In C# I would go:
string UserName;
string Password;
But now, in Python:
class User:
UserName
Password
I receive an error that UserName isn't defined. Can I not declare a variable without a variable?
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1The Python tutorial is here: docs.python.org/tutorial/index.html It's pretty good if you have done programming before.Lennart Regebro– Lennart Regebro2010-01-19 16:14:45 +00:00Commented Jan 19, 2010 at 16:14
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1Why would you want to do that?sykloid– sykloid2010-01-19 16:33:27 +00:00Commented Jan 19, 2010 at 16:33
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6 Answers
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
8 Comments
Sergio Tapia
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.
Fred Larson
@Sergio Tapia: See this: stackoverflow.com/questions/54867/…
Kenan Banks
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.
eric
@SergioTapia yes. See docs.python.org/2/tutorial/…
michcio1234
@Triptych Python definitely doesn't assign None value "from time to time". If a function/method doesn't contain a
return statement, then it always returns None. I know I am ~10 years late with this comment, but maybe it'll still be useful to someone. |
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.
1 Comment
S.Lott
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.@dataclass
class User:
UserName: str
Password: str
Comments
In your code:
class User:
UserName
Password
print(UserName, Password)
UserName and Password and print(UserName, 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.
Comments
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.
1 Comment
Kenan Banks
+1. Always good to remind folks that "assigning value" in Python is a bit of a dubious concept.
Just assign the value of None:
self.username = None
3 Comments
Lyndsey Ferguson
Yeah, you're right. But the answer has already been selected.
S.Lott
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.
Lyndsey Ferguson
Thanks for pushing S.Lott. I've tested it and updated my answer.