The answer, in a few words
In your example, itsProblem
is a local variable.
Your must use self
to set and get instance variables. You can set it in the __init__
method. Then your code would be:
class Example[object]:
def __init__[self]:
self.itsProblem = "problem"
theExample = Example[]
print[theExample.itsProblem]
But if you want a true class variable, then use the class name directly:
class Example[object]:
itsProblem = "problem"
theExample = Example[]
print[theExample.itsProblem]
print [Example.itsProblem]
But be careful with this one, as theExample.itsProblem
is automatically set to be equal to Example.itsProblem
, but is not the same variable at all and can be changed
independently.
Some explanations
In Python, variables can be created dynamically. Therefore, you can do the following:
class Example[object]:
pass
Example.itsProblem = "problem"
e = Example[]
e.itsSecondProblem = "problem"
print Example.itsProblem == e.itsSecondProblem
prints
True
Therefore, that's exactly what you do with the previous examples.
Indeed, in Python we use self
as this
, but it's a bit more than that. self
is the the first argument to any object method because the first argument is always the object reference. This is automatic,
whether you call it self
or not.
Which means you can do:
class Example[object]:
def __init__[self]:
self.itsProblem = "problem"
theExample = Example[]
print[theExample.itsProblem]
or:
class Example[object]:
def __init__[my_super_self]:
my_super_self.itsProblem = "problem"
theExample = Example[]
print[theExample.itsProblem]
It's exactly the same. The first argument of ANY object method is the current object, we only call it self
as a convention. And you add just a variable to this object, the same way you would do it from outside.
Now, about the class variables.
When you do:
class Example[object]:
itsProblem = "problem"
theExample = Example[]
print[theExample.itsProblem]
You'll notice we first set a class variable, then we access an object [instance] variable. We never set this object variable but it works, how is that possible?
Well, Python tries to get first the object variable, but if it can't find it, will give you the class variable. Warning: the class variable is shared among instances, and the object variable is not.
As a conclusion, never use class variables to set default values to object variables. Use __init__
for that.
Eventually, you will learn that Python classes are instances and therefore objects themselves, which gives new insight to understanding the above. Come back and read this again later, once you realize that.
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In Python, we can define the variable outside the class, inside the class, and even inside the methods. Let’s see, how to use and access these variables throughout the program.
Variable defined outside the class:
The variables that are defined outside the class can be accessed by any class or any methods in the class by just writing the variable name.
outVar
=
'outside_class'
print
[
"Outside_class1"
, outVar]
class
Geek:
print
[
"Outside_class2"
, outVar]
def
access_method[
self
]:
print
[
"Outside_class3"
, outVar]
uac
=
Geek[]
uac.access_method[]
class
Another_Geek_class:
print
[
"Outside_class4"
, outVar]
def
another_access_method[
self
]:
print
[
"Outside_class5"
, outVar]
uaac
=
Another_Geek_class[]
uaac.another_access_method[]
Output:
Outside_class1 outside_class Outside_class2 outside_class Outside_class3 outside_class Outside_class4 outside_class Outside_class5 outside_class
Variable defined inside the class:
The variables that are defined inside the class but outside the method can be accessed within the class[all methods
included] using the instance of a class. For Example – self.var_name.
If you want to use that variable even outside the class, you must declared that variable as a global. Then the variable can be accessed using its name inside and outside the class and not using the instance of the class.
class
Geek:
inVar
=
'inside_class'
print
[
"Inside_class2"
, inVar]
def
access_method[
self
]:
print
[
"Inside_class3"
,
self
.inVar]
uac
=
Geek[]
uac.access_method[]
class
another_Geek_class:
print
[]
def
another_access_method[
self
]:
print
[]
uaac
=
another_Geek_class[]
uaac.another_access_method[]
Output:
Inside_class2 inside_class Inside_class3 inside_class
The statements which are marked as error will produce an error upon execution as the variable is not accessible there.
Variable defined inside the method:
The variables that are defined inside the methods can be accessed within that method only by simply using the variable name. Example – var_name.
If you want to use that variable outside the method or class, you have to declared that variable as a global.
class
Geek:
print
[]
def
access_method[
self
]:
inVar
=
'inside_method'
print
[
"Inside_method3"
, inVar]
uac
=
Geek[]
uac.access_method[]
class
AnotherGeek:
print
[]
def
access_method[
self
]:
print
[]
uaac
=
AnotherGeek[]
uaac.access_method[]
Output:
Inside_method3 inside_method
The statements which are marked as error will produce error upon execution as the variable is not accessible there.
Summary: