🐍 Python
Cloning Objects

Updated at 2018-06-10 22:50

Use collection factory functions to create shallow copies.

new_list = list(original_list)
new_dict = dict(original_dict)
new_set = set(original_set)

Use copy package to create collection deep copies.

import copy

xs = [[1, 2], [4, 5], [7, 8]]
ys = copy.deepcopy(xs)

assert xs == ys
assert xs is not ys

xs[1][0] = 'X'
assert xs != ys

Use copy package to create object copies. You can customize the copy behavior with __copy__ and __deepcopy__.

import copy

class Point:

    def __init__(self, x: float, y: float) -> None:
        self.x: float = x
        self.y: float = y

class Rectangle:

    def __init__(self, top_left: Point, bottom_right: Point) -> None:
        self.top_left: Point = top_left
        self.bottom_right: Point = bottom_right

a = Point(10, 20)
b = copy.copy(a)
assert a.x == b.x
assert a.y == b.y
assert a is not b

c = Rectangle(a, b)
d = copy.copy(c)      # BAD, doesn't duplicate the points
e = copy.deepcopy(c)  # GOOD, duplicates the points
assert a is d.top_left and b is d.bottom_right
assert a is not e.top_left and b is not e.bottom_right


  • Python Tricks The Book, Dan Bader
  • Fluent Python, Luciano Ramalho