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Python List copy() Function

The List copy() method creates a Shallow copy of the list.

Syntax​

my_list.copy()

copy() Parameters​

Python List copy() function does not take any parameters.

copy() Return Value​

Python List copy() function returns a new list. It doesn't modify the original list.

Examples​

Example 1: Copy a List with copy()​

Consider this example in which:

  • original_list.copy() creates a shallow copy of original_list.
  • When a string in the copied_list is modified, it does not affect original_list because the values in the lists are references to the same objects.
# create a list
original_list = ['Tom', 'Ryan', 'David']

# create a copy of the original_list
copied_list = original_list.copy() 

print("Original List:", original_list)
print("Copied List:", copied_list)

# Modifying the copied list
copied_list[1] =  'Anna'

print("Original List after modification:", original_list)
print("Copied List after modification:", copied_list)

output

Original List: ['Tom', 'Ryan', 'David']
Copied List: ['Tom', 'Ryan', 'David']
Original List after modification: ['Tom', 'Ryan', 'David']
Copied List after modification: ['Tom', 'Anna', 'David']

Example 2: Shallow Copy of a List with Mutable Values​

The copy() method creates a Shallow copy!

In the following example the values in original_list are lists (which are mutable) and so modifying one of the lists in copied_list also affects the corresponding list in original_list.

original_list = [['Tom', 'Ryan'], ['David']]
copied_list = original_list.copy()

print("Original List:", original_list)
print("Copied List:", copied_list)

# Modifying the copied dictionary
copied_list[1].append('Anna')

print("Original List after modification:", original_list)
print("Copied List after modification:", copied_list)

output

Original List: [['Tom', 'Ryan'], ['David']]
Copied List: [['Tom', 'Ryan'], ['David']]
Original List after modification: [['Tom', 'Ryan'], ['David', 'Anna']]
Copied List after modification: [['Tom', 'Ryan'], ['David', 'Anna']]

copy() vs Assignment statement​

  • When the copy() method is used, a new list is created which is filled with a copy of the references from the original list.
  • When the = operator is used, a new reference to the original list is created.

For example, an assignment statement does not copy objects.

old_list = ['Ryan', 'David', 'Tom']
new_list = old_list
new_list[0] = 'Anna'

print(old_list) # Output: ['Anna', 'David', 'Tom']
print(new_list) # Output: ['Anna', 'David', 'Tom']

output

['Anna', 'David', 'Tom']
['Anna', 'David', 'Tom']

When you execute new_list = old_list, you don’t actually have two lists. The assignment just makes the two variables point to the one list in memory.

So, when you change new_list, old_list is also modified. If you want to change one copy without changing the other, use copy() method.

old_list = ['Ryan', 'David', 'Tom']
new_list = old_list
new_list[0] = 'Anna'

print(old_list) # Output: ['Anna', 'David', 'Tom']
print(new_list) # Output: ['Anna', 'David', 'Tom']

output

{'name': 'Tom', 'age': 25}
{'name': 'xx', 'age': 25}

Equivalent Method using Slicing Operator​

An alternative way to copy a list is assigning a slice of the entire list to a variable:

old_list = ['Anna', 'David', 'Tom']
new_list = old_list[:]

print(new_list) # Output: ['Anna', 'David', 'Tom']

output

['Anna', 'David', 'Tom']

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