Why are lists linked in Python in a persistent way? -

a variable set. variable set first. first changes value. second not. has been nature of programming since dawn of time.

>>> = 1 >>> b = >>> b = b - 1 >>> b 0 >>> 1 

i extend python lists. list declared , appended. list declared equal first. values in second list change. mysteriously, values in first list, though not acted upon directly, change.

>>> alist = list() >>> blist = list() >>> alist.append(1) >>> alist.append(2) >>> alist [1, 2] >>> blist [] >>> blist = alist >>> alist.remove(1) >>> alist [2] >>> blist [2] >>>  

why this?

and how prevent happening -- want alist unfazed changes blist (immutable, if will)?

variable binding in python works way: assign object variable.

a = 4 b = 

both point 4.

b = 9 

now b points somewhere else.

exactly same happens lists:

a = [] b = b = [9] 

now, b has new value, while a has old one.

till now, clear , have same behaviour mutable , immutable objects.

now comes misunderstanding: modifying objects.

lists mutable, if mutate list, modifications visible via variables ("name bindings") exist:

a = [] b =  # same list c = [] # empty 1  a.append(3) print a, b, c # b = [3], c = [] different 1  d = a[:] # copy b.append(9) # = b = [3, 9], c = [], d = [3], copy of old resp. b