What is the difference between doing

class a:

def __init__(self):

self.val=1

to doing

class a:

val=1

def __init__(self):

pass

解决方案

class a:

def __init__(self):

self.val=1

this creates a class (in Py2, a cruddy, legacy, old-style, don't do that! class; in Py3, the nasty old legacy classes have finally gone away so this would be a class of the one and only kind -- the **good* kind, which requires class a(object): in Py2) such that each instance starts out with its own reference to the integer object 1.

class a:

val=1

def __init__(self):

pass

this creates a class (of the same kind) which itself has a reference to the integer object 1 (its instances start out with no per-instance reference).

For immutables like int values, it's hard to see a practical difference. For example, in either case, if you later do self.val = 2 on one instance of a, this will make an instance reference (the existing answer is badly wrong in this respect).

The distinction is important for mutable objects, because they have mutator methods, so it's pretty crucial to know if a certain list is unique per-instance or shared among all instances. But for immutable objects, since you can never change the object itself but only assign (e.g. to self.val, which will always make a per-instance reference), it's pretty minor.

Just about the only relevant difference for immutables: if you later assign a.val = 3, in the first case this will affect what's seen as self.val by each instance (except for instances that had their own self.val assigned to, or equivalent actions); in the second case, it will not affect what's seen as self.val by any instance (except for instances for which you had performed del self.val or equivalent actions).