Understanding Python Lists

This article is about Python lists. I want to show you some examples of unexpected behavior (for non-Python programmers) of lists in Python.

Imagine you have the following Python source code:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import copy

example1 = [[1, 5, 7], [3, 6], [], [8, 1, 6]]
example2 = example1[:]
example3 = list(example1)
example4 = copy.deepcopy(example1)

example1[1][0] = 0
example1.append(1)

print(example1)
print(example2)
print(example3)
print(example4)

How should the output look like? Think about it for a moment, then scroll down.

[[1,5,7],[0,6],[],[8,1,6],1]
[[1,5,7],[0,6],[],[8,1,6]]
[[1,5,7],[0,6],[],[8,1,6]]
[[1,5,7],[3,6],[],[8,1,6]]

Explanation

The reason for this behavior is how lists are handled in Python. The variable itself is basically only a pointer to the list. When you slice the list (myList[:]), you copy each value of the list into another list. If myList was a nested list, it contained pointers to the sublists. So, if you want to make a deep copy, you have to use the copy module.

• example1: Original list, modified directly
• example2: Shallow copy via slicing - shares references to inner lists
• example3: Shallow copy via list() constructor - same as example2
• example4: Deep copy - completely independent copy

Scoping (Historical Note)

Important: This behavior was changed in Python 3! In modern Python, list comprehensions have their own scope.

phimuemue added this example in my old blog:

In Python 2, there was an issue with scoping:

i = 0
[i for i in [1, 2, 3]]
print(i)  # yields 3 in Python 2, but 0 in Python 3

In Python 2, Python didn't create a new variable for the list comprehension but used the outer i. This was fixed in Python 3, where list comprehensions have their own scope.

Recursive Lists

Python allows you to create recursive data structures, which can lead to interesting output:

a = [1, 2, 3]
b = [4, 5, 6]
a.append(b)
print(a)
# Output: [1, 2, 3, [4, 5, 6]]

b.append(a)
print(b)
# Output: [4, 5, 6, [1, 2, 3, [...]]]

print(a)
# Output: [1, 2, 3, [4, 5, 6, [...]]]

The [...] represents the recursive reference to avoid infinite printing.

Modern Python Alternatives

For modern Python development, consider using:

• List comprehensions: More Pythonic than traditional loops
• copy.deepcopy(): For creating independent copies
• collections.deque: For frequent append/pop operations
• Type hints: For better code documentation