Let’s you have a list in Python:

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a = [1, 2, 3, 4]

a = [1, 2, 3, 4]

And you have another list in Python:

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b = [5, 6, 7, 8]

b = [5, 6, 7, 8]

You can concatenate two lists by simply using + operator, which will leave both lists untouched and return a copy of the concatenated list.

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a + b  # [1, 2, 3, 4, 5, 6, 7, 8]

a + b  # [1, 2, 3, 4, 5, 6, 7, 8]

You can use extend() method of the array, which allows us to append all the elements from another list to it. This will modify the original list.

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# c is None
c = a.extend(b)
# a is now [1, 2, 3, 4, 5, 6, 7, 8]

# c is None
c = a.extend(b)
# a is now [1, 2, 3, 4, 5, 6, 7, 8]

The append() on the other hand, appends an element to the list. For example,

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a = [1, 2, 3, 4]
a.append(5)
# a is now [1, 2, 3, 4, 5]

a = [1, 2, 3, 4]
a.append(5)
# a is now [1, 2, 3, 4, 5]

The append returns None. You can use append to achieve what the extend does.

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def extend(a, b):
  for x in b:
    a.append(x)

def extend(a, b):
  for x in b:
    a.append(x)

–EOF (The Ultimate Computing & Technology Blog) —