Develop Reference

Python Class Script Running Errors?

I am trying to make a class using Python 2.7.9 and it keeps running into errors.
Here is my script:
class Hero():
def __init__(self, name):
self.health=50
def eat(self, food):
if(food=='apple'):
self.health+=10
self.name=Jeff
Jeff=Hero('Jeff')
def introduce(self, name):
print Jeff.name
def checkAtt():
print Jeff.health
introduce()
It keeps saying name 'Jeff' is not defined.

Your code has numerous problems. The first, that is causing the specific error, is that you attempt to assign:
self.name = Jeff
before you have defined either self or Jeff. self is conventionally only used inside instance methods (like your Hero.eat), where it is the name of the first parameter.
Secondly, your Hero.__init__ doesn't actually assign the name parameter to the name attribute; it should look like:
class Hero(object): # Note inheritance from 'object' for new-style class
def __init__(self, name):
self.name = name # Note assignment of instance attribute
self.health = 50
...
jeff = Hero("Jeff") will call Hero.__init__, creating new Hero instance, setting its name attribute to "Jeff" (and health attribute to 50) and assigning that instance to the name jeff.
Thirdly, you have two standalone functions (introduce and checkAtt) that should probably also be instance methods:
def Hero(object):
...
def introduce(self):
print self.name
...
jeff = Hero("Jeff")
jeff.introduce() # equivalent to 'Hero.introduce(jeff)'
or, if remaining as standalone functions, take a single parameter, the Hero instance to operate on (which should not be called self, again by convention) - there is not much point writing a function that only works if it's run in a scope where the name Jeff is available!
class Hero(object):
...
def introduce(hero):
print hero.name
jeff = Hero("Jeff")
introduce(jeff)
Note the indentation in these two different cases - it is very important in Python. Also, note the different ways of calling introduce depending on whether it's an instance method or a function.
I suggest you read the tutorial on classes and the style guide.

How to add a classmethod in Python dynamically

I'm using Python 3.
I know about the #classmethod decorator. Also, I know that classmethods can be called from instances.
class HappyClass(object):
#classmethod
def say_hello():
print('hello')
HappyClass.say_hello() # hello
HappyClass().say_hello() # hello
However, I don't seem to be able to create class methods dynamically AND let them be called from instances. Let's say I want something like
class SadClass(object):
def __init__(self, *args, **kwargs):
# create a class method say_dynamic
SadClass.say_dynamic() # prints "dynamic!"
SadClass().say_dynamic() # prints "dynamic!"
I've played with cls.__dict__ (which produces exceptions), and with setattr(cls, 'say_dynamic', blahblah) (which only makes the thingie callable from the class and not the instance).
If you ask me why, I wanted to make a lazy class property. But it cannot be called from instances.
#classmethod
def search_url(cls):
if hasattr(cls, '_search_url'):
setattr(cls, '_search_url', reverse('%s-search' % cls._meta.model_name))
return cls._search_url
Maybe because the property hasn't been called from the class yet...
In summary, I want to add a lazy, class method that can be called from the instance... Can this be achieved in an elegant (nottoomanylines) way?
Any thoughts?
How I achieved it
Sorry, my examples were very bad ones :\
Anyway, in the end I did it like this...
#classmethod
def search_url(cls):
if not hasattr(cls, '_search_url'):
setattr(cls, '_search_url', reverse('%s-search' % cls._meta.model_name))
return cls._search_url
And the setattr does work, but I had made a mistake when testing it...

You can add a function to a class at any point, a practice known as monkey-patching:
class SadClass:
pass
#classmethod
def say_dynamic(cls):
print('hello')
SadClass.say_dynamic = say_dynamic
>>> SadClass.say_dynamic()
hello
>>> SadClass().say_dynamic()
hello
Note that you are using the classmethod decorator, but your function accepts no arguments, which indicates that it's designed to be a static method. Did you mean to use staticmethod instead?

If you want to create class methods, do not create them in the __init__ function as it is then recreated for each instance creation. However, following works:
class SadClass(object):
pass
def say_dynamic(cls):
print("dynamic")
SadClass.say_dynamic = classmethod(say_dynamic)
# or
setattr(SadClass, 'say_dynamic', classmethod(say_dynamic))
SadClass.say_dynamic() # prints "dynamic!"
SadClass().say_dynamic() # prints "dynamic!"
Of course, in the __init__ method the self argument is an instance, and not the class: to put the method in the class there, you can hack something like
class SadClass(object):
def __init__(self, *args, **kwargs):
#classmethod
def say_dynamic(cls):
print("dynamic!")
setattr(self.__class__, 'say_dynamic', say_dynamic)
But it will again reset the method for each instance creation, possibly needlessly. And notice that your code most probably fails because you are calling the SadClass.say_dynamic() before any instances are created, and thus before the class method is injected.
Also, notice that a classmethod gets the implicit class argument cls; if you do want your function to be called without any arguments, use the staticmethod decorator.

As a side note, you can just use an instance attribute to hold a function:
>>> class Test:
... pass
...
>>> t=Test()
>>> t.monkey_patch=lambda s: print(s)
>>> t.monkey_patch('Hello from the monkey patch')
Hello from the monkey patch

How I achieved it:
#classmethod
def search_url(cls):
if not hasattr(cls, '_search_url'):
setattr(cls, '_search_url', reverse('%s-search' % cls._meta.model_name))
return cls._search_url

How does one write a method to call after a variable or object, such as "string {0}".format(stringy)?

For different data types, like string, there are methods that you call by adding a dot after, such as:
"string {0}".format(stringy)
or
listx.remove(x)
How is the information being passed to the method? How can I write a function like that?

class YourObject(object):
def do_something(self):
print('doing something')
Then you can use your object:
your_object = YourObject()
your_object.do_something()
This shows how to create an object, and call a method on it (like theexamples you provided in your post).
There are way more in-depth tutorials/blogs about object creation and custom classes. A good place to start is always the standard documentation.

You can create a custom class and then include whatever methods you want. Below is an example:
>>> class MyClass(object): # Define class MyClass
... def __init__(self): # Define MyClass' constructor method
... self.name = "Me" # Make an attribute
... def getName(self): # Define method getName
... return self.name # Return MyClass' attribute name (self.name)
...
>>> test = MyClass() # Initialize (create an instance of) MyClass
>>> print test.getName() # Print the name attribute by calling the getName method
Me
>>>
Basically, you are working with OOP (Object-Oriented Programming). However, since this concept is so large, I can't demonstrate/explain everything you can do with it here (otherwise my post would be enormous). My advice is to research OOP and Python classes. There are many good tutorials you can find. I gave one above; here is another:

Difference between #classmethod and a method in python [duplicate]

This question already has answers here:
What's an example use case for a Python classmethod?
(7 answers)
Closed 9 years ago.
What is the difference between #classmethod and a 'classic' method in python,
When should I use the #classmethod and when should I use a 'classic' method in python.
Is the classmethod must be an method who is referred to the class (I mean it's only a method who handle the class) ?
And I know what is the difference between a #staticmethod and classic method
Thx

Let's assume you have a class Car which represents the Car entity within your system.
A classmethod is a method that works for the class Car not on one of any of Car's instances. The first parameter to a function decorated with #classmethod, usually called cls, is therefore the class itself. Example:
class Car(object):
colour = 'red'
#classmethod
def blue_cars(cls):
# cls is the Car class
# return all blue cars by looping over cls instances
A function acts on a particular instance of the class; the first parameter usually called self is the instance itself:
def get_colour(self):
return self.colour
To sum up:
use classmethod to implement methods that work on a whole class (and not on particular class instances):
Car.blue_cars()
use instance methods to implement methods that work on a particular instance:
my_car = Car(colour='red')
my_car.get_colour() # should return 'red'

If you define a method inside a class, it is handled in a special way: access to it wraps it in a special object which modifies the calling arguments in order to include self, a reference to the referred object:
class A(object):
def f(self):
pass
a = A()
a.f()
This call to a.f actually asks f (via the descriptor protocol) for an object to really return. This object is then called without arguments and deflects the call to the real f, adding a in front.
So what a.f() really does is calling the original f function with (a) as arguments.
In order to prevent this, we can wrap the function
with a #staticmethod decorator,
with a #classmethod decorator,
with one of other, similiar working, self-made decorators.
#staticmethod turns it into an object which, when asked, changes the argument-passing behaviour so that it matches the intentions about calling the original f:
class A(object):
def method(self):
pass
#staticmethod
def stmethod():
pass
#classmethod
def clmethod(cls):
pass
a = A()
a.method() # the "function inside" gets told about a
A.method() # doesn't work because there is no reference to the needed object
a.clmethod() # the "function inside" gets told about a's class, A
A.clmethod() # works as well, because we only need the classgets told about a's class, A
a.stmethod() # the "function inside" gets told nothing about anything
A.stmethod() # works as well
So #classmethod and #staticmethod have in common that they "don't care about" the concrete object they were called with; the difference is that #staticmethod doesn't want to know anything at all about it, while #classmethod wants to know its class.
So the latter gets the class object the used object is an instance of. Just replace self with cls in this case.
Now, when to use what?
Well, that is easy to handle:
If you have an access to self, you clearly need an instance method.
If you don't access self, but want to know about its class, use #classmethod. This may for example be the case with factory methods. datetime.datetime.now() is such an example: you can call it via its class or via an instance, but it creates a new instance with completely different data. I even used them once for automatically generating subclasses of a given class.
If you need neither self nor cls, you use #staticmethod. This can as well be used for factory methods, if they don't need to care about subclassing.

#classmethod takes the class as first argument while function takes instance of the class
>>> class Test(object):
... def func(self):
... print self
... #classmethod
... def meth(self):
... print self
>>> t = Test()
>>> t.func()
<__main__.Test object at 0x00000000027238D0>
>>> t.meth()
<class '__main__.Test'>
I've used self argument in meth intentionally so it would be very close in syntax to the func. But usually you'd better use cls as argument:
... #classmethod
... def meth(cls):
... print cls

Is there a way apply a decorator to a Python method that needs informations about the class?

When you decorate a method, it is not bound yet to the class, and therefor doesn't have the im_class attribute yet. I looking for a way to get the information about the class inside the decorator. I tried this:
import types
def decorator(method):
def set_signal(self, name, value):
print name
if name == 'im_class':
print "I got the class"
method.__setattr__ = types.MethodType(set_signal, method)
return method
class Test(object):
#decorator
def bar(self, foo):
print foo
But it doesn't print anything.
I can imagine doing this:
class Test(object):
#decorator(klass=Test)
def bar(self, foo):
print foo
But if I can avoid it, it would make my day.

__setattr__ is only called on explicit object.attribute = assignments; building a class does not use attribute assignment but builds a dictionary (Test.__dict__) instead.
To access the class you have a few different options though:
Use a class decorator instead; it'll be passed the completed class after building it, you could decorate individual methods on that class by replacing them (decorated) in the class. You could use a combination of a function decorator and a class decorator to mark which methods are to be decorated:
def methoddecoratormarker(func):
func._decorate_me = True
return func
def realmethoddecorator(func):
# do something with func.
# Note: it is still an unbound function here, not a method!
return func
def classdecorator(klass):
for name, item in klass.__dict__.iteritems():
if getattr(item, '_decorate_me', False):
klass.__dict__[name] = realmethoddecorator(item)
You could use a metaclass instead of a class decorator to achieve the same, of course.
Cheat, and use sys._getframe() to retrieve the class from the calling frame:
import sys
def methoddecorator(func):
callingframe = sys._getframe(1)
classname = callingframe.f_code.co_name
Note that all you can retrieve is the name of the class; the class itself is still being built at this time. You can add items to callingframe.f_locals (a mapping) and they'll be made part of the new class object.
Access self whenever the method is called. self is a reference to the instance after all, and self.__class__ is going to be, at the very least, a sub-class of the original class the function was defined in.

My strict answer would be: It's not possible, because the class does not yet exist when the decorator is executed.
The longer answer would depend on your very exact requirements. As I wrote, you cannot access the class if it does not yet exists. One solution would be, to mark the decorated method to be "transformed" later. Then use a metaclass or class decorator to apply your modifications after the class has been created.
Another option involves some magic. Look for the implementation of the implements method in zope.interfaces. It has some access to the information about the class which is just been parsed. Don't know if it will be enough for your use case.

You might want to take a look at descriptors. They let you implement a __get__ that is used when an attribute is accessed, and can return different things depending on the object and its type.

Use method decorators to add some marker attributes to the interesting methods, and use a metaclass which iterates over the methods, finds the marker attributes, and does the logic. The metaclass code is run when the class is created, so it has a reference to the newly created class.
class MyMeta(object):
def __new__(...):
...
cls = ...
... iterate over dir(cls), find methods having .is_decorated, act on them
return cls
def decorator(f):
f.is_decorated = True
return f
class MyBase(object):
__metaclass__ = MyMeta
class MyClass(MyBase):
#decorator
def bar(self, foo):
print foo
If you worry about that the programmer of MyClass forgets to use MyBase, you can forcibly set the metaclass in decorator, by exampining the globals dicitionary of the caller stack frame (sys._getframe()).