In C# I would go:
string UserName;
string Password;
But now, in Python:
class User:
UserName
Password
I recieve an error that UserName isn't defined. I can't declare a variable without a variable?
views:
126answers:
5
+6
A:
First of all, you should rewrite like this:
class User(object):
def __init__(self, username, password):
self.username = username
self.password = password
This way, username and password are instance variables instead of class variables (in your example, they are class variables -- all instance variables need to be defined in __init__ as properties of self). Then, you can initialize a User with whatever username and password you want, including None if you truly want them to have no value.
danben
2010-01-19 15:42:50
And, if you think you want to "declare" an attribute, you can't. You can, however, assign `None` so that you can create an attribute.
S.Lott
2010-01-19 15:44:00
+3
A:
In Python, and many other languages, there is a value that means "no value". In Python, that value is None. So you could do something like this:
class User:
UserName = None
PassWord = None
Those sure sound like instance variables though, and not class variables, so maybe do this:
class User(object):
def __init__(self):
self.UserName = None
self.PassWord = None
Note how Python assigns the None value implicitly from time to time:
def f():
pass
g = f() # g now has the value of None
Triptych
2010-01-19 15:43:29
Hi there, thanks for the answer. A couple of questions though, why do you put (object) at the end of the class name? Also, what is def __init__ ? If you have a good link that'll be great.
Sergio Tapia
2010-01-19 15:45:22
@Sergio Tapia: See this: http://stackoverflow.com/questions/54867/old-style-and-new-style-classes-in-python
Fred Larson
2010-01-19 15:47:13
extending `object` in Python is kind of a syntactic hack, though an extremely common one, that means you want to use new-style classes. If using Python 2.x, you should get in the habit of doing that. `__init__` is a python constructor.
Triptych
2010-01-19 15:47:42
`__init__` is Python's constructor method. It takes `self` as its first parameter, which is a reference to the object to be created, as well as a variable number of other parameters. You then construct the object by manipulating `self`.
danben
2010-01-19 15:49:05
Just another quick question. Class variables will be things that only my class uses, correct, like if I have a class for DB connection, the connectionstring will only be useful for itself. But instance classes are items that you want to be available if the class is instantiated, correct?
Sergio Tapia
2010-01-19 15:52:06
It helps to speak positively. When in doubt, run the simplest test to get confirmation. You can (and may) continue to get upvotes long after an answer was accepted.
S.Lott
2010-01-19 16:57:01
Thanks for pushing S.Lott. I've tested it and updated my answer.
Lyndsey Ferguson
2010-01-19 17:12:18
+2
A:
You can't. In Python, "Variables" are just names. A name always points ("is bound") to an object. It's convention to assign names that don't yet have a sensible value, but should be present, to None.
You might also be interested in my answer here.
balpha
2010-01-19 15:44:38
+1. Always good to remind folks that "assigning value" in Python is a bit of a dubious concept.
Triptych
2010-01-19 15:53:39
+1
A:
In your code:
class User:
UserName
Password
UserName and Password are parsed as expressions. Since they have not been assigned at this point, you get a NameError.
In Python, a variable must be defined with a assignment statement before it can be used in an expression, otherwise you get a NameError. Note that "before" here means "execution order", not "source code order". There's a bit more to it (import statements, globals, namespace hacks), but let's keep it simple here.
The idiomatic way to define a variable "without a value" is to assign it the value None.
Also, your code looks like it really wants instance members, and not class members. The idiomatic way to do it, as recognized by some static analysis tools such as pylint, is:
class User(object):
def __init__(self):
self.username = None
self.password = None
Also, it is good Python style to derive all classes from "object", so you use new-style classes, and to name instance variable with the lowercase_with_underscore or initialLowerWithCaps convention. The InitialCaps style is quite universally reserved to class names.
ddaa
2010-01-19 15:56:08