initializing an attribute in __init__

user4592357

creating a subclass of Qt class which has custom attributes.

Python complains when an attribute isn't constructed in __init__ and is constructed in another method, you get something like this:
AttributeError: 'Window' object has no attribute '_Window__menubar'

so what i do is:

# in __init__()
self.__menubar = self.menuBar()
self.__create_menubar()

# in __create_menubar()
menu = self.__menubar.addMenu('menu')
...

this code doesn't feel "comfortable" because i init the menu bar (or a widget), then add properties to it in another method.

is this the only way?

SGaist

Hi,

What about using self.__menubar = None in the constructor ?

With what version of python do you get that message ?

user4592357

@SGaist

oh i forgot about that, thanks. and 3.6.3. isn't is normal that i get the error?

JonB

@user4592357
PyCharm --- the IDE I use for Python/PyQt --- marks self. variables which are not created in __init__() with a harmless "warning underline". But it doesn't really matter. There is no "error message from Python".

user4592357

@JonB

i use visual studio code. anyways i think that initializing them in init isn't a bad idea, i was just confused about how to do it correctly

JonB

@user4592357
If you wish to ensure you always create the self. variables in __init__(), but that's before you are ready to actually create the objects --- e.g. you only create some member widget when the user performs some action --- then as @SGaist said do self.member = None in __init__(). That's effectively what happens for C++ code when member variables are declared (which of course you can't do in Python).

SGaist

Just a small correction, in C++, class members are not initialised by default, you have to explicitly set their value either in the constructor, constructor initialisation list or since C++11, you can do that at the same place as the declaration.

JonB

@SGaist
Oh, so in C++, for example:

class Fred
{
  private/protected/public:
    char * m_pChar;
}

m_pChar is not set to 0/NULL? (It is in C# :) )

user4592357

@JonB

in c++ members get some value assigned to them if you don't init them explicitly. so in your case the pointer will have some garbage value, but not NULL/nullptr

SGaist

No it's not.

See this excellent article from Arne Mertz on the subject.