python - How do numpy arrays change when passed to functions as parameters -

as can guess title i'm not quite sure how ask question. anyway problem following:

i want pass numpy array parameter function, inside function want make copy of array, pass array function manipulating array , in end replace array copy.

... arr = np.array([...]) arr_ocp = arr.copy() func1(arr) print np.array_equal(arr,arr_ocp) -> false ...  def func1(arr):   arr_cp = arr.copy()   func2(arr)   print np.array_equal(arr,arr_cp) -> false   arr = arr_cp   print np.array_equal(arr,arr_cp) -> true  def func2(arr):   ...   arr[x:x+l,y:y+w] += np.array(...)   ... 

now did few prints , turns out in func1 arr same @ beginning , end, i'd have expected.but if print after func1(arr), arr value of arr after func2. why manipulations of func2 apply arr not arr = arr_cp?

in hindsight it's better pass copy func2 instead of array itself, still i'd not expect happen this. missing here?

you need understand difference between mutating , rebinding. when arr = arr_cp, you're doing rebinding name arr whatever referred arr_cp. don't in way affect object bound arr.

however, when arr - example, arr[0] = 'foo' - mutation, , changes actual object. names bound same object continue so, changes visible them all.