Context: I'm making a Ren'py game. The value is Character(). Yes, I know this is a dumb idea outside of this context.
I need to create a variable from an input string inside of a class that exists outside of the class' scope:
class Test:
def __init__(self):
self.dict = {} # used elsewhere to give the inputs for the function below.
def create_global_var(self, variable, value):
# the equivalent of exec("global {0}; {0} = {1}".format(str(variable), str(value)))
# other functions in the class that require this.
Test().create_global_var("abc", "123") # hence abc = 123
I have tried vars()[], globals()[variable] = value, etc, and they simply do not work (they don't even define anything) Edit: this was my problem.
I know that the following would work equally as well, but I want the variables in the correct scope:
setattr(self.__class__, variable, value) # d.abc = 123, now. but incorrect scope.
How can I create a variable in the global scope from within a class, using a string as the variable name, without using attributes or exec in python?
And yes, i'll be sanity checking.
First things first: what we call the "global" scope in Python is actually the "module" scope
(on the good side, it diminishes the "evils" of using global vars).
Then, for creating a global var dynamically, although I still can't see why that would
be better than using a module-level dictionary, just do:
globals()[variable] = value
This creates a variable in the current module. If you need to create a module variable on the module from which the method was called, you can peek at the globals dictionary from the caller frame using:
from inspect import currentframe
currentframe(1).f_globals[variable] = name
Now, the this seems especially useless since you may create a variable with a dynamic name, but you can't access it dynamically (unless using the globals dictionary again)
Even in your test example, you create the "abc" variable passing the method a string, but then you have to access it by using a hardcoded "abc" - the language itself is designed to discourage this (hence the difference to Javascript, where array indexes and object attributes are interchangeable, while in Python you have distinct Mapping objects)
My suggestion is that you use a module-level explicit dictionary and create all your
dynamic variables as key/value pairs there:
names = {}
class Test(object):
def __init__(self):
self.dict = {} # used elsewhere to give the inputs for the function below.
def create_global_var(self, variable, value):
names[variable] = value
(on a side note, in Python 2 always inherit your classes from "object")
You can use setattr(__builtins__, 'abc', '123') for this.
Do mind you that this is most likely a design problem and you should rethink the design.
Related
i have some code that has this general structure:
def runSolver():
global a
global b
while (condition) :
# do some stuff
recalculateIntermediateParameters()
# do more stuff
return result
def recalculateIntermediateParameters():
# read globals
# calculate
return recalculatedParameter
# main
if __name__="__main__":
runSolver()
I'm wondering if this is the best implementation. For instance is it bad practice to have the globals declared in a function? I know in Java, a global variable is best declared outside any function definitions.
My thought was that this would be syntactically "better":
def runSolver():
a = foo
b = bar
# do some stuff
return result
def recalculateIntermediateParameters(a, b):
# do some stuff
return recalculatedParameter
But what if a and b are only read not manipulated by the function? Does that affect global placement? Further, what if a and b are lengthy lists? Does it make sense in a performance perspective to pass the values from function to function? Which paradigm offers the best compromise between "pythonic" code and performance?
You only need to state an identifier is global when you are changing it in a function. If you only reference a and b and not assign to them, you can omit the global statements.
Functions are globals too, for example. recalculateIntermediateParameters is not being assigned to, only referenced, and you didn't need to use global recalculateIntermediateParameters to be able to do so.
Python uses different opcodes to assign to locals versus globals. If you do:
def foo():
a = 10
versus
def foo():
global a
a = 10
Python uses two different operations to update a as a local or a global.
More generally speaking, you want to avoid using globals to pass around state. Pass values as parameters (using primitives, or a compound structure such as a dictionary or a custom class instance) around instead. It makes your code easier to read and debug as you can trace the state through the functions instead of having to keep track of a global separately.
What I am trying to do, is creating a module, with a class; and a function, which is an interface of that class; and a variable name on-the-fly in this function, which is pointing to an instance of that class. This function and the class itself should be in a separate module, and their usage should be in a different python file.
I think, it's much easier to understand what I am trying to do, when you are looking at my code:
This is the first.py:
class FirstClass:
def setID(self, _id):
self.id = _id
def func(self):
pass
# An 'interface' for FirstClass
def fst(ID):
globals()['%s' % ID] = FirstClass(ID)
return globals()['%s' % ID]
Now, if I'm calling fst('some_text') right in first.py, the result is pretty much what I dreamed of, because later on, any time I write some_text.func(), it will call the func(), because some_text is pointing to an instance of FirstClass.
But, when the second.py is something like this:
from first import fst
fst('sample_name')
sample_name.func()
Then the answer from python is going to be like this:
NameError: name 'sample_name' is not defined.
Which is somewhat reasonable.. So my question is: is there a "prettier" method or a completely different one to do this? Or do I have to change something small in my code to get this done?
Thank you!
Don't set it as a global in the function. Instead, just return the new instance from the function and set the global to that return value:
def fst(ID):
return FirstClass(ID)
then in second.py:
sample_name = fst('sample_name')
where, if inside a function, you declare sample_name a global.
The globals() method only ever returns the globals of the module in which you call it. It'll never return the globals of whatever is calling the function. If you feel you need to have access to those globals, rethink your code, you rarely, if ever, need to alter the globals of whatever is calling your function.
If you are absolutely certain you need access to the caller globals, you need to start hacking with stack frames:
# retrieve caller globals
import sys
caller_globals = sys._getframe(1).f_globals
But, as the documentation of sys._getframe() states:
CPython implementation detail: This function should be used for internal and specialized purposes only. It is not guaranteed to exist in all implementations of Python.
This question already has answers here:
What is the purpose of the `self` parameter? Why is it needed?
(26 answers)
Closed 6 months ago.
I'm learning Python and I have a question, more theoretical than practical, regarding access class variables from method of this class.
For example we have:
class ExampleClass:
x = 123
def example_method(self):
print(self.x)
Why is necessarily to write exactly self.x, not just x? x belongs to namespace of the class, and method using it belongs to it too. What am I missing? What a rationale stands behind such style?
In C++ you can write:
class ExampleClass {
public:
int x;
void example_method()
{
x = 123;
cout << x;
};
};
And it will work!
From The History of Python: Adding Support for User-defined Classes:
Instead, I decided to give up on the idea of implicit references to
instance variables. Languages like C++ let you write this->foo to
explicitly reference the instance variable foo (in case there’s a
separate local variable foo). Thus, I decided to make such explicit
references the only way to reference instance variables. In addition,
I decided that rather than making the current object ("this") a
special keyword, I would simply make "this" (or its equivalent) the
first named argument to a method. Instance variables would just always
be referenced as attributes of that argument.
With explicit references, there is no need to have a special syntax
for method definitions nor do you have to worry about complicated
semantics concerning variable lookup. Instead, one simply defines a
function whose first argument corresponds to the instance, which by
convention is named "self." For example:
def spam(self,y):
print self.x, y
This approach resembles something I had seen in Modula-3, which had
already provided me with the syntax for import and exception handling.
Modula-3 doesn’t have classes, but it lets you create record types
containing fully typed function pointer members that are initialized
by default to functions defined nearby, and adds syntactic sugar so
that if x is such a record variable, and m is a function pointer
member of that record, initialized to function f, then calling
x.m(args) is equivalent to calling f(x, args). This matches the
typical implementation of objects and methods, and makes it possible
to equate instance variables with attributes of the first argument.
So, stated by the BDFL himself, the only real reason he decided to use explicit self over implicit self is that:
it is explicit
it is easier to implement, since the lookup must be done at runtime(and not at compile time like other languages) and having implicit self could have increased the complexity(and thus cost) of the lookups.
Edit: There is also an answer in the Python FAQ.
It seems to be related to module vs. class scope handling, in Python:
COLOR = 'blue'
class TellColor(object):
COLOR = 'red'
def tell(self):
print self.COLOR # references class variable
print COLOR # references module variable
a = TellColor()
a.tell()
> red
> blue
Here's the content I did in an ancient answer concerning this feature:
The problem you encountered is due to this:
A block is a piece of Python program text that is executed as a unit.
The following are blocks: a module, a function body, and a class
definition.
(...)
A scope defines the visibility of a name within a
block.
(...)
The scope of names defined in a class block is limited to
the class block; it does not extend to the code blocks of methods –
this includes generator expressions since they are implemented using a
function scope. This means that the following will fail:
class A:
a = 42
b = list(a + i for i in range(10))
http://docs.python.org/reference/executionmodel.html#naming-and-binding
The above means:
a function body is a code block and a method is a function, then names defined out of the function body present in a class definition do not extend to the function body.
It appeared strange to me, when I was reading this, but that's how Python is crafted:
The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods
That's the official documentation that says this.
.
EDIT
heltonbiker wrote an interesting code:
COLOR = 'blue'
class TellColor(object):
COLOR = 'red'
def tell(self):
print self.COLOR # references class variable
print COLOR # references module variable
a = TellColor()
a.tell()
> red
> blue
It made me wonder how the instruction print COLOR written inside the method tell() provokes the printing of the value of the global object COLOR defined outside the class.
I found the answer in this part of the official documentation:
Methods may reference global names in the same way as ordinary
functions. The global scope associated with a method is the module
containing its definition. (A class is never used as a global scope.)
While one rarely encounters a good reason for using global data in a
method, there are many legitimate uses of the global scope: for one
thing, functions and modules imported into the global scope can be
used by methods, as well as functions and classes defined in it.
Usually, the class containing the method is itself defined in this
global scope (...)
http://docs.python.org/2/tutorial/classes.html#method-objects
When the interpreter has to execute print self.COLOR, as COLOR isn't an instance attribute (that is to say the identifier 'COLOR' doesn't belong to the namespace of the instance), the interpreter goes in the namespace of the class of the instance in search for the identifier 'COLOR' and find it, so it prints the value of TellColor.COLOR
When the interpreter has to execute print COLOR, as there is no attribute access written in this instruction, it will search for the identifier 'COLOR' in the global namespace, which the official documentation says it's the module's namespace.
What attribute names are attached to an object (and its class, and the ancestors of that class) is not decidable at compile time. So you either make attribute lookup explicit, or you:
eradicate local variables (in methods) and always use instance variables. This does no good, as it essentially removes local variables with all their advantages (at least in methods).
decide whether a base x refers to an attribute or local at runtime (with some extra rules to decide when x = ... adds a new attribute if there's no self.x). This makes code less readable, as you never know which one a name is supposed to be, and essentially turns every local variable in all methods into part of the public interface (as attaching an attribute of that name changes the behavior of a method).
Both have the added disadvantage that they require special casing for methods. Right now, a "method" is just a regular function that happens to be accessible through a class attribute. This is very useful for a wide variety of use good cases.
I want to clarify how variables are declared in Python.
I have seen variable declaration as
class writer:
path = ""
sometimes, there is no explicit declaration but just initialization using __init__:
def __init__(self, name):
self.name = name
I understand the purpose of __init__, but is it advisable to declare variable in any other functions?
How can I create a variable to hold a custom type?
class writer:
path = "" # string value
customObj = ??
Okay, first things first.
There is no such thing as "variable declaration" or "variable initialization" in Python.
There is simply what we call "assignment", but should probably just call "naming".
Assignment means "this name on the left-hand side now refers to the result of evaluating the right-hand side, regardless of what it referred to before (if anything)".
foo = 'bar' # the name 'foo' is now a name for the string 'bar'
foo = 2 * 3 # the name 'foo' stops being a name for the string 'bar',
# and starts being a name for the integer 6, resulting from the multiplication
As such, Python's names (a better term than "variables", arguably) don't have associated types; the values do. You can re-apply the same name to anything regardless of its type, but the thing still has behaviour that's dependent upon its type. The name is simply a way to refer to the value (object). This answers your second question: You don't create variables to hold a custom type. You don't create variables to hold any particular type. You don't "create" variables at all. You give names to objects.
Second point: Python follows a very simple rule when it comes to classes, that is actually much more consistent than what languages like Java, C++ and C# do: everything declared inside the class block is part of the class. So, functions (def) written here are methods, i.e. part of the class object (not stored on a per-instance basis), just like in Java, C++ and C#; but other names here are also part of the class. Again, the names are just names, and they don't have associated types, and functions are objects too in Python. Thus:
class Example:
data = 42
def method(self): pass
Classes are objects too, in Python.
So now we have created an object named Example, which represents the class of all things that are Examples. This object has two user-supplied attributes (In C++, "members"; in C#, "fields or properties or methods"; in Java, "fields or methods"). One of them is named data, and it stores the integer value 42. The other is named method, and it stores a function object. (There are several more attributes that Python adds automatically.)
These attributes still aren't really part of the object, though. Fundamentally, an object is just a bundle of more names (the attribute names), until you get down to things that can't be divided up any more. Thus, values can be shared between different instances of a class, or even between objects of different classes, if you deliberately set that up.
Let's create an instance:
x = Example()
Now we have a separate object named x, which is an instance of Example. The data and method are not actually part of the object, but we can still look them up via x because of some magic that Python does behind the scenes. When we look up method, in particular, we will instead get a "bound method" (when we call it, x gets passed automatically as the self parameter, which cannot happen if we look up Example.method directly).
What happens when we try to use x.data?
When we examine it, it's looked up in the object first. If it's not found in the object, Python looks in the class.
However, when we assign to x.data, Python will create an attribute on the object. It will not replace the class' attribute.
This allows us to do object initialization. Python will automatically call the class' __init__ method on new instances when they are created, if present. In this method, we can simply assign to attributes to set initial values for that attribute on each object:
class Example:
name = "Ignored"
def __init__(self, name):
self.name = name
# rest as before
Now we must specify a name when we create an Example, and each instance has its own name. Python will ignore the class attribute Example.name whenever we look up the .name of an instance, because the instance's attribute will be found first.
One last caveat: modification (mutation) and assignment are different things!
In Python, strings are immutable. They cannot be modified. When you do:
a = 'hi '
b = a
a += 'mom'
You do not change the original 'hi ' string. That is impossible in Python. Instead, you create a new string 'hi mom', and cause a to stop being a name for 'hi ', and start being a name for 'hi mom' instead. We made b a name for 'hi ' as well, and after re-applying the a name, b is still a name for 'hi ', because 'hi ' still exists and has not been changed.
But lists can be changed:
a = [1, 2, 3]
b = a
a += [4]
Now b is [1, 2, 3, 4] as well, because we made b a name for the same thing that a named, and then we changed that thing. We did not create a new list for a to name, because Python simply treats += differently for lists.
This matters for objects because if you had a list as a class attribute, and used an instance to modify the list, then the change would be "seen" in all other instances. This is because (a) the data is actually part of the class object, and not any instance object; (b) because you were modifying the list and not doing a simple assignment, you did not create a new instance attribute hiding the class attribute.
This might be 6 years late, but in Python 3.5 and above, you can give a hint about a variable type like this:
variable_name: type_name
or this:
variable_name # type: shinyType
This hint has no effect in the core Python interpreter, but many tools will use it to aid the programmer in writing correct code.
So in your case(if you have a CustomObject class defined), you can do:
customObj: CustomObject
See this or that for more info.
There's no need to declare new variables in Python. If we're talking about variables in functions or modules, no declaration is needed. Just assign a value to a name where you need it: mymagic = "Magic". Variables in Python can hold values of any type, and you can't restrict that.
Your question specifically asks about classes, objects and instance variables though. The idiomatic way to create instance variables is in the __init__ method and nowhere else — while you could create new instance variables in other methods, or even in unrelated code, it's just a bad idea. It'll make your code hard to reason about or to maintain.
So for example:
class Thing(object):
def __init__(self, magic):
self.magic = magic
Easy. Now instances of this class have a magic attribute:
thingo = Thing("More magic")
# thingo.magic is now "More magic"
Creating variables in the namespace of the class itself leads to different behaviour altogether. It is functionally different, and you should only do it if you have a specific reason to. For example:
class Thing(object):
magic = "Magic"
def __init__(self):
pass
Now try:
thingo = Thing()
Thing.magic = 1
# thingo.magic is now 1
Or:
class Thing(object):
magic = ["More", "magic"]
def __init__(self):
pass
thing1 = Thing()
thing2 = Thing()
thing1.magic.append("here")
# thing1.magic AND thing2.magic is now ["More", "magic", "here"]
This is because the namespace of the class itself is different to the namespace of the objects created from it. I'll leave it to you to research that a bit more.
The take-home message is that idiomatic Python is to (a) initialise object attributes in your __init__ method, and (b) document the behaviour of your class as needed. You don't need to go to the trouble of full-blown Sphinx-level documentation for everything you ever write, but at least some comments about whatever details you or someone else might need to pick it up.
For scoping purpose, I use:
custom_object = None
Variables have scope, so yes it is appropriate to have variables that are specific to your function. You don't always have to be explicit about their definition; usually you can just use them. Only if you want to do something specific to the type of the variable, like append for a list, do you need to define them before you start using them. Typical example of this.
list = []
for i in stuff:
list.append(i)
By the way, this is not really a good way to setup the list. It would be better to say:
list = [i for i in stuff] # list comprehension
...but I digress.
Your other question.
The custom object should be a class itself.
class CustomObject(): # always capitalize the class name...this is not syntax, just style.
pass
customObj = CustomObject()
As of Python 3, you can explicitly declare variables by type.
For instance, to declare an integer one can do it as follows:
x: int = 3
or:
def f(x: int):
return x
see this question for more detailed info about it:
Explicitly declaring a variable type in Python
Both of these blocks of code work. Is there a "right" way to do this?
class Stuff:
def __init__(self, x = 0):
global globx
globx = x
def inc(self):
return globx + 1
myStuff = Stuff(3)
print myStuff.inc()
Prints "4"
class Stuff:
def __init__(self, x = 0):
self.x = x
def inc(self):
return self.x + 1
myStuff = Stuff(3)
print myStuff.inc()
Also prints "4"
I'm a noob, and I'm working with a lot of variables in a class. Started wondering why I was putting "self." in front of everything in sight.
Thanks for your help!
You should use the second way, then every instance has a separate x
If you use a global variable then you may find you get surprising results when you have more than one instance of Stuff as changing the value of one will affect all the others.
It's normal to have explicit self's all over your Python code. If you try tricks to avoid that you will be making your code difficult to read for other Python programmers (and potentially introducing extra bugs)
There are 2 ways for "class scope variables". One is to use self, this is called instance variable, each instance of a class has its own copy of instance variables; another one is to define variables in the class definition, this could be achieved by:
class Stuff:
globx = 0
def __init__(self, x = 0):
Stuff.globx = x
...
This is called class attribute, which could be accessed directly by Stuff.globx, and owned by the class, not the instances of the class, just like the static variables in Java.
you should never use global statement for a "class scope variable", because it is not. A variable declared as global is in the global scope, e.g. the namespace of the module in which the class is defined.
namespace and related concept is introduced in the Python tutorial here.
Those are very different semantically. self. means it's an instance variable, i.e. each instance has its own. This is propably the most common kind, but not the only one. And then there are class variables, defined at class level (and therefore by the time the class definition is executed) and accessable in class methods. The equivalent to most uses of static methods, and most propably what you want when you need to share stuff between instances (this is perfectly valid, although not automatically teh one and only way for a given problem). You propably want one of those, depending on what you're doing. Really, we can't read your mind and tell you which one fits your problem.
Globals variables are a different story. They're, well, global - everyone has the same one. This is almost never a good idea (for reasons explained on many occasions), but if you're just writing a quick and dirty script and need share something between several places, they can be acceptable.