Concatenate lists in Python (extend, append, addition operator, slicing)

A Python list can be extended by extend(), a list method.

x = ['a', 'b', 'c']
y = [1, 2]

x.extend(y)

print(x)  # ['a', 'b', 'c', 1, 2]
print(y)  # [1, 2]

It updates x and takes only one list.

x = ['a', 'b', 'c']
y = [1, 2]
z = [3, 4]

x.extend(y, z)
# TypeError: extend() takes exactly one argument (2 given)

The difference of extend() and append()

extend() may be confused with append(), which appends an element to a list. append() adds an element but extend() adds a list.

a = [1, 2, 3]
a.append(4)

print(a)  # [1, 2, 3, 4]

The difference of extend() and addition operator

Python lists can be concatenated by + operator.

a = [1, 2, 3]
b = [4, 5]

c = a + b

print(c)  # [1, 2, 3, 4, 5]

The addition operator creates a new list and old lists doesn't change. List addition is a typical example of overloading operator.

Unpacking operator

a = [1, 2, 3]
b = [4, 5]

c = [*a, *b]

print(c)  # [1, 2, 3, 4, 5]

* is an unpacking operator that enables concatenating combined with brackets used to form a Python list.

extend() is equivalent to slicing

As stated in Python document, extend() can be substituted with a slice format:

a = [1, 2, 3]
b = [4, 5]

a[len(a):] = b

print(a)  # [1, 2, 3, 4, 5]

It's helpful to know list methods but it's also important to understand the equivalence of the iterable methods (like extend()) and slice. append() is actually equivalent to this slice:

a = [1, 2, 3]

a[len(a):] = [5]

print(a)  # [1, 2, 3, 5]

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