copy

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

  • A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.
  • A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.
In [1]: import copy

In [2]: a = [1, 2, 3, ['a', 'b', 'c']]

In [3]: b = a

In [4]: c = copy.copy(a)

In [5]: d = copy.deepcopy(a)

In [6]: a, b, c, d
Out[6]:
([1, 2, 3, ['a', 'b', 'c']],
 [1, 2, 3, ['a', 'b', 'c']],
 [1, 2, 3, ['a', 'b', 'c']],
 [1, 2, 3, ['a', 'b', 'c']])

In [7]: a.append(4)

In [8]: a[3].append('d')

In [9]: a, b, c, d
Out[9]:
([1, 2, 3, ['a', 'b', 'c', 'd'], 4],
 [1, 2, 3, ['a', 'b', 'c', 'd'], 4],
 [1, 2, 3, ['a', 'b', 'c', 'd']],
 [1, 2, 3, ['a', 'b', 'c']])

dict.copy()

help(dict.copy)

Help on method_descriptor:

copy(...)
    D.copy() -> a shallow copy of D

Examples:

In [1]: dt = {'a': 1, 'b': {'c': 3}}

In [2]: dt1 = dt.copy()

In [3]: dt1
Out[3]: {'a': 1, 'b': {'c': 3}}

In [4]: dt['c'] = 3

In [5]: dt1
Out[5]: {'a': 1, 'b': {'c': 3}}

In [6]: dt['b']['d'] = 4

In [7]: dt1
Out[7]: {'a': 1, 'b': {'c': 3, 'd': 4}}

References

[1] [email protected], 8.17. copy — Shallow and deep copy operations

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