I need to define a variable of a class a class object. How can I do it?
if for example I have a class like this :
class A:
def __init__(self, a, b):
self.a = a
self.b = b
and I want to create another class B that have a variable as instance of class A like :
class B:
def __init__(self, c = A(), d):
self.c = c
self.d = d
How can I do it ? I need to do particular operation or simply declarate c as object of class A when I create the object of class B ?
class B:
def __init__(self, a, b, d):
self.c = A(a, b)
self.d = d
or
class B:
def __init__(self, c, d):
self.c = c
self.d = d
or
class B:
def __init__(self, d):
self.c = A(a, b) # a and b can be values
self.d = d
a and b which won't work since they haven't been defined). The critical thing is that you can't just say A() because A.__init__ requires two arguments.What you wrote mostly works:
def __init__(self, c = A(), d):
self.c = c
But there's a "gotcha" which you really want to avoid.
The A constructor will be evaluated just once,
at def time, rather than each time you construct
a new B object.
That's typically not what a novice coder wants.
That signature mentions a mutable default arg,
something it's usually best to avoid,
if only to save future maintainers from
doing some frustrating debugging.
https://dollardhingra.com/blog/python-mutable-default-arguments/
https://towardsdatascience.com/python-pitfall-mutable-default-arguments-9385e8265422
Instead, phrase it this way:
class B:
def __init__(self, c = None, d):
self.c = A(1, 2) if c is None else c
...
That way the A constructor will be evaluated afresh each time.
(Also, it would be good to supply both of A's mandatory arguments.)
A() will raise an error because it's missing the a and b arguments.
if c is not None: self.c = c to self.c = c or .... If c can only be some class instance, which is normally True in boolean context, this will work ok, but more generally, it would prevent passing anything falsy for c, like 0, an empty string or list...
self.c = A(). Default values are evaluated when the function is defined, not when it's called.Aas a default argument toB.__init__?