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macOS Python constructor syntax

Analog Kid

Analog Kid

macrumors G3
Original poster
Mar 4, 2003
8,869
11,410
Can someone explain why this is behaving how it is:


Code: 
class TestClass(object):
    def __init__(self,x):
            if isinstance(x,TestClass):
                    self.x=x.x
            else:
                    self.x=x
    def foo(self,y):
            self.x=self.x+y

t=TestClass(5)
t.foo(4)
t.x         #returns 9

v=TestClass(t)
v.foo(3)
v.x        #returns 12

m=TestClass(t).foo(3)
m.x       #Attribute Error: 'NoneType" object has no attribute 'x'

Why does m.x not behave the same as v.x?
 
D

dylanryan

macrumors member
Aug 21, 2011
57
2
Because you are assigning the result of TestClass(t).foo(3) to m. So, m will be the number 12 (EDIT: Not the number 12, but rather None. I don't use Python, so I assume that is like undefined, in which case you need to return something from that function if you want to use it like that). And that doesn't have an x attribute.

Compare to the other cases, where you assign just TestClass(t) and then as a separate line call foo.
 
Analog Kid

Analog Kid

macrumors G3
Original poster
Mar 4, 2003
8,869
11,410
dylanryan said:
Because you are assigning the result of TestClass(t).foo(3) to m. So, m will be the number 12 (EDIT: Not the number 12, but rather None. I don't use Python, so I assume that is like undefined, in which case you need to return something from that function if you want to use it like that). And that doesn't have an x attribute.

Compare to the other cases, where you assign just TestClass(t) and then as a separate line call foo.
Click to expand...

Doh! Of course...

Thanks!
 
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