup
Point = namedgroup('Point', ['x', 'y'])
p = Point(0, 0)
p.x = 10
>>> p
Point(x=10, y=0)
>>> p.x *= 10
Point(x=100, y=0)
It must be possible to pickle the resulting object. And per the characteristics of named tuple, the ordering of the output when represented must match the order of the parameter list when constructing the object.
There is a mutable alternative to collections.namedtuple – recordclass.
It can be installed from PyPI:
pip3 install recordclass
It has the same API and memory footprint as namedtuple and it supports assignments (It should be faster as well). For example:
from recordclass import recordclass
Point = recordclass('Point', 'x y')
>>> p = Point(1, 2)
>>> p
Point(x=1, y=2)
>>> print(p.x, p.y)
1 2
>>> p.x += 2; p.y += 3; print(p)
Point(x=3, y=5)
recordclass (since 0.5) support typehints:
from recordclass import recordclass, RecordClass
class Point(RecordClass):
x: int
y: int
>>> Point.__annotations__
{'x':int, 'y':int}
>>> p = Point(1, 2)
>>> p
Point(x=1, y=2)
>>> print(p.x, p.y)
1 2
>>> p.x += 2; p.y += 3; print(p)
Point(x=3, y=5)
There is a more complete example (it also includes performance comparisons).
Recordclass library now provides another variant -- recordclass.make_dataclass factory function. It support dataclasses-like API (there are module level functions update, make, replace instead of self._update, self._replace, self._asdict, cls._make methods).
from recordclass import dataobject, make_dataclass
Point = make_dataclass('Point', [('x', int), ('y',int)])
Point = make_dataclass('Point', {'x':int, 'y':int})
class Point(dataobject):
x: int
y: int
>>> p = Point(1, 2)
>>> p
Point(x=1, y=2)
>>> p.x = 10; p.y += 3; print(p)
Point(x=10, y=5)
recordclass and make_dataclass can produce classes, whose instances occupy less memory than __slots__-based instances. This can be important for the instances with attribute values, which has not intended to have reference cycles. It may help reduce memory usage if you need to create millions of instances. Here is an illustrative example.
types.SimpleNamespace was introduced in Python 3.3 and supports the requested requirements.
from types import SimpleNamespace
t = SimpleNamespace(foo='bar')
t.ham = 'spam'
print(t)
namespace(foo='bar', ham='spam')
print(t.foo)
'bar'
import pickle
with open('/tmp/pickle', 'wb') as f:
pickle.dump(t, f)
As a Pythonic alternative for this task, since Python-3.7, you can use
dataclasses module that not only behaves like a mutable NamedTuple, because they use normal class definitions, they also support other class features.
From PEP-0557:
Although they use a very different mechanism, Data Classes can be thought of as "mutable namedtuples with defaults". Because Data Classes use normal class definition syntax, you are free to use inheritance, metaclasses, docstrings, user-defined methods, class factories, and other Python class features.
A class decorator is provided which inspects a class definition for variables with type annotations as defined in PEP 526, "Syntax for Variable Annotations". In this document, such variables are called fields. Using these fields, the decorator adds generated method definitions to the class to support instance initialization, a repr, comparison methods, and optionally other methods as described in the Specification section. Such a class is called a Data Class, but there's really nothing special about the class: the decorator adds generated methods to the class and returns the same class it was given.
This feature is introduced in PEP-0557 that you can read about it in more details on provided documentation link.
Example:
In [20]: from dataclasses import dataclass
In [21]: #dataclass
...: class InventoryItem:
...: '''Class for keeping track of an item in inventory.'''
...: name: str
...: unit_price: float
...: quantity_on_hand: int = 0
...:
...: def total_cost(self) -> float:
...: return self.unit_price * self.quantity_on_hand
...:
Demo:
In [23]: II = InventoryItem('bisc', 2000)
In [24]: II
Out[24]: InventoryItem(name='bisc', unit_price=2000, quantity_on_hand=0)
In [25]: II.name = 'choco'
In [26]: II.name
Out[26]: 'choco'
In [27]:
In [27]: II.unit_price *= 3
In [28]: II.unit_price
Out[28]: 6000
In [29]: II
Out[29]: InventoryItem(name='choco', unit_price=6000, quantity_on_hand=0)
The latest namedlist 1.7 passes all of your tests with both Python 2.7 and Python 3.5 as of Jan 11, 2016. It is a pure python implementation whereas the recordclass is a C extension. Of course, it depends on your requirements whether a C extension is preferred or not.
Your tests (but also see the note below):
from __future__ import print_function
import pickle
import sys
from namedlist import namedlist
Point = namedlist('Point', 'x y')
p = Point(x=1, y=2)
print('1. Mutation of field values')
p.x *= 10
p.y += 10
print('p: {}, {}\n'.format(p.x, p.y))
print('2. String')
print('p: {}\n'.format(p))
print('3. Representation')
print(repr(p), '\n')
print('4. Sizeof')
print('size of p:', sys.getsizeof(p), '\n')
print('5. Access by name of field')
print('p: {}, {}\n'.format(p.x, p.y))
print('6. Access by index')
print('p: {}, {}\n'.format(p[0], p[1]))
print('7. Iterative unpacking')
x, y = p
print('p: {}, {}\n'.format(x, y))
print('8. Iteration')
print('p: {}\n'.format([v for v in p]))
print('9. Ordered Dict')
print('p: {}\n'.format(p._asdict()))
print('10. Inplace replacement (update?)')
p._update(x=100, y=200)
print('p: {}\n'.format(p))
print('11. Pickle and Unpickle')
pickled = pickle.dumps(p)
unpickled = pickle.loads(pickled)
assert p == unpickled
print('Pickled successfully\n')
print('12. Fields\n')
print('p: {}\n'.format(p._fields))
print('13. Slots')
print('p: {}\n'.format(p.__slots__))
Output on Python 2.7
1. Mutation of field values
p: 10, 12
2. String
p: Point(x=10, y=12)
3. Representation
Point(x=10, y=12)
4. Sizeof
size of p: 64
5. Access by name of field
p: 10, 12
6. Access by index
p: 10, 12
7. Iterative unpacking
p: 10, 12
8. Iteration
p: [10, 12]
9. Ordered Dict
p: OrderedDict([('x', 10), ('y', 12)])
10. Inplace replacement (update?)
p: Point(x=100, y=200)
11. Pickle and Unpickle
Pickled successfully
12. Fields
p: ('x', 'y')
13. Slots
p: ('x', 'y')
The only difference with Python 3.5 is that the namedlist has become smaller, the size is 56 (Python 2.7 reports 64).
Note that I have changed your test 10 for in-place replacement. The namedlist has a _replace() method which does a shallow copy, and that makes perfect sense to me because the namedtuple in the standard library behaves the same way. Changing the semantics of the _replace() method would be confusing. In my opinion the _update() method should be used for in-place updates. Or maybe I failed to understand the intent of your test 10?
It seems like the answer to this question is no.
Below is pretty close, but it's not technically mutable. This is creating a new namedtuple() instance with an updated x value:
Point = namedtuple('Point', ['x', 'y'])
p = Point(0, 0)
p = p._replace(x=10)
On the other hand, you can create a simple class using __slots__ that should work well for frequently updating class instance attributes:
class Point:
__slots__ = ['x', 'y']
def __init__(self, x, y):
self.x = x
self.y = y
To add to this answer, I think __slots__ is good use here because it's memory efficient when you create lots of class instances. The only downside is that you can't create new class attributes.
Here's one relevant thread that illustrates the memory efficiency - Dictionary vs Object - which is more efficient and why?
The quoted content in the answer of this thread is a very succinct explanation why __slots__ is more memory efficient - Python slots
The following is a good solution for Python 3: A minimal class using __slots__ and Sequence abstract base class; does not do fancy error detection or such, but it works, and behaves mostly like a mutable tuple (except for typecheck).
from collections import Sequence
class NamedMutableSequence(Sequence):
__slots__ = ()
def __init__(self, *a, **kw):
slots = self.__slots__
for k in slots:
setattr(self, k, kw.get(k))
if a:
for k, v in zip(slots, a):
setattr(self, k, v)
def __str__(self):
clsname = self.__class__.__name__
values = ', '.join('%s=%r' % (k, getattr(self, k))
for k in self.__slots__)
return '%s(%s)' % (clsname, values)
__repr__ = __str__
def __getitem__(self, item):
return getattr(self, self.__slots__[item])
def __setitem__(self, item, value):
return setattr(self, self.__slots__[item], value)
def __len__(self):
return len(self.__slots__)
class Point(NamedMutableSequence):
__slots__ = ('x', 'y')
Example:
>>> p = Point(0, 0)
>>> p.x = 10
>>> p
Point(x=10, y=0)
>>> p.x *= 10
>>> p
Point(x=100, y=0)
If you want, you can have a method to create the class too (though using an explicit class is more transparent):
def namedgroup(name, members):
if isinstance(members, str):
members = members.split()
members = tuple(members)
return type(name, (NamedMutableSequence,), {'__slots__': members})
Example:
>>> Point = namedgroup('Point', ['x', 'y'])
>>> Point(6, 42)
Point(x=6, y=42)
In Python 2 you need to adjust it slightly - if you inherit from Sequence, the class will have a __dict__ and the __slots__ will stop from working.
The solution in Python 2 is to not inherit from Sequence, but object. If isinstance(Point, Sequence) == True is desired, you need to register the NamedMutableSequence as a base class to Sequence:
Sequence.register(NamedMutableSequence)
Tuples are by definition immutable.
You can however make a dictionary subclass where you can access the attributes with dot-notation;
In [1]: %cpaste
Pasting code; enter '--' alone on the line to stop or use Ctrl-D.
:class AttrDict(dict):
:
: def __getattr__(self, name):
: return self[name]
:
: def __setattr__(self, name, value):
: self[name] = value
:--
In [2]: test = AttrDict()
In [3]: test.a = 1
In [4]: test.b = True
In [5]: test
Out[5]: {'a': 1, 'b': True}
If you want similar behavior as namedtuples but mutable try namedlist
Note that in order to be mutable it cannot be a tuple.
Let's implement this with dynamic type creation:
import copy
def namedgroup(typename, fieldnames):
def init(self, **kwargs):
attrs = {k: None for k in self._attrs_}
for k in kwargs:
if k in self._attrs_:
attrs[k] = kwargs[k]
else:
raise AttributeError('Invalid Field')
self.__dict__.update(attrs)
def getattribute(self, attr):
if attr.startswith("_") or attr in self._attrs_:
return object.__getattribute__(self, attr)
else:
raise AttributeError('Invalid Field')
def setattr(self, attr, value):
if attr in self._attrs_:
object.__setattr__(self, attr, value)
else:
raise AttributeError('Invalid Field')
def rep(self):
d = ["{}={}".format(v,self.__dict__[v]) for v in self._attrs_]
return self._typename_ + '(' + ', '.join(d) + ')'
def iterate(self):
for x in self._attrs_:
yield self.__dict__[x]
raise StopIteration()
def setitem(self, *args, **kwargs):
return self.__dict__.__setitem__(*args, **kwargs)
def getitem(self, *args, **kwargs):
return self.__dict__.__getitem__(*args, **kwargs)
attrs = {"__init__": init,
"__setattr__": setattr,
"__getattribute__": getattribute,
"_attrs_": copy.deepcopy(fieldnames),
"_typename_": str(typename),
"__str__": rep,
"__repr__": rep,
"__len__": lambda self: len(fieldnames),
"__iter__": iterate,
"__setitem__": setitem,
"__getitem__": getitem,
}
return type(typename, (object,), attrs)
This checks the attributes to see if they are valid before allowing the operation to continue.
So is this pickleable? Yes if (and only if) you do the following:
>>> import pickle
>>> Point = namedgroup("Point", ["x", "y"])
>>> p = Point(x=100, y=200)
>>> p2 = pickle.loads(pickle.dumps(p))
>>> p2.x
100
>>> p2.y
200
>>> id(p) != id(p2)
True
The definition has to be in your namespace, and must exist long enough for pickle to find it. So if you define this to be in your package, it should work.
Point = namedgroup("Point", ["x", "y"])
Pickle will fail if you do the following, or make the definition temporary (goes out of scope when the function ends, say):
some_point = namedgroup("Point", ["x", "y"])
And yes, it does preserve the order of the fields listed in the type creation.
I can't believe nobody's said this before, but it seems to me Python just wants you to write your own simple, mutable class instead of using a namedtuple whenever you need the "namedtuple" to be mutable.
Quick summary
Just jump straight down to Approach 5 below. It's short and to-the-point, and by far the best of these options.
Various, detailed approaches:
Approach 1 (good): simple, callable class with __call__()
Here is an example of a simple Point object for (x, y) points:
class Point():
def __init__(self, x, y):
self.x = x
self.y = y
def __call__(self):
"""
Make `Point` objects callable. Print their contents when they
are called.
"""
print("Point(x={}, y={})".format(self.x, self.y))
Now use it:
p1 = Point(1,2)
p1()
p1.x = 7
p1()
p1.y = 8
p1()
Here is the output:
Point(x=1, y=2)
Point(x=7, y=2)
Point(x=7, y=8)
This is pretty similar to a namedtuple, except it is fully mutable, unlike a namedtuple. Also, a namedtuple isn't callable, so to see its contents, just type the object instance name withOUT parenthesis after it (as p2 in the example below, instead of as p2()). See this example and output here:
>>> from collections import namedtuple
>>> Point2 = namedtuple("Point2", ["x", "y"])
>>> p2 = Point2(1, 2)
>>> p2
Point2(x=1, y=2)
>>> p2()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'Point2' object is not callable
>>> p2.x = 7
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: can't set attribute
Approach 2 (better): use __repr__() in place of __call__()
I just learned you can use __repr__() in place of __call__(), to get more namedtuple-like behavior. Defining the __repr__() method allows you to define "the 'official' string representation of an object" (see the official documentation here). Now, just calling p1 is the equivalent of calling the __repr__() method, and you get identical behavior to the namedtuple. Here is the new class:
class Point():
def __init__(self, x, y):
self.x = x
self.y = y
def __repr__(self):
"""
Obtain the string representation of `Point`, so that just typing
the instance name of an object of this type will call this method
and obtain this string, just like `namedtuple` already does!
"""
return "Point(x={}, y={})".format(self.x, self.y)
Now use it:
p1 = Point(1,2)
p1
p1.x = 7
p1
p1.y = 8
p1
Here is the output:
Point(x=1, y=2)
Point(x=7, y=2)
Point(x=7, y=8)
Approach 3 (better still, but a little awkward to use): make it a callable which returns an (x, y) tuple
The original poster (OP) would also like something like this to work (see his comment below my answer):
x, y = Point(x=1, y=2)
Well, for simplicity, let's just make this work instead:
x, y = Point(x=1, y=2)()
# OR
p1 = Point(x=1, y=2)
x, y = p1()
While we are at it, let's also condense this:
self.x = x
self.y = y
...into this (source where I first saw this):
self.x, self.y = x, y
Here is the class definition for all of the above:
class Point():
def __init__(self, x, y):
self.x, self.y = x, y
def __repr__(self):
"""
Obtain the string representation of `Point`, so that just typing
the instance name of an object of this type will call this method
and obtain this string, just like `namedtuple` already does!
"""
return "Point(x={}, y={})".format(self.x, self.y)
def __call__(self):
"""
Make the object callable. Return a tuple of the x and y components
of the Point.
"""
return self.x, self.y
Here are some test calls:
p1 = Point(1,2)
p1
p1.x = 7
x, y = p1()
x2, y2 = Point(10, 12)()
x
y
x2
y2
I won't show pasting the class definition into the interpreter this time, but here are those calls with their output:
>>> p1 = Point(1,2)
>>> p1
Point(x=1, y=2)
>>> p1.x = 7
>>> x, y = p1()
>>> x2, y2 = Point(10, 12)()
>>> x
7
>>> y
2
>>> x2
10
>>> y2
12
Approach 4 (best so far, but a lot more code to write): make the class also an iterator
By making this into an iterator class, we can get this behavior:
x, y = Point(x=1, y=2)
# OR
x, y = Point(1, 2)
# OR
p1 = Point(1, 2)
x, y = p1
Let's get rid of the __call__() method, but to make this class an iterator we will add the __iter__() and __next__() methods. Read more about these things here:
https://treyhunner.com/2018/06/how-to-make-an-iterator-in-python/
How to build a basic iterator?
https://docs.python.org/3/library/exceptions.html#StopIteration
Here is the solution:
class Point():
def __init__(self, x, y):
self.x, self.y = x, y
self._iterator_index = 0
self._num_items = 2 # counting self.x and self.y
def __repr__(self):
"""
Obtain the string representation of `Point`, so that just typing
the instance name of an object of this type will call this method
and obtain this string, just like `namedtuple` already does!
"""
return "Point(x={}, y={})".format(self.x, self.y)
def __iter__(self):
return self
def __next__(self):
self._iterator_index += 1
if self._iterator_index == 1:
return self.x
elif self._iterator_index == 2:
return self.y
else:
raise StopIteration
And here are some test calls and their output:
>>> x, y = Point(x=1, y=2)
>>> x
1
>>> y
2
>>> x, y = Point(3, 4)
>>> x
3
>>> y
4
>>> p1 = Point(5, 6)
>>> x, y = p1
>>> x
5
>>> y
6
>>> p1
Point(x=5, y=6)
Approach 5 (USE THIS ONE) (Perfect!--best and cleanest/shortest approach): make the class an iterable, with the yield generator keyword
Study these references:
https://treyhunner.com/2018/06/how-to-make-an-iterator-in-python/
What does the "yield" keyword do?
Here is the solution. It relies on a fancy "iterable-generator" (AKA: just "generator") keyword/Python mechanism, called yield.
Basically, the first time an iterable calls for the next item, it calls the __iter__() method, and stops and returns the contents of the first yield call (self.x in the code below). The next time an iterable calls for the next item, it picks up where it last left off (just after the first yield in this case), and looks for the next yield, stopping and returning the contents of that yield call (self.y in the code below). Each "return" from a yield actually returns a "generator" object, which is an iterable itself, so you can iterate on it. Each new iterable call for the next item continues this process, starting up where it last left off, just after the most-recently-called yield, until no more yield calls exist, at which point the iterations are ended and the iterable has been fully iterated. Therefore, once this iterable has called for two objects, both yield calls have been used up, so the iterator ends. The end result is that calls like this work perfectly, just as they did in Approach 4, but with far less code to write!:
x, y = Point(x=1, y=2)
# OR
x, y = Point(1, 2)
# OR
p1 = Point(1, 2)
x, y = p1
Here is the solution (a part of this solution can also be found in the treyhunner.com reference just above). Notice how short and clean this solution is!
Just the class definition code; no docstrings, so you can truly see how short and simple this is:
class Point():
def __init__(self, x, y):
self.x, self.y = x, y
def __repr__(self):
return "Point(x={}, y={})".format(self.x, self.y)
def __iter__(self):
yield self.x
yield self.y
With descriptive docstrings:
class Point():
def __init__(self, x, y):
self.x, self.y = x, y
def __repr__(self):
"""
Obtain the string representation of `Point`, so that just typing
the instance name of an object of this type will call this method
and obtain this string, just like `namedtuple` already does!
"""
return "Point(x={}, y={})".format(self.x, self.y)
def __iter__(self):
"""
Make this `Point` class an iterable. When used as an iterable, it will
now return `self.x` and `self.y` as the two elements of a list-like,
iterable object, "generated" by the usages of the `yield` "generator"
keyword.
"""
yield self.x
yield self.y
Copy and paste the exact same test code as used in the previous approach (Approach 4) just above, and you will get the exact same output as above as well!
References:
https://docs.python.org/3/library/collections.html#collections.namedtuple
Approach 1:
What is the difference between __init__ and __call__?
Approach 2:
https://www.tutorialspoint.com/What-does-the-repr-function-do-in-Python-Object-Oriented-Programming
Purpose of __repr__ method?
https://docs.python.org/3/reference/datamodel.html#object.__repr__
Approach 4:
*****[EXCELLENT!] https://treyhunner.com/2018/06/how-to-make-an-iterator-in-python/
How to build a basic iterator?
https://docs.python.org/3/library/exceptions.html#StopIteration
Approach 5:
See links from Approach 4, plus:
*****[EXCELLENT!] What does the "yield" keyword do?
What is the meaning of single and double underscore before an object name?
Provided performance is of little importance, one could use a silly hack like:
from collection import namedtuple
Point = namedtuple('Point', 'x y z')
mutable_z = Point(1,2,[3])
If you want to be able to create classes "on-site", I find the following very convenient:
class Struct:
def __init__(self, **kw):
self.__dict__.update(**kw)
That allows me to write:
p = Struct(x=0, y=0)
P.x = 10
stats = Struct(count=0, total=0.0)
stats.count += 1
The most elegant way I can think of doesn't require a 3rd party library and lets you create a quick mock class constructor with default member variables without dataclasses cumbersome type specification. So it's better for roughing out some code:
# copy-paste 3 lines:
from inspect import getargvalues, stack
from types import SimpleNamespace
def DefaultableNS(): return SimpleNamespace(**getargvalues(stack()[1].frame)[3])
# then you can make classes with default fields on the fly in one line, eg:
def Node(value,left=None,right=None): return DefaultableNS()
node=Node(123)
print(node)
#[stdout] namespace(value=123, left=None, right=None)
print(node.value,node.left,node.right) # all fields exist
A plain SimpleNamespace is clumsier, it breaks DRY:
def Node(value,left=None,right=None):
return SimpleNamespace(value=value,left=left,right=right)
# breaks DRY as you need to repeat the argument names twice
I will share my solution to this question. I needed a way to save attributes in the case that my program crashed or was stopped for some reason so that it would know where where in a list of inputs to resume from. Based on #GabrielStaples's answer:
import pickle, json
class ScanSession:
def __init__(self, input_file: str = None, output_file: str = None,
total_viable_wallets: int = 0, total: float = 0,
report_dict: dict = {}, wallet_addresses: list = [],
token_map: list = [], token_map_file: str = 'data/token.maps.json',
current_batch: int = 0):
self.initialized = time.time()
self.input_file = input_file
self.output_file = output_file
self.total_viable_wallets = total_viable_wallets
self.total = total
self.report_dict = report_dict
self.wallet_addresses = wallet_addresses
self.token_map = token_map
self.token_map_file = token_map_file
self.current_batch = current_batch
#property
def __dict__(self):
"""
Obtain the string representation of `Point`, so that just typing
the instance name of an object of this type will call this method
and obtain this string, just like `namedtuple` already does!
"""
return {'initialized': self.initialized, 'input_file': self.input_file,
'output_file': self.output_file, 'total_viable_wallets': self.total_viable_wallets,
'total': self.total, 'report_dict': self.report_dict,
'wallet_addresses': self.wallet_addresses, 'token_map': self.token_map,
'token_map_file':self.token_map_file, 'current_batch': self.current_batch
}
def load_session(self, session_file):
with open(session_file, 'r') as f:
_session = json.loads(json.dumps(f.read()))
_session = dict(_session)
for key, value in _session.items():
setattr(self, key, value)
def dump_session(self, session_file):
with open(session_file, 'w') as f:
json.dump(self.__dict__, fp=f)
Using it:
session = ScanSession()
session.total += 1
session.__dict__
{'initialized': 1670801774.8050613, 'input_file': None, 'output_file': None, 'total_viable_wallets': 0, 'total': 10, 'report_dict': {}, 'wallet_addresses': [], 'token_map': [], 'token_map_file': 'data/token.maps.json', 'current_batch': 0}
pickle.dumps(session)
b'\x80\x04\x95\xe8\x00\x00\x00\x00\x00\x00\x00\x8c\x08__main__\x94\x8c\x0bScanSession\x94\x93\x94)\x81\x94}\x94(\x8c\x0binitialized\x94GA\xd8\xe5\x9a[\xb3\x86 \x8c\ninput_file\x94N\x8c\x0boutput_file\x94N\x8c\x14total_viable_wallets\x94K\x00\x8c\x05total\x94K\n\x8c\x0breport_dict\x94}\x94\x8c\x10wallet_addresses\x94]\x94\x8c\ttoken_map\x94]\x94\x8c\x0etoken_map_file\x94\x8c\x14data/token.maps.json\x94\x8c\rcurrent_batch\x94K\x00ub.'
I am new to Python so please don't flame me if I ask something too noobish :)
1.
Consider I have a class:
class Test:
def __init__(self, x, y):
self.x = x
self.y = y
def wow():
print 5 * 5
Now I try to create an object of the class:
x = Test(3, 4)
This works as expected. However, when I try to call the method wow(), it returns an error, which is fixed by changing wow() to:
def wow(self)
Why do I need to include self and if I don't, what does the method mean?2. In the definition of __init__:
def __init__(self, x, y):
self.x = x
self.y = y
Why do I need to declare x and y, when I can do this:
def __init__(self):
self.x = x
self.y = y
I hope I am being clear...
Thanks for your time.
If you do that :
def __init__(self):
self.x = x
self.y = y
you assign the gobal vars x and y (it they exists ) to your instance
with :
def __init__(self, x, y):
self.x = x
self.y = y
you assign what you give as parameter to the constructor
and that is a lot more flexible :-)
The instance reference in Python is explicit. That way it can be manipulated by e.g. decorators before finally being passed to the method.
We need to declare x and y as arguments to the function so that we can use their names within the function, bound to the arguments passed in the corresponding function call.
Just to be clear
Why do I need to declare x and y, when
I can do this:
def __init__(self):
self.x = x
self.y = y
This ^ will only work if x and y can be found at runtime - if they haven't been passed in then they must have been set elsewhere (globally) or it will generate an error.
>>> class c:
def __init__(self):
self.x = x
>>> x = 1
>>> q = c()
>>> q.x
1
>>> del x
>>> q.x
1
>>> w = c()
Traceback (most recent call last):
File "<pyshell#24>", line 1, in <module>
w = c()
File "<pyshell#14>", line 3, in __init__
self.x = x
NameError: global name 'x' is not defined
>>>
>>> w = c(2)
Traceback (most recent call last):
File "<pyshell#19>", line 1, in <module>
w = c(2)
TypeError: __init__() takes exactly 1 argument (2 given)
This is why you want / need to specify them as parameters - it might work with the global lookup but it would probably violate the "principle of least astonishment"
self is a "magic" name - it can really be anything, but self is used for consistency and clarity. To answer your question, each class method/function requires an explicit reference to the class as the first parameter. Using Ipython:
In [66]: class Test:
....: def __init__(self):
....: pass
....: def wow(self):
....: print self
....:
....:
In [67]: x = Test()
In [68]: x.wow()
<__main__.Test instance at 0x0159FDF0>
Your second example won't actually work unless you already have an x and y in your namespace.
For instance, if you defined your class:
class Test:
def __init__(self):
self.x = x
self.y = y
and tried
x = Test()
it will throw a NameError.
However if you write:
x = 3
y = 4
test = Test()
then it will work. However, it's not a good idea to do such a thing. For the reason why read line 2:
In [72]: import this
The Zen of Python, by Tim Peters
Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!
In Python, methods should always take "one extra" argument, which is the reference to the instance the method is being called on. This is automatic in other languages such as Java, C#, etc. but Python is verbose about it.
That doesn't make sense. Where are x and y in that example? If you want the constructor to take two arguments which populate the object, define it as such. Otherwise, you're doing something else.
Python is different from languages like C++ and Java in that the object instance reference is passed explicitly.
That is, if you have an object which is an instance of the class and you want to invoke a method that operates on that instance (e.g., reads its fields), you use the self references as the object.
In c++ and Java, you have an implicit "this" reference that is present in the compiled version of your program but not in the source code. You use the static keyword to make it into a class method that does not have a "this".