I have a class definition which defines a static method. I have a field which I would like to initialize with the static method. My default thinking led me to this:
class SomeConcreteClass(object):
some_data = SomeConcreteClass.create_default_data()
#staticmethod
def create_default_data():
return 'Foo'
The problem is that when I run this, I get a NameError: name 'SomeConcreteClass' is not defined. It makes sense as the SomeConcreteClass is just being built. Does this mean I cannot use static init functions? Is there an alternate way which is recommended to handle such a situation?
The appropriate place for create_default_data would be outside the class entirely. Then your problems go away:
def create_default_data():
return 'Foo'
class SomeConcreteClass(object):
some_data = create_default_data()
If you really do want it as a static method inside the class that's alright too:
def _create_default_data():
return 'Foo'
class SomeConcreteClass(object):
some_data = _create_default_data()
create_default_data = staticmethod(_create_default_data)
but static methods aren't often used in Python because there's no need to put a function inside a class unless it operates on the class in some way.
If some_data is exactly the output of create_default_data (and assuming the latter is deterministic in the context of your call) then why not just make some_data a #property?
class SomeConcreteClass(object):
#property
def some_data():
return 'Foo'
Alternatively, but not equivalently, you could initialize some_data for each instance within __init__.
I don't think you want to do this. Don't forget that Python is not Java ™... attributes defined at class level are class attributes, not instance attributes. You almost certainly want the data to be instance-specific, so you should do this in the __init__ method. You can certainly call the classmethod from within that method, if you want to, or (better) just put the code in __init__.
Related
Assuming I have a class which requires a function (or should I say method) which is:
independent from my class instance - doesn't need self argument;
is called only inside my class object
I won't need access to it at any point (to override it for example);
should I (A) place it inside the class and mark it as a #staticmethod or should I (B) define it outside my class object (but in the same namespace)? Why?
Example:
class A:
def __init__(self, my_int):
self.my_int = my_int
def my_int_and_4(self):
print(self.adder(self.my_int,4))
#staticmethod
def adder(a,b):
return a+b
or
def adder(a,b):
return a+b
class B:
def __init__(self, my_int):
self.my_int = my_int
def my_int_and_4(self):
print(adder(self.my_int,4))
EDIT: maybe the example is a bit oversimplified. I should have added that my version of "adder" is specificly used with my class and in no other case.
This is a textbook use case for a private static method.
They key point here is that you should make it a private method of that class. That way you're certain nothing else will use it and depend on its implementation. You'll be free to change it in the future, or even delete it, without breaking anything outside that class.
And yeah, make it static, because you can.
In Python, there is no way to make a method truly private, but by convention, prefixing the method name by a _ means it should be treated as private.
#staticmethod
def _adder(a,b): ## <-- note the _
return a+b
If at some point you suddenly need to use it outside the class, then exposing it will be no trouble at all, e.g. using a public wrapper method.
The reverse, however, isn't true; once exposed, it's difficult to retract that exposure.
I would definitely use a private static method in this case, for the reasons described by Jean-Francois Corbett. There are two types of methods in Python that belong to the class itself, rather than an instance: class methods and static methods.
The first parameter of a class method (created with #classmethod) references the class in exactly the same manner that the first parameter of an instance method (self) references an instance. It is the equivalent of static methods in most other languages. If your method requires access to other class members, use a class method.
A static method (created with #staticmethod) does not contain a reference to the class, and therefore cannot reference other class members. It's generally used for private helper methods and the like.
For your adder method, I would definitely use a static method. However, in this modified (and rather useless) version, a class method is necessary:
class A:
x = 1
def __init__(self, my_int):
self.my_int = my_int
def my_int_and_4(self):
print(self._adder(self.my_int,4))
#staticmethod
def _adder(a,b):
return a+b
#classmethod
def _increment(cls, n):
return n + cls.x
Both approaches will work, so it's the matter of readability and following conventions.
Does the method need to look at the instance's private attributes? If yes, it's a good reason to keep it in the class.
Is the method only used as a helper for one of different methods? If yes, it's a good reason to put it right after the calling method so that the code can be read top-down.
Does the method seem to make sense outside of the context of your class? If yes, it's a good reason to make it a free function or even move it to a different file, like utils.
I have a python class which has multiple methods. I have defined my methods via #staticmethod instance and I want to call other methods of my class from inside my main function(main_function). I think I need self parameter for calling my other functions from my main function and I want to pass this parameter to my main_function when I create an instance of my class.
class myclass:
#staticmethod
def function1(param1)
print "function1"
#staticmethod
def main_function(self, param1)
function1(param1)
my_object = myclass()
my_object.main_function(param1)
I got this error:
TypeError: main_function() takes exactly 2 arguments (1 given)
The problem is that I have not self parameter when I create my instance. I tried to remove #staticmethod keyword from my method definition and remove all self parameter using, but this does not work.
Only use #staticmethod if you are creating a function that you'd normally want to tie to specific classes but do not need any other context. For example, the str.maketrans() function is a static method because it is a utility function you'd often use when working with strings, namespacing it to the already-existing str type (which pre-exists as a class) makes sense there.
You appear to be using classes as a namespace instead. Don't do that. Use a module for your functions, and you don't have to worry about the special scoping rules that apply to classes. Only use a class when you need to bundle state with functionality.
If you insist on using classes with static methods anyway, you are stuck with hardcoding the class name everywhere:
class myclass:
#staticmethod
def function1(param1)
print "function1"
#staticmethod
def main_function(param1)
# Want to use other functions in this class? Then you will
# have to use the full name of the class as a prefix:
myclass.function1(param1)
You could make use of classmethods instead so you have a reference to the class object:
class myclass:
#staticmethod
def function1(param1)
print "function1"
#classmethod
def main_function(cls, param1)
# Now you can use the `cls` reference to access other attributes
cls.function1(param1)
This has the added advantage that you can use inheritance.
However, using a module is the correct way to organise a set of functions into a namespace. Put everything into a my_module.py file in your package, and use importing;
import my_module
my_module.main_function(param1)
Now all globals in my_module are bundled into one module object, and no prefixing or cls references are needed.
I'm trying to create a class/static method within a class. I need SignInForm to have the defined instance variables, as they are rendered by Django's template. How does one call a method within a class from an instance variable? My objective is to update each variable with the custom widget for creating a consistent style.
class SignInForm(forms.Form):
#classmethod
def get_text_input_with_attributes():
return forms.TextInput(attrs= {'class':'form-style'})
first_name = forms.CharField(max_length=50,required=True)
first_name.widget = SignInForm.get_text_input_with_attributes()
last_name = forms.CharField(max_length=50,error_messages={'required': ''})
last_name.widget = SignInForm.get_text_input_with_attributes()
....lots of other custom fields
Error:
name 'SignInForm' is not defined
First, the short version:
You can't call methods of a class—even classmethods and staticmethods—while you're in the middle of defining that class, at least not easily.
So, what can you do? Well, really, you don't want a class method or a static method here. You want a regular old function. Like this:
class Spam(object):
def _func():
return 42
class_attr = _func()
del _func
After the class is defined, _func would be an instance method—and an un-callable instance method, since it doesn't take a self. (That's why I prefixed it with an underscore, and also del'd it, to make it harder to accidentally call it later…)
But while it's being defined, it's a normal function, and can be called as such.
I should mention that usually, wanting to do this is a sign that there's something off about your design, and the thing you're trying to write as a classmethod or staticmethod should actually be a method of a base class, or a free function or a class constructor, or maybe even a metaclass method.
As David Sanders' answer explains, the specific thing you're trying to do is common enough that there's an idiomatic way to write it: as a class constructor for a TextField subclass.
How does this work?
Clearly, while you're in the middle of executing the Spam class definition, there is nothing called Spam, so you definitely can't call Spam._func.
But why can you call _func? You have to understand how class definitions are executed. Oversimplifying a bit: Python creates an empty global dictionary, runs all the code inside the class definition as if it were a script, then it goes back to the real globals and runs Spam = type('Spam', (object,), that_global_dict). So when we do class_attr = _func(), we're inside that temporary global environment, and _func is a function in that environment, so we can call it.
So, why can't we do this with a classmethod or staticmethod?
For one thing, you need a cls object to call a classmethod, and we don't have one.
For another, classmethod and staticmethod objects aren't callable. Function objects are callable as themselves, and they're also descriptors that can be used to construct bound methods. Class and static methods are not callable, they're just descriptors that can be used to construct bound methods in special ways (bound to a class or to nothing, instead of to an instance).
So, what if you wanted to write something that was usable as a class-definition-time function, and also as a static method later? The simplest way is:
class Spam(object):
def _func():
return 42
smeth = staticmethod(_func)
class_attr = func()
del _func
A decorator like #staticmethod just does, in effect, _func = staticmethod(_func). We can do the same thing, but give the result a different name, and now we've got a static method, while still having the original function to call directly.
While abernet's answer does a good job of explaining why you were getting that error, the idiomatic way to do this with django would be something like this:
from django import forms
class SignInFormCharField(forms.CharField):
def __init__(self, *args, **kwargs):
kwargs.setdefault('widget', forms.TextInput(attrs={'class': 'form-style'}))
super(SignInFormCharField, self).__init__(*args, **kwargs)
class SignInForm(forms.Form):
first_name = SignInFormCharField(max_length=50, required=True)
last_name = SignInFormCharField(max_length=50, error_messages={'required': ''})
# ...
I've become aware of #staticmethod - next question, are you supposed to use the class name to refer to these methods from within the class?
class C:
#staticmethod
def imstatic():
print("i'm static")
#staticmethod
def anotherstatic():
# Is this the proper python way?
C.imstatic()
#staticmethod
def brokenstatic():
# This doesn't work..
self.imstatic()
Yes, as you don't have any other reference to the class from within a static method. You could make these class methods instead, using the classmethod decorator:
class C:
#staticmethod
def imstatic():
print("i'm static")
#classmethod
def anotherstatic(cls):
cls.imstatic()
A class method does have a reference to the class.
If you need to refer to the class within a static method you should probably be using a classmethod instead:
class C:
#staticmethod
def imstatic():
print("i'm static")
#classmethod
def imclass(cls):
cls.imstatic()
In the same way that instance methods are "magically" given a reference to the instance as the first argument, class methods are given a reference to the class. You can call them either from an instance or from the class directly, for example both of the following are valid and have the same behavior:
C().imclass()
C.imclass()
That being said, if you do still want to use a static method your current approach is correct, just refer to the class by name.
If you always want to call the static method of that specific class, yes, you must specify it by name. If you want to support overriding the static methods, what you want is a classmethod instead: it passes the class on which the method is being called as the first parameter, analogous to self on regular instance methods, so you can call the overridden method. In general I'd suggest using classmethods.
Look at this code:
class MyClass():
# Why does this give me "NameError: name 'self' is not defined":
mySelf = self
# But this does not?
def myFunction(self):
mySelf2 = self
Basically I want a way for a class to refer to itself without needing to name itself specifically, hence I want self to work for the class, not just methods/functions. How can I achieve this?
EDIT: The point of this is that I'm trying to refer to the class name from inside the class itself with something like self.class._name_ so that the class name isn't hardcoded anywhere in the class's code, and thus it's easier to re-use the code.
EDIT 2: From what I've learned from the answers below, what I'm trying to do is impossible. I'll have to find a different way. Mission abandoned.
EDIT 3: Here is specifically what I'm trying to do:
class simpleObject(object):
def __init__(self, request):
self.request = request
#view_defaults(renderer='string')
class Test(simpleObject):
# this line throws an error because of self
myClassName = self.__class__.__name__
#view_config(route_name=myClassName)
def activateTheView(self):
db = self.request.db
foo = 'bar'
return foo
Note that self is not defined at the time when you want the class to refer to itself for the assignment to work. This is because (in addition to being named arbitrarily), self refers to instances and not classes. At the time that the suspect line of code attempts to run, there is as of yet no class for it to refer to. Not that it would refer to the class if there was.
In a method, you can always use type(self). That will get the subclass of MyClass that created the current instance. If you want to hard-code to MyClass, that name will be available in the global scope of the methods. This will allow you to do everything that your example would allow if it actually worked. E.g, you can just do MyClass.some_attribute inside your methods.
You probably want to modify the class attributes after class creation. This can be done with decorators or on an ad-hoc basis. Metaclasses may be a better fit. Without knowing what you actually want to do though, it's impossible to say.
UPDATE:
Here's some code to do what you want. It uses a metaclass AutoViewConfigMeta and a new decorator to mark the methods that you want view_config applied to. I spoofed the view_config decorator. It prints out the class name when it's called though to prove that it has access to it. The metaclass __new__ just loops through the class dictionary and looks for methods that were marked by the auto_view_config decorator. It cleans off the mark and applies the view_config decorator with the appropriate class name.
Here's the code.
# This just spoofs the view_config decorator.
def view_config(route=''):
def dec(f):
def wrapper(*args, **kwargs):
print "route={0}".format(route)
return f(*args, **kwargs)
return wrapper
return dec
# Apply this decorator to methods for which you want to call view_config with
# the class name. It will tag them. The metaclass will apply view_config once it
# has the class name.
def auto_view_config(f):
f.auto_view_config = True
return f
class AutoViewConfigMeta(type):
def __new__(mcls, name, bases, dict_):
#This is called during class creation. _dict is the namespace of the class and
# name is it's name. So the idea is to pull out the methods that need
# view_config applied to them and manually apply them with the class name.
# We'll recognize them because they will have the auto_view_config attribute
# set on them by the `auto_view_config` decorator. Then use type to create
# the class and return it.
for item in dict_:
if hasattr(dict_[item], 'auto_view_config'):
method = dict_[item]
del method.auto_view_config # Clean up after ourselves.
# The next line is the manual form of applying a decorator.
dict_[item] = view_config(route=name)(method)
# Call out to type to actually create the class with the modified dict.
return type.__new__(mcls, name, bases, dict_)
class simpleObject(object):
__metaclass__ = AutoViewConfigMeta
class Test(simpleObject):
#auto_view_config
def activateTheView(self):
foo = 'bar'
print foo
if __name__=='__main__':
t = Test()
t.activateTheView()
Let me know if you have any questions.
Python has an "explict is better than implicit" design philosophy.
Many languages have an implicit pointer or variable in the scope of a method that (e.g. this in C++) that refers to the object through which the method was invoked. Python does not have this. Here, all bound methods will have an extra first argument that is the object through which the method was invoked. You can call it anything you want (self is not a keyword like this in C++). The name self is convention rather than a syntactic rule.
Your method myFunction defines the variable self as a parameter so it works. There's no such variable at the class level so it's erroring out.
So much for the explanation. I'm not aware of a straightforward way for you to do what you want and I've never seen such requirement in Python. Can you detail why you want to do such a thing? Perhaps there's an assumption that you're making which can be handled in another way using Python.
self is just a name, your self in this case is a class variable and not this for the object using which it is called,
self is treated as a normal variable and it is not defined, where as the self in the function comes from the object used for calling.
you want to treat the object reference in self as a class variable which is not possible.
self isn't a keyword, it's just a convention. The methods are attributes of the class object (not the instance), but they receive the instance as their first argument. You could rename the argument to xyzzy if you wanted and it would still work the same way.
But (as should be obvious) you can't refer to a method argument outside the body of the method. Inside a class block but outside of any method, self is undefined. And the concept wouldn't even make sense -- at the time the class block is being evaluated, no instance of the class can possibly exist yet.
Because the name self is explicitly defined as part of the arguments to myFunction. The first argument to a method is the instance that the method was called on; in the class body, there isn't an "instance we're dealing with", because the class body deals with every possible instance of the class (including ones that don't necessarily exist yet) - so, there isn't a particular object that could be called self.
If you want to refer to the class itself, rather than some instance of it, this is spelled self.__class__ (or, for new-style classes in Py2 and all classes in Py3, type(self)) anywhere self exists. If you want to be able to deal with this in situations where self doesn't exist, then you may want to look at class methods which aren't associated with any particular instance, and so take the class itself in place of self. If you really need to do this in the class body (and, you probably don't), you'll just have to call it by name.
You can't refer to the class itself within the class body because the class doesn't exist at the time that the class body is executed. (If the previous sentence is confusing, reading up about metaclasses will either clear this up or make you more confused.)
Within an instance method, you can refer to the class of the instance with self.__class__, but be careful here. This will be the instance's actual class, which through the power of inheritance might not be the class in which the method was defined.
Within a class method, the class is passed in as the first argument, much like instances are the first argument to instance methods:
class MyClass(object):
#classmethod
def foo(cls):
print cls.__name__
MyClass.foo() # Should print "MyClass"
As with instance methods, the actual class might differ due to inheritance.
class OtherClass(MyClass):
pass
OtherClass.foo() # Should print "OtherClass"
If you really need to refer to MyClass within a method of MyClass, you're pretty much going to have to refer to it as MyClass unless you use magic. This sort of magic is more trouble than it is worth.