Wandering around

It took me ~1 hour to find a silly bug because of the "call-by-value" thing in python: >> a = b = {} >> a[0] = 1 >> print a {0:1} >>b[0] = 9 >> print a {0:9} >> print id(a) 4297396640 >> print id(b) 4297396640 When you change the value of a copied variable, the formal one changes, too. So yes, as it is confusing usually how parameters are passed in different programming languages. What happens when a variable is passed and whether its value will be changed or not. In python, a variable actually holds a reference (pointer, address) of an object. When the variable is passed as a parameter, its value is copied into the formal parameter variable. >>x = 1 >>print id(x) 39030144 >>def f(x): >> print id(x) 39030144 >> x = 2 >> print id(x) 39030132 >>f(x) >>print x 1